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1.1F: Branches and Subdisciplines of Biology - Biology


The field of biology can be divided into various branches and subdisciplines, which leads to careers that result in more focused fields.

Learning Objectives

  • Recognize the various subfields of biology; e.g. microbiology, genetics, evolutionary, etc.

Key Points

  • Biology is broad and focuses on the study of life from various perspectives.
  • The branches and subdisciplines of biology, which are highly focused areas, have resulted in the development of careers that are specific to these branches and subdisciplines.
  • Branches of biological study include microbiology, physiology, ecology and genetics; subdisciplines within these branches can include: microbial physiology, microbial ecology and microbial genetics.

Key Terms

  • genetic engineering: the deliberate modification of the genetic structure of an organism
  • forensic: Relating to the use of science and technology in the investigation and establishment of facts or evidence in a court of law.

Branches of Biological Study

The scope of biology is broad and therefore contains many branches and subdisciplines. Biologists may pursue one of those subdisciplines and work in a more focused field. The biological branches are divided according to the focus of the discipline and can even be divided based on the types of techniques and tools used to study that specific focus. However, with the increasing amount of basic biological information growing due to advances in technology and databases, there is often cross-discipline and collaboration between branches. For instance, molecular biology and biochemistry study biological processes at the molecular and chemical level, respectively, including interactions among molecules such as DNA, RNA, and proteins, as well as the way they are regulated. Microbiology, the study of microorganisms, is the study of the structure and function of single-celled organisms. It is quite a broad branch itself, and depending on the subject of study, there are also microbial physiologists, ecologists, and geneticists, among others.

Biological Disciplines and Careers

Forensic science is the application of science to answer questions related to the law. Biologists as well as chemists and biochemists can be forensic scientists. Forensic scientists provide scientific evidence for use in courts, and their job involves examining trace materials associated with crimes.Their job activities are primarily related to crimes against people such as murder, rape, and assault. Their work involves analyzing samples such as hair, blood, and other body fluids, including the processing of DNA found in many different environments and materials associated with the crime scenes.

Another field of biological study, neurobiology, is the study of the nervous system, and although it is considered a branch of biology, it is also recognized as an interdisciplinary field of study known as neuroscience. Because of its interdisciplinary nature, this subdiscipline focuses on different functions of the nervous system using molecular, cellular, developmental, medical, and computational approaches.

Additional branches of biology include paleontology, which uses fossils to study life’s history; zoology, which studies animals; and botany, which studies plants. Biologists can also specialize as biotechnologists, ecologists, or physiologists. This is just a small sample of the many fields that biologists can pursue.

Biology is the culmination of the achievements of the natural sciences from their inception to today. Excitingly, it is the cradle of emerging sciences such as the biology of brain activity, genetic engineering of custom organisms, and the biology of evolution that uses the laboratory tools of molecular biology to retrace the earliest stages of life on earth. A scan of news headlines—whether reporting on immunizations, a newly discovered species, sports doping, or a genetically-modified food—demonstrates the way biology is active in and important to our everyday world.


What are the Principal Branches of Biology and What Do They Study?

Below you will find a list of the Branches of biology And explain what they study.

Biology is the study of life and living organisms, including their structure, function, growth, evolution, distribution, identification and taxonomy.

Modern biology is a vast and eclectic field, composed of many branches and subdisciplines. However, despite the broad scope of biology, there are certain unifying concepts within it that consolidate it into a single coherent field.

In general, biology recognizes the cell as the basic unit of life, genes as the basic unit of heredity, and evolution as the engine that propels the creation of new species.

The subdisciplines of biology are defined by the scale at which organisms are studied, the types of organisms studied and the methods used to study them.

Aerobiology

According to Belmonte (2016) this branch of biology studies everything related to living airborne organisms: their diversity, how they affect the environment, ways of life, characteristics, etc.

Farming

It is dedicated to the study and development of techniques and activities for the cultivation of land: soil treatment, planting and harvesting crops to obtain vegetable products (vegetables, fruits, grains, grasses) for human and livestock feed. The science that deals with the study of agriculture as a practice is agronomy.

Anatomy

Science that studies the internal structures of living beings (animals, humans, plants).

Histology

Branch of biology that studies everything related to the organic tissues of living beings.

Astrobiology

It studies life in the universe as well as its origin, evolution, distribution, search for extraterrestrial life, etc.

Cellular Biology

Also called cell biochemistry, it studies the fundamental unit of living beings: the cell, at the microscopic and / or molecular level, its properties, structures, organs, interactions in the environment, life cycle, etc.

Developmental biology

Study of the process by which an organism develops at molecular, cellular, genetic and evolutionary levels.

Embryology

Study the formation and development of the embryo.

Evolutionary Biology

It studies the origin and the changes of the species over time (its evolution).

Marine biology

It studies the plants, animals and other living organisms that inhabit the marine ecosystem, as well as their conservation, interaction, etc.

Molecular biology

A branch of science that studies the formation, structure and function of molecules essential for life, such as nucleic acids and proteins, especially their role in cell replication and the transmission of genetic information.

Bioclimatology

It analyzes how the climate is related to the different territories of the earth to be able to predict the influence of the same on the living beings and the effect that causes in the morphological and structural characteristics of the flora, vegetation and landscape.

Biophysics

Also known as physical biology, it is an interdisciplinary science that applies theories, methods and precepts of physics for the analysis of biological phenomena.

Biogeography

It studies the distribution of the living beings on the terrestrial surface and the different patterns that arise from this.

Bioinformatics

Application of computer technologies in the field of biology More specifically everything related to the study, management and data collection.

Bioengineering

Also called biological engineering or biomedical engineering, it is a relatively new discipline that deals with the biological or medical application of the principles of engineering for the creation of new technologies and therapies.

Biomathematics

Interdisciplinary science that models natural and biological processes through the use of mathematical tools and techniques.

Biochemistry

Scientific discipline that studies the structure and chemical composition of living matter, especially proteins, carbohydrates, lipids, nucleic acids and other molecules, in addition to their functions and the transformations they experience throughout their lives.

Biotechnology

Biological technology, uses biological machinery, living organisms or biological systems for the creation of processes and products intended for a specific use.

Botany

Scientific study of plants.

Chronobiology

Study the time and biological rhythms of living organisms.

Biological conservation

It is concerned with the preservation of habitat, ecosystems and species to avoid possible extinctions and maintain biodiversity.

Cryobiology

It studies the effects of low temperatures on living organisms and what are the processes that occur when biological material is subjected to low temperatures.

Ecology

Science that studies the relationships of different living beings with each other and the environment in which they live.

Epidemiology

It investigates matters concerning public health and factors affecting health in populations.

Ethnobiology

Study of the dynamic relations between human beings, flora fauna and the environment.

Pharmacology

Study the preparation and use of different drugs and synthetic medicines.

Physiology

It is dedicated to the analysis of the functions of the living organism and how it performs its vital functions and activities.

Genetics

Study of the genes and mechanisms that regulate the transmission of the hereditary load.

Geobiology

It combines geology and biology to determine the interactions that occur between organisms and the environment.

Hematology

It studies the organs that produce the blood and the immunological elements present in it, as well as the diseases that are manifested by the alteration of the normal hematological values.

Immunobiology

It studies the structure and function of the immune system and the interaction with different antigens and antibodies.

Medicine

Science that studies everything related to health, prevention of diseases, cures, alleviating physical pain, etc.

Microbiology

Branch of biology that studies the microorganisms and the effects of these on other living organisms.

Bacteriology

Mycology

Parasitology

Virology

Study of viruses and other viral agents.

Neurobiology

It studies the anatomy, physiology and pathology of the nervous system.

Paleontology

It studies the fossils and analyzes the evidence that confirms the existence of life in prehistory.

Pathology

According to Robertson (2016), it is a branch of medical science that is responsible for studying the causes, origin, development and nature of diseases.

Primatology

Science that is in charge of studying primates.

Psychobiology

Functional study and mental behavior in relation to other biological processes.

Sociobiology

He studies biology on the foundations of sociology.

Toxicology

It is dedicated to studying how natural poisons are created that cause undesirable effects on living organisms

Zoology

It treats animals and animal life, including its structure, physiology, development, behavior and classification.


Various Branches of Biology

Biology is an overarching science, but within it are several branches – both pure (botany and zoology) and applied (biotechnology and biomedical engineering) – each with its unique focus.

Let’s take a look at some of the important branches of biology.

Based On Approach of Study

This includes the branches that focus on specific biological processes, such as organismal interaction either between different organisms or, within the same organism.

1. Anatomy (Greek anatomē = ‘dissection’)

Father of anatomy: Andreas Vesalius

Anatomy focuses on the study of physical structures and organs of living beings. It has many sub-disciplines, the major being gross or macroscopic anatomy and microscopic anatomy. Macroscopic anatomy deals with structures that can be viewed by the naked eye, while microscopic anatomy involves the cellular level study of internal organs and structures.

2. Physiology (Greek physis = “nature/origin and logia = “study of”)

Father of modern experimental physiology: Claude Bernard

Physiology is the study of how the human body functions. It describes the chemistry and physics behind body functions, from signal transduction in cells to how organ system works together.

Physiology forms the bridge between all of the other biomedical sciences. It enables information gained by molecular biology, genetics, pharmacology, biophysics, and biochemistry to be described in an integrated manner which can then be applied for improving quality of life.

3. Taxonomy (Greek taxis = arrangement or division, and nomos = law)

Father of Taxonomy: Carolus Linnaeus

Taxonomy is the science of naming and classifying living organisms based on their morphological, genetic, and biochemical characteristics. It also focuses on identifying both the similarities and differences between species.

4. Pathology (Greek pathos = “suffering”, and ology=”study of”)

Father of microscopic pathology: Rudolf Virchow

Pathology focuses on the cause, origin, and nature of diseases. It underpins everything from prevention to treatment and patient care.

5. Paleontology (Greek palaios = “ancient”, ontos = “being” and logy = study)

Father of Paleontology: Georges Cuvier

Paleontology is the study of ancient life on Earth based on fossils. Fossils are the remains of plants, animals, fungi, bacteria, and other single-celled organisms that have been replaced by sedimentary rocks or impressions of organisms preserved in rock. It’s an interdisciplinary science involving geology, archaeology, chemistry, biology, archaeology, and anthropology.

6. Immunology (a hybrid from immune + – ology)

Father of Immunology: Louis Pasteur

Immunology is a relatively new branch of biology that covers how the immune system of an organism works against foreign bodies including pathogens and also experiments on how immunity can be built.

7. Genetics (Greek Genesis = origin)

Father of Genetics: Gregor Mendel

Genetics is the branch of biology that studies how different qualities, called traits, are passed down from parents to kids. Genetics is majorly all about genes, DNA, and RNA. It also gives an insight into what makes an organism unique, why certain diseases run in families, and more.

8. Eugenics (Greek Eu = “good” and genes = “born”)

Father of Eugenics: Francis Galton

Another relatively new branch of biology originated in the early part of the 20th century. Eugenics advocates improving human life through genetic selection. Modern eugenics, also called human genetic engineering, has come a long way and offers hope for treating many genetic disorders. Even so, it remains a hot debate topic.

9. Ecology (Greek “oikos” = household and “logos” = study)

Father of Modern Ecology: Eugene Odum

Ecology is an interdisciplinary field that includes biology and Earth science. It studies interactions among living things and their environment. It gives insights into the benefits of the ecosystem, the impact of ecosystem destruction, sustainability, etc., Ecological studies also provide information about how we can use natural resources in ways that leave the Earth healthy for future generations.

10. Embryology (Greek embryon = the unborn and logy = study of)

Father of modern comparative embryology: Karl Ernst von Baer

Embryology is the branch of biology concerned with embryogenesis, the development of the embryo from a fertilized egg cell. In its widest sense, embryology involves two aspects: 1) ontogeny, which deals with the development of an organism and 2) phylogeny, which concerns itself with organisms and their evolutionary development. Embryological studies also help better understand congenital disorders and developing effective treatments.

11. Sociobiology

Father of sociobiology: E. O. Wilson

A relatively interesting concept introduced back in the 1970s, sociobiology deals with how natural selection drives behavioral patterns in all organisms, including human beings. It is also called behavioral ecology.

This category focuses on the study of individual branches of life.

1. Botany (Greek botanē = pasture/grass/fodder)

Father of Botany: Theophrastus

Botany is the study of plant life. It a broader sense, it deals with names, structure, ecology, habitat, and importance of Kingdom Plantae. Some of the branches of Botany are,

  • Agricultural Science – the study of economically important plants and their production
  • Agronomy – production of crops and management of soil
  • Plant breeding – deals with the development of improved varieties of plants
  • Agrostology – the study of grasses
  • Bryology – study of Bryophytes such as mosses, liverworts, and hornworts
  • Dendrology – study of woody plants such as shrubs and trees
  • Ethnobotany – study of plants and their relationship with humans
  • Horticulture – study of the Cultivation of plants.
  • Palynology – study of pollen grains and spores
  • Phenology – study of the timing of root and shoot germination flowering and fruiting

2. Zoology (Greek “Zoon” = animal and “logos” = study)

Father of zoology: Aristotle.

Zoology is a branch of biology that deals with the study of Kingdom Animalia. It focuses on everything from the development, behavior, and physiology of animals. Some sub-divisions are,

  • Entomology – study of insects
  • Herpetology – study of amphibians and reptiles
  • Ichthyology – study of fish
  • Mammalogy – study of mammals
  • Ornithology – study of birds
  • Nothology – study of animal and plant hybrids
  • Primatology – study of primates

3. Microbiology (Greek mīkros = “small”, bios = “life” and logia)

Father of microbiology: Antonie van Leeuwenhoek

Microbiology is the study of microorganisms that are too small to be seen with the naked eye. It is a wide area of science that includes, bacteriology (study of bacteria), virology (study of virus), mycology (study of fungi), phycology (study of algae), parasitology (study of parasites), and other branches of biology.

Based on Evolutionary Point of View

1. Evolutionary Biology

It is the broader branch of biology concerned with the origin, diversification, and adaptation of life forms on Earth. It also encompasses other disciplines of biology such as systematics, genetics, ecology, and paleontology. Some of the hot topics currently being studied are evolution speciation, genetic drift, and molecular evolution.

2. Developmental Biology

Developmental biology is a sub-field of evolutionary biology. It studies the fundamental processes starting from fertilization of the egg to its transformation into a whole organism. Modern developmental biology also focuses on the growth and development of organisms at cellular, genetic, and evolutionary levels.

3. Ichnology

It is a fascinating branch of biology that solves mysteries of fossils. Ichnologists follow plants and animal footprints in an attempt to reconstruct a sequence of events to identify organisms long dead. It is multi-discipline encompassing paleontology, biology, sedimentology, stratigraphy, pedology, and geomorphology.

Recently, Neoichnology – the study of modern and experimentally produced traces – is having a moment.

4. Epigenetics

It is an emerging field of biology. Traditional genetics describes the way the DNA in our genes are passed from parents to children. Whereas epigenetics describes how turning genes on or off makes different cell types in the body. It also studies the impact of environmental factors and lifestyle on genes. The literal meaning of the term epigenetics is ‘above the genes’.


The Scientific Method

The scientific method is a process by which observations are questioned hypotheses are created and tested and the results are analyzed.

Learning Objectives

Discuss hypotheses and the components of a scientific experiment as part of the scientific method

Key Takeaways

Key Points

  • In the scientific method, observations lead to questions that require answers.
  • In the scientific method, the hypothesis is a testable statement proposed to answer a question.
  • In the scientific method, experiments (often with controls and variables) are devised to test hypotheses.
  • In the scientific method, analysis of the results of an experiment will lead to the hypothesis being accepted or rejected.

Key Terms

  • scientific method: a way of discovering knowledge based on making falsifiable predictions (hypotheses), testing them, and developing theories based on collected data
  • hypothesis: an educated guess that usually is found in an “if…then…” format
  • control group: a group that contains every feature of the experimental group except it is not given the manipulation that is hypothesized

The Scientific Method

Biologists study the living world by posing questions about it and seeking science -based responses. This approach is common to other sciences as well and is often referred to as the scientific method. The scientific method was used even in ancient times, but it was first documented by England’s Sir Francis Bacon (1561–1626) who set up inductive methods for scientific inquiry. The scientific method can be applied to almost all fields of study as a logical, rational, problem-solving method.

Sir Francis Bacon: Sir Francis Bacon (1561–1626) is credited with being the first to define the scientific method.

The scientific process typically starts with an observation (often a problem to be solved) that leads to a question. Let’s think about a simple problem that starts with an observation and apply the scientific method to solve the problem. A teenager notices that his friend is really tall and wonders why. So his question might be, “Why is my friend so tall? ”

The Scientific Method: The scientific method consists of a series of well-defined steps. If a hypothesis is not supported by experimental data, a new hypothesis can be proposed.

Proposing a Hypothesis

Recall that a hypothesis is an educated guess that can be tested. Hypotheses often also include an explanation for the educated guess. To solve one problem, several hypotheses may be proposed. For example, the student might believe that his friend is tall because he drinks a lot of milk. So his hypothesis might be “If a person drinks a lot of milk, then they will grow to be very tall because milk is good for your bones.” Generally, hypotheses have the format “If…then…” Keep in mind that there could be other responses to the question therefore, other hypotheses may be proposed. A second hypothesis might be, “If a person has tall parents, then they will also be tall, because they have the genes to be tall. ”

Once a hypothesis has been selected, the student can make a prediction. A prediction is similar to a hypothesis but it is truly a guess. For instance, they might predict that their friend is tall because he drinks a lot of milk.

Testing a Hypothesis

A valid hypothesis must be testable. It should also be falsifiable, meaning that it can be disproven by experimental results. Importantly, science does not claim to “prove” anything because scientific understandings are always subject to modification with further information. This step—openness to disproving ideas—is what distinguishes sciences from non-sciences. The presence of the supernatural, for instance, is neither testable nor falsifiable. To test a hypothesis, a researcher will conduct one or more experiments designed to eliminate one or more of the hypotheses. Each experiment will have one or more variables and one or more controls. A variable is any part of the experiment that can vary or change during the experiment. The control group contains every feature of the experimental group except it is not given the manipulation that is hypothesized. For example, a control group could be a group of varied teenagers that did not drink milk and they could be compared to the experimental group, a group of varied teenagers that did drink milk. Thus, if the results of the experimental group differ from the control group, the difference must be due to the hypothesized manipulation rather than some outside factor. To test the first hypothesis, the student would find out if drinking milk affects height. If drinking milk has no affect on height, then there must be another reason for the height of the friend. To test the second hypothesis, the student could check whether or not his friend has tall parents. Each hypothesis should be tested by carrying out appropriate experiments. Be aware that rejecting one hypothesis does not determine whether or not the other hypotheses can be accepted. It simply eliminates one hypothesis that is not valid. Using the scientific method, the hypotheses that are inconsistent with experimental data are rejected.

While this “tallness” example is based on observational results, other hypotheses and experiments might have clearer controls. For instance, a student might attend class on Monday and realize she had difficulty concentrating on the lecture. One hypothesis to explain this occurrence might be, “If I eat breakfast before class, then I am better able to pay attention.” The student could then design an experiment with a control to test this hypothesis.

The scientific method may seem too rigid and structured. It is important to keep in mind that although scientists often follow this sequence, there is flexibility. Many times, science does not operate in a linear fashion. Instead, scientists continually draw inferences and make generalizations, finding patterns as their research proceeds. Scientific reasoning is more complex than the scientific method alone suggests.


Chapter Summary

Biology is the science that studies living organisms and their interactions with one another and their environments. Science attempts to describe and understand the nature of the universe in whole or in part by rational means. Science has many fields those fields related to the physical world and its phenomena are considered natural sciences.

Science can be basic or applied. The main goal of basic science is to expand knowledge without any expectation of short-term practical application of that knowledge. The primary goal of applied research, however, is to solve practical problems.

Two types of logical reasoning are used in science. Inductive reasoning uses particular results to produce general scientific principles. Deductive reasoning is a form of logical thinking that predicts results by applying general principles. The common thread throughout scientific research is the use of the scientific method, a step-based process that consists of making observations, defining a problem, posing hypotheses, testing these hypotheses, and drawing one or more conclusions. The testing uses proper controls. Scientists present their results in peer-reviewed scientific papers published in scientific journals. A scientific research paper consists of several well-defined sections: introduction, materials and methods, results, and, finally, a concluding discussion. Review papers summarize the research done in a particular field over a period of time.

1.2 Themes and Concepts of Biology

Biology is the science of life. All living organisms share several key properties such as order, sensitivity or response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing. Living things are highly organized parts of a hierarchy that includes atoms, molecules, organelles, cells, tissues, organs, and organ systems. Organisms, in turn, are grouped as populations, communities, ecosystems, and the biosphere. The great diversity of life today evolved from less-diverse ancestral organisms over billions of years. A diagram called a phylogenetic tree can be used to show evolutionary relationships among organisms.

Biology is very broad and includes many branches and subdisciplines. Examples include molecular biology, microbiology, neurobiology, zoology, and botany, among others.


Contents

    – Natural, physical, or material world and its phenomena, or Natural environment – All living and non-living things occurring naturally, generally on Earth – Community of living organisms together with the nonliving components of their environment, or Biome – Plants and animals associated with an environment
      – Associated populations of species in a given area, or Biocoenosis – The interacting organisms living together in a habitat in the field of palaeontology
        – Basic unit of taxonomic classification
          – All the organisms of a given species that live in the specified region
            – Any individual living physical system
            – Natural interconnection of food chains

          Ecology can also be classified on the basis of:

          • the primary kinds of organism under study, e.g. animal ecology, plant ecology, insect ecology
          • the biomes principally studied, e.g. forest ecology, grassland ecology, desert ecology, benthic ecology, marine ecology, urban ecology
          • the geographic or climatic area, e.g. arctic ecology, tropical ecology
          • the spatial scale under consideration, e.g. macroecology, landscape ecology
          • the philosophical approach, e.g. systems ecology which adopts a holistic approach
          • the methods used, e.g. molecular ecology.

          Ecology is a broad discipline comprising many subdisciplines. The field of ecology can be subdivided according to several classification schemes:

          By level of complexity or scope Edit

          Arranged from lowest to highest complexity:

            – The study of adaptation of an organism's physiology to environmental conditions, or Behavioral ecology – Study of the evolutionary basis for animal behavior due to ecological pressures , also known as autoecology – Study of the dynamics of species populations and how these populations interact with the environment , also known as synecology – Associated populations of species in a given area – The study of living and non-living components of ecosystems and their interactions – Holistic approach to the study of ecological systems – The science of relationships between ecological processes in the environment and particular ecosystems

          By organisms under study Edit

            – Scientific study of the relationships between living animals and their environment – Study of the evolutionary basis for animal behavior due to ecological pressures – The study of the distribution of species and ecosystems in geographic space and through geological time – The study of how insects interact with the surrounding environment – Study of the relationship of microorganisms with their environment – The study of interactions between organisms and their environments across geologic timescales – The study of effect of the environment on the abundance and distribution of plants

          By biome under study Edit

            – The study of the interaction of sea-floor organisms with each other and with the environment – The study of interactions between both biotic and abiotic components of desert environments – The study of interactions between the biota and environment in forets – The study of the interactions between organisms and environment in the sea – The study of interactions between organisms and the environment in water – The study of the relation of living organisms with each other and their surroundings in the context of an urban environment.

          By geographic or climatic area under study Edit

            – The study of the relationships between biotic and abiotic factors in the arctic – The relationship between plants and animals and a polar environment – The study of the relationships between the biotic and abiotic components of the tropics

          By spatial scale under study Edit

          By ecological aspects or phenomena under investigation Edit

            – which deals with the ecological role of biological chemicals used in a wide range of areas including defense against predators and attraction of mates – The study of adaptation of an organism's physiology to environmental conditions – which studies the interaction of physiological traits with the abiotic environment – which looks at the ecological role of toxic chemicals (often pollutants, but also naturally occurring compounds) – Study of how interactions among species and between species and their environment affect species through selection and adaptation – or ecoevolution which looks at evolutionary changes in the context of the populations and communities in which the organisms exist – which looks at the role of fire in the environment of plants and animals and its effect on ecological communities – the study of the roles, or functions, that certain species (or groups thereof) play in an ecosystem – Study of genetic material in the environment – the ecology of the pedosphere

          By technique used for investigation Edit

          By environmental approach Edit

            – The practice of employing ecological principles and understanding to solve real world problems (includes agroecology and conservation biology). – The study of threats to biological diversity – which studies how to reduce the risk of species extinction. – Ecological and environmental philosophy – an ecological and environmental philosophy promoting the inherent worth of living beings regardless of their instrumental utility to human needs, plus a radical restructuring of modern human societies in accordance with such ideas. – Philosophy of ecological harmony or equilibrium as developed by Arne Næss or Félix Guattari – philosophy of ecological harmony or equilibrium. – Scientific study of renewing and restoring ecosystems – which attempts to understand the ecological basis needed to restore impaired or damaged ecosystems. – Special consideration to individuals solely on the basis of their species membership – involves the assignment of different values, rights, or special consideration to individuals solely on the basis of their species membership. – value system that is centered on technology and its ability to control and protect the environment.

          Ecology-involved interdisciplinary fields Edit

            – Study of ecological processes in agriculture – The study of chemical cycles of the earth that are either driven by or influence biological activity – Design that minimizes environmentally destructive impacts by integrating itself with living processes – Use of ecology and engineering to predict, design, construct or restore, and manage ecosystems that integrate "human society with its natural environment for the benefit of both" – Study of the relationships between the symbolism and the ecology of the plants, fungi and animals associated with cultural events – Study of the relationship between humans and their natural, social, and built environments – The study of cultural adaptations to environments – Study of relationships between people and their environment – study of matter and energy flow in industrial systems – The study of interactions between organisms and their environments across geologic timescales

          Other subdisciplines Edit

          Other branches of ecology include:

            , which focuses on understanding directed vegetation change , the study of the relationship between temperature and organisms

          Ecology has also inspired (and lent its name to) other non-biological disciplines such as


          Contents

          "Biology" derives from the Ancient Greek words of βίος romanized bíos meaning "life" and -λογία romanized logía (-logy) meaning "branch of study" or "to speak". [11] [12] Those combined make the Greek word βιολογία romanized biología meaning biology. Despite this, the term βιολογία as a whole didn't exist in Ancient Greek. The first to borrow it was the English and French (biologie). Historically there was another term for "biology" in English, lifelore it is rarely used today.

          The Latin-language form of the term first appeared in 1736 when Swedish scientist Carl Linnaeus (Carl von Linné) used biologi in his Bibliotheca Botanica. It was used again in 1766 in a work entitled Philosophiae naturalis sive physicae: tomus III, continens geologian, biologian, phytologian generalis, by Michael Christoph Hanov, a disciple of Christian Wolff. The first German use, Biologie, was in a 1771 translation of Linnaeus' work. In 1797, Theodor Georg August Roose used the term in the preface of a book, Grundzüge der Lehre van der Lebenskraft. Karl Friedrich Burdach used the term in 1800 in a more restricted sense of the study of human beings from a morphological, physiological and psychological perspective (Propädeutik zum Studien der gesammten Heilkunst). The term came into its modern usage with the six-volume treatise Biologie, oder Philosophie der lebenden Natur (1802–22) by Gottfried Reinhold Treviranus, who announced: [13]

          The objects of our research will be the different forms and manifestations of life, the conditions and laws under which these phenomena occur, and the causes through which they have been affected. The science that concerns itself with these objects we will indicate by the name biology [Biologie] or the doctrine of life [Lebenslehre].

          The earliest of roots of science, which included medicine, can be traced to ancient Egypt and Mesopotamia in around 3000 to 1200 BCE. [14] [15] Their contributions later entered and shaped Greek natural philosophy of classical antiquity. [14] [15] [16] [17] Ancient Greek philosophers such as Aristotle (384–322 BCE) contributed extensively to the development of biological knowledge. His works such as History of Animals were especially important because they revealed his naturalist leanings, and later more empirical works that focused on biological causation and the diversity of life. Aristotle's successor at the Lyceum, Theophrastus, wrote a series of books on botany that survived as the most important contribution of antiquity to the plant sciences, even into the Middle Ages. [18]

          Scholars of the medieval Islamic world who wrote on biology included al-Jahiz (781–869), Al-Dīnawarī (828–896), who wrote on botany, [19] and Rhazes (865–925) who wrote on anatomy and physiology. Medicine was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought, especially in upholding a fixed hierarchy of life.

          Biology began to quickly develop and grow with Anton van Leeuwenhoek's dramatic improvement of the microscope. It was then that scholars discovered spermatozoa, bacteria, infusoria and the diversity of microscopic life. Investigations by Jan Swammerdam led to new interest in entomology and helped to develop the basic techniques of microscopic dissection and staining. [20]

          Advances in microscopy also had a profound impact on biological thinking. In the early 19th century, a number of biologists pointed to the central importance of the cell. Then, in 1838, Schleiden and Schwann began promoting the now universal ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, although they opposed the idea that (3) all cells come from the division of other cells. Thanks to the work of Robert Remak and Rudolf Virchow, however, by the 1860s most biologists accepted all three tenets of what came to be known as cell theory. [21] [22]

          Meanwhile, taxonomy and classification became the focus of natural historians. Carl Linnaeus published a basic taxonomy for the natural world in 1735 (variations of which have been in use ever since), and in the 1750s introduced scientific names for all his species. [23] Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility of common descent. Although he was opposed to evolution, Buffon is a key figure in the history of evolutionary thought his work influenced the evolutionary theories of both Lamarck and Darwin. [24]

          Serious evolutionary thinking originated with the works of Jean-Baptiste Lamarck, who was the first to present a coherent theory of evolution. [26] He posited that evolution was the result of environmental stress on properties of animals, meaning that the more frequently and rigorously an organ was used, the more complex and efficient it would become, thus adapting the animal to its environment. Lamarck believed that these acquired traits could then be passed on to the animal's offspring, who would further develop and perfect them. [27] However, it was the British naturalist Charles Darwin, combining the biogeographical approach of Humboldt, the uniformitarian geology of Lyell, Malthus's writings on population growth, and his own morphological expertise and extensive natural observations, who forged a more successful evolutionary theory based on natural selection similar reasoning and evidence led Alfred Russel Wallace to independently reach the same conclusions. [28] [29] Darwin's theory of evolution by natural selection quickly spread through the scientific community and soon became a central axiom of the rapidly developing science of biology.

          The basis for modern genetics began with the work of Gregor Mendel, who presented his paper, "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), in 1865, [30] which outlined the principles of biological inheritance, serving as the basis for modern genetics. [31] However, the significance of his work was not realized until the early 20th century when evolution became a unified theory as the modern synthesis reconciled Darwinian evolution with classical genetics. [32] In the 1940s and early 1950s, a series of experiments by Alfred Hershey and Martha Chase pointed to DNA as the component of chromosomes that held the trait-carrying units that had become known as genes. A focus on new kinds of model organisms such as viruses and bacteria, along with the discovery of the double-helical structure of DNA by James Watson and Francis Crick in 1953, marked the transition to the era of molecular genetics. From the 1950s to the present times, biology has been vastly extended in the molecular domain. The genetic code was cracked by Har Gobind Khorana, Robert W. Holley and Marshall Warren Nirenberg after DNA was understood to contain codons. Finally, the Human Genome Project was launched in 1990 with the goal of mapping the general human genome. This project was essentially completed in 2003, [33] with further analysis still being published. The Human Genome Project was the first step in a globalized effort to incorporate accumulated knowledge of biology into a functional, molecular definition of the human body and the bodies of other organisms.

          Chemical basis

          Atoms and molecules

          All living organisms are made up of matter and all matter is made up of elements. [34] Oxygen, carbon, hydrogen, and nitrogen are the four elements that account for 96% of all living organisms, with calcium, phosphorus, sulfur, sodium, chlorine, and magnesium accounting for the remaining 3.7%. [34] Different elements can combine to form compounds such as water, which is fundamental to life. [34] Life on Earth began from water and remained there for about three billions years prior to migrating onto land. [35] Matter can exist in different states as a solid, liquid, or gas.

          The smallest unit of an element is an atom, which is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Individual atoms can be held together by chemical bonds to form molecules and ionic compounds. [34] Common types of chemical bonds include ionic bonds, covalent bonds, and hydrogen bonds. Ionic bonding involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, [36] and is the primary interaction occurring in ionic compounds. Ions are atoms (or groups of atoms) with an electrostatic charge. Atoms that gain electrons make negatively charged ions (called anions) whereas those that lose electrons make positively charged ions (called cations).

          Unlike ionic bonds, a covalent bond involves the sharing of electron pairs between atoms. These electron pairs and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding. [37]

          A hydrogen bond is primarily an electrostatic force of attraction between a hydrogen atom which is covalently bound to a more electronegative atom or group such as oxygen. A ubiquitous example of a hydrogen bond is found between water molecules. In a discrete water molecule, there are two hydrogen atoms and one oxygen atom. Two molecules of water can form a hydrogen bond between them. When more molecules are present, as is the case with liquid water, more bonds are possible because the oxygen of one water molecule has two lone pairs of electrons, each of which can form a hydrogen bond with a hydrogen on another water molecule.

          Organic compounds

          With the exception of water, nearly all the molecules that make up each living organism contain carbon. [38] [39] Carbon can form very long chains of interconnecting carbon–carbon bonds, which are strong and stable. The simplest form of an organic molecule is the hydrocarbon, which is a large family of organic compounds that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other atoms. When combined with other elements such as oxygen, hydrogen, phosphorus, and sulfur, carbon can form many groups of important biological compounds such as sugars, fats, amino acids, and nucleotides.

          Macromolecules

          Molecules such as sugars, amino acids, and nucleotides can act as single repeating units called monomers to form chain-like molecules called polymers via a chemical process called condensation. [40] For example, amino acids can form polypeptides whereas nucleotides can form strands of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Polymers make up three of the four macromolecules (polysaccharides, lipids, proteins, and nucleic acids) that are found in all living organisms. Each macromolecule plays a specialized role within any given cell. Some polysaccharides, for instance, can function as storage material that can be hydrolyzed to provide cells with sugar. Lipids are the only class of macromolecules that are not made up of polymers and the most biologically important lipids are fats, phospholipids, and steroids. [40] Proteins are the most diverse of the macromolecules, which include enzymes, transport proteins, large signaling molecules, antibodies, and structural proteins. Finally, nucleic acids store, transmit, and express hereditary information. [40]

          Cells

          Cell theory states that cells are the fundamental units of life, that all living things are composed of one or more cells, and that all cells arise from preexisting cells through cell division. [41] Most cells are very small, with diameters ranging from 1 to 100 micrometers and are therefore only visible under a light or electron microscope. [42] There are generally two types of cells: eukaryotic cells, which contain a nucleus, and prokaryotic cells, which do not. Prokaryotes are single-celled organisms such as bacteria, whereas eukaryotes can be single-celled or multicellular. In multicellular organisms, every cell in the organism's body is derived ultimately from a single cell in a fertilized egg.

          Cell structure

          Every cell is enclosed within a cell membrane that separates its cytoplasm from the extracellular space. [43] A cell membrane consists of a lipid bilayer, including cholesterols that sit between phospholipids to maintain their fluidity at various temperatures. Cell membranes are semipermeable, allowing small molecules such as oxygen, carbon dioxide, and water to pass through while restricting the movement of larger molecules and charged particles such as ions. [44] Cell membranes also contains membrane proteins, including integral membrane proteins that go across the membrane serving as membrane transporters, and peripheral proteins that loosely attach to the outer side of the cell membrane, acting as enzymes shaping the cell. [45] Cell membranes are involved in various cellular processes such as cell adhesion, storing electrical energy, and cell signalling and serve as the attachment surface for several extracellular structures such as a cell wall, glycocalyx, and cytoskeleton.

          Within the cytoplasm of a cell, there are many biomolecules such as proteins and nucleic acids. [46] In addition to biomolecules, eukaryotic cells have specialized structures called organelles that have their own lipid bilayers or are spatially units. These organelles include the cell nucleus, which contains a cell's genetic information, or mitochondria, which generates adenosine triphosphate (ATP) to power cellular processes. Other organelles such as endoplasmic reticulum and Golgi apparatus play a role in the synthesis and packaging of proteins, respectively. Biomolecules such as proteins can be engulfed by lysosomes, another specialized organelle. Plant cells have additional organelles that distinguish them from animal cells such as a cell wall, chloroplasts, and vacuole.

          Metabolism

          All cells require energy to sustain cellular processes. Energy is the capacity to do work, which, in thermodynamics, can be calculated using Gibbs free energy. According to the first law of thermodynamics, energy is conserved, i.e., cannot be created or destroyed. Hence, chemical reactions in a cell do not create new energy but are involved instead in the transformation and transfer of energy. [47] Nevertheless, all energy transfers lead to some loss of usable energy, which increases entropy (or state of disorder) as stated by the second law of thermodynamics. As a result, living organisms such as cells require continuous input of energy to maintain a low state of entropy. In cells, energy can be transferred as electrons during redox (reduction–oxidation) reactions, stored in covalent bonds, and generated by the movement of ions (e.g., hydrogen, sodium, potassium) across a membrane.

          Metabolism is the set of life-sustaining chemical reactions in organisms. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes the conversion of food/fuel to building blocks for proteins, lipids, nucleic acids, and some carbohydrates and the elimination of metabolic wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolic reactions may be categorized as catabolic – the breaking down of compounds (for example, the breaking down of glucose to pyruvate by cellular respiration) or anabolic – the building up (synthesis) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy.

          The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts – they allow a reaction to proceed more rapidly without being consumed by it – by reducing the amount of activation energy needed to convert reactants into products. Enzymes also allow the regulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.

          Cellular respiration

          Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. [48] The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy because weak high-energy bonds, in particular in molecular oxygen, [49] are replaced by stronger bonds in the products. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are redox reactions. Although cellular respiration is technically a combustion reaction, it clearly does not resemble one when it occurs in a living cell because of the slow, controlled release of energy from the series of reactions.

          Sugar in the form of glucose is the main nutrient used by animal and plant cells in respiration. Cellular respiration involving oxygen is called aerobic respiration, which has four stages: glycolysis, citric acid cycle (or Krebs cycle), electron transport chain, and oxidative phosphorylation. [50] Glycolysis is a metabolic process that occurs in the cytoplasm whereby glucose is converted into two pyruvates, with two net molecules of ATP being produced at the same time. [50] Each pyruvate is then oxidized into acetyl-CoA by the pyruvate dehydrogenase complex, which also generates NADH and carbon dioxide. Acetyl-Coa enters the citric acid cycle, which takes places inside the mitochondrial matrix. At the end of the cycle, the total yield from 1 glucose (or 2 pyruvates) is 6 NADH, 2 FADH2, and 2 ATP molecules. Finally, the next stage is oxidative phosphorylation, which in eukaryotes, occurs in the mitochondrial cristae. Oxidative phosphorylation comprises the electron transport chain, which is a series of four protein complexes that transfer electrons from one complex to another, thereby releasing energy from NADH and FADH2 that is coupled to the pumping of protons (hydrogen ions) across the inner mitochondrial membrane (chemiosmosis), which generates a proton motive force. [50] Energy from the proton motive force drives the enzyme ATP synthase to synthesize more ATPs by phosphorylating ADPs. The transfer of electrons terminates with molecular oxygen being the final electron acceptor.

          If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion but remains in the cytoplasm, where it is converted to waste products that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD + so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD + for glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product is lactic acid. This type of fermentation is called lactic acid fermentation. In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD + regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD + attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol fermentation. The ATP generated in this process is made by substrate-level phosphorylation, which does not require oxygen.

          Photosynthesis

          Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organism's metabolic activities via cellular respiration. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water. [51] [52] [53] In most cases, oxygen is also released as a waste product. Most plants, algae, and cyanobacteria perform photosynthesis, which is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth. [54]

          Photosynthesis has four stages: Light absorption, electron transport, ATP synthesis, and carbon fixation. [50] Light absorption is the initial step of photosynthesis whereby light energy is absorbed by chlorophyll pigments attached to proteins in the thylakoid membranes. The absorbed light energy is used to remove electrons from a donor (water) to a primary electron acceptor, a quinone designated as Q. In the second stage, electrons move from the quinone primary electron acceptor through a series of electron carriers until they reach a final electron acceptor, which is usually the oxidized form of NADP + , which is reduced to NADPH, a process that takes place in a protein complex called photosystem I (PSI). The transport of electrons is coupled to the movement of protons (or hydrogen) from the stroma to the thylakoid membrane, which forms a pH gradient across the membrane as hydrogen becomes more concentrated in the lumen than in the stroma. This is analogous to the proton-motive force generated across the inner mitochondrial membrane in aerobic respiration. [50]

          During the third stage of photosynthesis, the movement of protons down their concentration gradients from the thylakoid lumen to the stroma through the ATP synthase is coupled to the synthesis of ATP by that same ATP synthase. [50] The NADPH and ATPs generated by the light-dependent reactions in the second and third stages, respectively, provide the energy and electrons to drive the synthesis of glucose by fixing atmospheric carbon dioxide into existing organic carbon compounds, such as ribulose bisphosphate (RuBP) in a sequence of light-independent (or dark) reactions called the Calvin cycle. [55]

          Cell signaling

          Cell communication (or signaling) is the ability of cells to receive, process, and transmit signals with its environment and with itself. [56] [57] Signals can be non-chemical such as light, electrical impulses, and heat, or chemical signals (or ligands) that interact with receptors, which can be found embedded in the cell membrane of another cell or located deep inside a cell. [58] [57] There are generally four types of chemical signals: autocrine, paracrine, juxtacrine, and hormones. [58] In autocrine signaling, the ligand affects the same cell that releases it. Tumor cells, for example, can reproduce uncontrollably because they release signals that initiate their own self-division. In paracrine signaling, the ligand diffuses to nearby cells and affect them. For example, brain cells called neurons release ligands called neurotransmitters that diffuse across a synaptic cleft to bind with a receptor on an adjacent cell such as another neuron or muscle cell. In juxtacrine signaling, there is direct contact between the signaling and responding cells. Finally, hormones are ligands that travel through the circulatory systems of animals or vascular systems of plants to reach their target cells. Once a ligand binds with a receptor, it can influence the behavior of another cell, depending on the type of receptor. For instance, neurotransmitters that bind with an inotropic receptor can alter the excitability of a target cell. Other types of receptors include protein kinase receptors (e.g., receptor for the hormone insulin) and G protein-coupled receptors. Activation of G protein-coupled receptors can initiate second messenger cascades. The process by which a chemical or physical signal is transmitted through a cell as a series of molecular events is called signal transduction

          Cell cycle

          The cell cycle is a series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA and some of its organelles, and the subsequent partitioning of its cytoplasm into two daughter cells in a process called cell division. [59] In eukaryotes (i.e., animal, plant, fungal, and protist cells), there are two distinct types of cell division: mitosis and meiosis. [60] Mitosis is part of the cell cycle, in which replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of interphase (during which the DNA is replicated) and is often followed by telophase and cytokinesis which divides the cytoplasm, organelles and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components. The different stages of mitosis all together define the mitotic phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells. [61] The cell cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, skin, blood cells, and some internal organs are renewed. After cell division, each of the daughter cells begin the interphase of a new cycle. In contrast to mitosis, meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions. [62] Homologous chromosomes are separated in the first division (meiosis I), and sister chromatids are separated in the second division (meiosis II). Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor.

          Prokaryotes (i.e., archaea and bacteria) can also undergo cell division (or binary fission). Unlike the processes of mitosis and meiosis in eukaryotes, binary fission takes in prokaryotes takes place without the formation of a spindle apparatus on the cell. Before binary fission, DNA in the bacterium is tightly coiled. After it has uncoiled and duplicated, it is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting. Growth of a new cell wall begins to separate the bacterium (triggered by FtsZ polymerization and "Z-ring" formation) [63] The new cell wall (septum) fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods, ribosomes, and plasmids.

          Genetics

          Inheritance

          Genetics is the scientific study of inheritance. [64] [65] [66] Mendelian inheritance, specifically, is the process by which genes and traits are passed on from parents to offspring. [31] It was formulated by Gregor Mendel, based on his work with pea plants in the mid-nineteenth century. Mendel established several principles of inheritance. The first is that genetic characteristics, which are now called alleles, are discrete and have alternate forms (e.g., purple vs. white or tall vs. dwarf), each inherited from one of two parents. Based on his law of dominance and uniformity, which states that some alleles are dominant while others are recessive an organism with at least one dominant allele will display the phenotype of that dominant allele. [67] Exceptions to this rule include penetrance and expressivity. [31] Mendel noted that during gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene, which is stated by his law of segregation. Heterozygotic individuals produce gametes with an equal frequency of two alleles. Finally, Mendel formulated the law of independent assortment, which states that genes of different traits can segregate independently during the formation of gametes, i.e., genes are unlinked. An exception to this rule would include traits that are sex-linked. Test crosses can be performed to experimentally determine the underlying genotype of an organism with a dominant phenotype. [68] A Punnett square can be used to predict the results of a test cross. The chromosome theory of inheritance, which states that genes are found on chromosomes, was supported by Thomas Morgans's experiments with fruit flies, which established the sex linkage between eye color and sex in these insects. [69] In humans and other mammals (e.g., dogs), it is not feasible or practical to conduct test cross experiments. Instead, pedigrees, which are genetic representations of family trees, [70] are used instead to trace the inheritance of a specific trait or disease through multiple generations. [71]

          Deoxyribonucleic acid (DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic hereditary information. The two DNA strands are known as polynucleotides as they are composed of monomers called nucleotides. [72] [73] Each nucleotide is composed of one of four nitrogenous bases (cytosine [C], guanine [G], adenine [A] or thymine [T]), a sugar called deoxyribose, and a phosphate group. The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. It is the sequence of these four bases along the backbone that encodes genetic information. Bases of the two polynucleotide strands are bound together by hydrogen bonds, according to base pairing rules (A with T and C with G), to make double-stranded DNA. The bases are divided into two groups: pyrimidines and purines. In DNA, the pyrimidines are thymine and cytosine whereas the purines are adenine and guanine. The two strands of DNA run in opposite directions to each other and are thus antiparallel. DNA is replicated once the two strands separate.

          A gene is a unit of heredity that corresponds to a region of DNA that influences the form or function of an organism in specific ways. DNA is found as linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. A chromosome is an organized structure consisting of DNA and histones. The set of chromosomes in a cell and any other hereditary information found in the mitochondria, chloroplasts, or other locations is collectively known as a cell's genome. In eukaryotes, genomic DNA is localized in the cell nucleus, or with small amounts in mitochondria and chloroplasts. [74] In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid. [75] The genetic information in a genome is held within genes, and the complete assemblage of this information in an organism is called its genotype. [76] Genes encode the information needed by cells for the synthesis of proteins, which in turn play a central role in influencing the final phenotype of the organism.

          Gene expression

          Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The process is summarized in the central dogma of molecular biology first formulated by Francis Crick in 1958. [77] [78] [79] Gene expression is the most fundamental level at which a genotype gives rise to a phenotype, i.e., observable trait. The genetic information stored in DNA represents the genotype, whereas the phenotype results from the synthesis of proteins that control an organism's structure and development, or that act as enzymes catalyzing specific metabolic pathways. A large part of DNA (e.g., >98% in humans) is non-coding, meaning that these sections do not serve as patterns for protein sequences. Messenger RNA (mRNA) strands are created using DNA strands as a template in a process called transcription, where DNA bases are exchanged for their corresponding bases except in the case of thymine (T), for which RNA substitutes uracil (U). [80] Under the genetic code, these mRNA strands specify the sequence of amino acids within proteins in a process called translation, which occurs in ribosomes. This process is used by all life—eukaryotes (including multicellular organisms), prokaryotes (bacteria and archaea), and utilized by viruses—to generate the macromolecular machinery for life. Gene products are often proteins, but in non-protein-coding genes such as transfer RNA (tRNA) and small nuclear RNA (snRNA), the product is a functional non-coding RNA. [81] [82] All steps in the gene expression process can be regulated, including the transcription, RNA splicing, translation, and post-translational modification of a protein. Regulation of gene expression gives control over the timing, location, and amount of a given gene product (protein or ncRNA) present in a cell and can have a profound effect on cellular structure and function.

          Genomes

          A genome is an organism's complete set of DNA, including all of its genes. [83] Sequencing and analysis of genomes can be done using high throughput DNA sequencing and bioinformatics to assemble and analyze the function and structure of entire genomes. [84] [85] [86] Many genes encode more than one protein, with posttranslational modifications increasing the diversity of proteins within a cell. A cell's proteome is its entire set of proteins expressed by its genome. [87] The genomes of prokaryotes are small, compact, and diverse. In contrast, the genomes of eukaryotes are larger and more complex such as having more regulatory sequences and much of its genome are made up of non-coding DNA sequences for functional RNA (rRNA, tRNA, and mRNA) or regulatory sequences. The genomes of various model organisms such as arabidopsis, fruit fly, mice, nematodes, and yeast have been sequenced. The sequencing of the entire human genome has yielded practical applications such as DNA fingerprinting, which can be used for paternity testing and forensics. In medicine, sequencing of the entire human genome has allowed for the identification mutations that cause tumors as well as genes that cause a specific genetic disorder. [87]

          Biotechnology

          Biotechnology is the use of cells or living organisms to develop products for humans. [88] It includes tools such as recombinant DNA, which are DNA molecules formed by laboratory methods of genetic recombination such as molecular cloning, which bring together genetic material from multiple sources, creating sequences that would otherwise not be found in a genome. Other tools include the use of genomic libraries, DNA microarrays, expression vectors, synthetic genomics, and CRISPR gene editing. [88] [89] Many of these tools have wide applications such as creating medically useful proteins, or improving plant cultivation and animal husbandry. [88] Human insulin, for example, was the first medicine to be made using recombinant DNA technology. Other approaches such as pharming can produce large quantities of medically useful products through the use of genetically modified organisms. [88]

          Genes, development, and evolution

          Development is the process by which a multicellular organism (plant or animal) goes through a series of a changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle. [90] There are four key processes that underlie development: Determination, differentiation, morphogenesis, and growth. Determination sets the developmental fate of a cell, which becomes more restrictive during development. Differentiation is the process by which specialized cells from less specialized cells such as stem cells. [91] [92] Stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. [93] Cellular differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals, which are largely due to highly controlled modifications in gene expression and epigenetics. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. [94] Thus, different cells can have very different physical characteristics despite having the same genome. Morphogenesis, or development of body form, is the result of spatial differences in gene expression. [90] Specially, the organization of differentiated tissues into specific structures such as arms or wings, which is known as pattern formation, is governed by morphogens, signaling molecules that move from one group of cells to surrounding cells, creating a morphogen gradient as described by the French flag model. Apoptosis, or programmed cell death, also occurs during morphogenesis, such as the death of cells between digits in human embryonic development, which frees up individual fingers and toes. Expression of transcription factor genes can determine organ placement in a plant and a cascade of transcription factors themselves can establish body segmentation in a fruit fly. [90]

          A small fraction of the genes in an organism's genome called the developmental-genetic toolkit control the development of that organism. These toolkit genes are highly conserved among phyla, meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. Among the most important toolkit genes are the Hox genes. Hox genes determine where repeating parts, such as the many vertebrae of snakes, will grow in a developing embryo or larva. [95] Variations in the toolkit may have produced a large part of the morphological evolution of animals. The toolkit can drive evolution in two ways. A toolkit gene can be expressed in a different pattern, as when the beak of Darwin's large ground-finch was enlarged by the BMP gene, [96] or when snakes lost their legs as Distal-less (Dlx) genes became under-expressed or not expressed at all in the places where other reptiles continued to form their limbs. [97] Or, a toolkit gene can acquire a new function, as seen in the many functions of that same gene, distal-less, which controls such diverse structures as the mandible in vertebrates, [98] [99] legs and antennae in the fruit fly, [100] and eyespot pattern in butterfly wings. [101] Given that small changes in toolbox genes can cause significant changes in body structures, they have often enabled convergent or parallel evolution.

          Evolution

          Evolutionary processes

          A central organizing concept in biology is that life changes and develops through evolution, which is the change in heritable characteristics of populations over successive generations. [102] [103] Evolution is now used to explain the great variations of life on Earth. The term evolution was introduced into the scientific lexicon by Jean-Baptiste de Lamarck in 1809, [104] and fifty years later Charles Darwin and Alfred Russel Wallace formulated the theory of evolution by natural selection. [105] [106] [107] [108] According to this theory, individuals differ from each other with respect to their heritable traits, resulting in different rates of survival and reproduction. As a results, traits that are better adapted to their environment are more likely to be passed on to subsequent generations. [109] [110] Darwin was not aware of Mendel's work of inheritance and so the exact mechanism of inheritance that underlie natural selection was not well-understood [111] until the early 20th century when the modern synthesis reconciled Darwinian evolution with classical genetics, which established a neo-Darwinian perspective of evolution by natural selection. [112] This perspective holds that evolution occurs when there are changes in the allele frequencies within a population of interbreeding organisms. In the absence of any evolutionary process acting on a large random mating population, the allele frequencies will remain constant across generations as described by the Hardy–Weinberg principle. [113]

          Another process that drives evolution is genetic drift, which is the random fluctuations of allele frequencies within a population from one generation to the next. [114] When selective forces are absent or relatively weak, allele frequencies are equally likely to drift upward or downward at each successive generation because the alleles are subject to sampling error. [115] This drift halts when an allele eventually becomes fixed, either by disappearing from the population or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone.

          Speciation

          Speciation is the process of splitting one lineage into two lineages that evolve independently from each other. [116] For speciation to occur, there has to be reproductive isolation. [116] Reproductive isolation can result from incompatibilities between genes as described by Bateson–Dobzhansky–Muller model. Reproductive isolation also tends to increase with genetic divergence. Speciation can occur when there are physical barriers that divide an ancestral species, a process known as allopatric speciation. [116] In contrast, sympatric speciation occurs in the absence of physical barriers.

          Pre-zygotic isolation such as mechanical, temporal, behavioral, habitat, and gametic isolations can prevent different species from hybridizing. [116] Similarly, post-zygotic isolations can result in hybridization being selected against due to the lower viability of hybrids or hybrid infertility (e.g., mule). Hybrid zones can emerge if there were to be incomplete reproductive isolation between two closely related species.

          Phylogenies

          A phylogeny is an evolutionary history of a specific group of organisms or their genes. [117] A phylogeny can be represented using a phylogenetic tree, which is a diagram showing lines of descent among organisms or their genes. Each line drawn on the time axis of a tree represents a lineage of descendents of a particular species or population. When a lineage divides into two, it is represented as a node (or split) on the phylogenetic tree. The more splits there are over time, the more branches there will be on the tree, with the common ancestor of all the organisms in that tree being represented by the root of that tree. Phylogenetic trees may portray the evolutionary history of all life forms, a major evolutionary group (e.g., insects), or an even smaller group of closely related species. Within a tree, any group of species designated by a name is a taxon (e.g., humans, primates, mammals, or vertebrates) and a taxon that consists of all its evolutionary descendants is a clade. Closely related species are referred to as sister species and closely related clades are sister clades.

          Phylogenetic trees are the basis for comparing and grouping different species. [117] Different species that share a feature inherited from a common ancestor are described as having homologous features. Homologous features may be any heritable traits such as DNA sequence, protein structures, anatomical features, and behavior patterns. A vertebral column is an example of a homologous feature shared by all vertebrate animals. Traits that have a similar form or function but were not derived from a common ancestor are described as analogous features. Phylogenies can be reconstructed for a group of organisms of primary interests, which are called the ingroup. A species or group that is closely related to the ingroup but is phylogenetically outside of it is called the outgroup, which serves a reference point in the tree. The root of the tree is located between the ingroup and the outgroup. [117] When phylogenetic trees are reconstructed, multiple trees with different evolutionary histories can be generated. Based on the principle of Parsimony (or Occam's razor), the tree that is favored is the one with the fewest evolutionary changes needed to be assumed over all traits in all groups. Computational algorithms can be used to determine how a tree might have evolved given the evidence. [117]

          Phylogeny provides the basis of biological classification, which is based on Linnaean taxonomy that was developed by Carl Linnaeus in the 18th century. [117] This classification system is rank-based, with the highest rank being the domain followed by kingdom, phylum, class, order, family, genus, and species. [117] All living organisms can be classified as belonging to one of three domains: Archaea (originally Archaebacteria) bacteria (originally eubacteria), or eukarya (includes the protist, fungi, plant, and animal kingdoms). [118] A binomial nomenclature is used to classify different species. Based on this system, each species is given two names, one for its genus and another for its species. [117] For example, humans are Homo sapiens, with Homo being the genus and sapiens being the species. By convention, the scientific names of organisms are italicized, with only the first letter of the genus capitalized. [119] [120]

          History of life

          The history of life on Earth traces the processes by which organisms have evolved from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago and all life on Earth, both living and extinct, descended from a last universal common ancestor that lived about 3.5 billion years ago. [121] [122] The similarities among all known present-day species indicate that they have diverged through the process of evolution from their common ancestor. [123] Biologists regard the ubiquity of the genetic code as evidence of universal common descent for all bacteria, archaea, and eukaryotes. [124] [10] [125] [126]

          Microbal mats of coexisting bacteria and archaea were the dominant form of life in the early Archean Epoch and many of the major steps in early evolution are thought to have taken place in this environment. [127] The earliest evidence of eukaryotes dates from 1.85 billion years ago, [128] [129] and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 billion years ago, multicellular organisms began to appear, with differentiated cells performing specialised functions. [130]

          Algae-like multicellular land plants are dated back even to about 1 billion years ago, [131] although evidence suggests that microorganisms formed the earliest terrestrial ecosystems, at least 2.7 billion years ago. [132] Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event. [133]

          Ediacara biota appear during the Ediacaran period, [134] while vertebrates, along with most other modern phyla originated about 525 million years ago during the Cambrian explosion. [135] During the Permian period, synapsids, including the ancestors of mammals, dominated the land, [136] but most of this group became extinct in the Permian–Triassic extinction event 252 million years ago. [137] During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates [138] one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods. [139] After the Cretaceous–Paleogene extinction event 66 million years ago killed off the non-avian dinosaurs, [140] mammals increased rapidly in size and diversity. [141] Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify. [142]

          Diversity

          Bacteria and Archaea

          Bacteria are a type of cell that constitute a large domain of prokaryotic microorganisms. Typically a few micrometers in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, [143] and the deep biosphere of the earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the bacterial phyla have species that can be grown in the laboratory. [144]

          Archaea constitute the other domain of prokaryotic cells and were initially classified as bacteria, receiving the name archaebacteria (in the Archaebacteria kingdom), a term that has fallen out of use. [145] Archaeal cells have unique properties separating them from the other two domains, Bacteria and Eukaryota. Archaea are further divided into multiple recognized phyla. Archaea and bacteria are generally similar in size and shape, although a few archaea have very different shapes, such as the flat and square cells of Haloquadratum walsbyi. [146] Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably for the enzymes involved in transcription and translation. Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes, [147] including archaeols. Archaea use more energy sources than eukaryotes: these range from organic compounds, such as sugars, to ammonia, metal ions or even hydrogen gas. Salt-tolerant archaea (the Haloarchaea) use sunlight as an energy source, and other species of archaea fix carbon, but unlike plants and cyanobacteria, no known species of archaea does both. Archaea reproduce asexually by binary fission, fragmentation, or budding unlike bacteria, no known species of Archaea form endospores.

          The first observed archaea were extremophiles, living in extreme environments, such as hot springs and salt lakes with no other organisms. Improved molecular detection tools led to the discovery of archaea in almost every habitat, including soil, oceans, and marshlands. Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet.

          Archaea are a major part of Earth's life. They are part of the microbiota of all organisms. In the human microbiome, they are important in the gut, mouth, and on the skin. [148] Their morphological, metabolic, and geographical diversity permits them to play multiple ecological roles: carbon fixation nitrogen cycling organic compound turnover and maintaining microbial symbiotic and syntrophic communities, for example. [149]

          Protists

          Protists are eukaryotic organism that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), [150] the exclusion of other eukaryotes means that protists do not form a natural group, or clade. [a] So some protists may be more closely related to animals, plants, or fungi than they are to other protists however, like algae, invertebrates, or protozoans, the grouping is used for convenience. [151]

          The taxonomy of protists is still changing. Newer classifications attempt to present monophyletic groups based on morphological (especially ultrastructural), [152] [153] [154] biochemical (chemotaxonomy) [155] [156] and DNA sequence (molecular research) information. [157] [158] Because protists as a whole are paraphyletic, new systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes.

          Plant diversity

          Plants are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the kingdom Plantae. Botany is the study of plant life, which would exclude fungi and some algae. Botanists have studied approximately 410,000 species of land plants of which some 391,000 species are vascular plants (including approximately 369,000 species of flowering plants), [159] and approximately 20,000 are bryophytes. [160]

          Algae is a large and diverse group of photosynthetic eukaryotic organisms. Included organisms range from unicellular microalgae, such as Chlorella, Prototheca and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga. Most are aquatic and autotrophic and lack many of the distinct cell and tissue types, such as stomata, xylem and phloem, which are found in land plants. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta.

          Nonvascular plants are plants without a vascular system consisting of xylem and phloem. Instead, they may possess simpler tissues that have specialized functions for the internal transport of water. Vascular plants, on the other hand, are a large group of plants (c. 300,000 accepted known species) [161] that are defined as land plants with lignified tissues (the xylem) for conducting water and minerals throughout the plant. [162] They also have a specialized non-lignified tissue (the phloem) to conduct products of photosynthesis. Vascular plants include the clubmosses, horsetails, ferns, gymnosperms (including conifers) and angiosperms (flowering plants).

          Seed plants (or spermatophyte) comprise five divisions, four of which are grouped as gymnosperms and one is angiosperms. Gymnosperms includes conifers, cycads, Ginkgo, and gnetophytes. Gymnosperm seeds develop either on the surface of scales or leaves, which are often modified to form cones, or solitary as in yew, Torreya, Ginkgo. [163] Angiosperms are the most diverse group of land plants, with 64 orders, 416 families, approximately 13,000 known genera and 300,000 known species. [161] Like gymnosperms, angiosperms are seed-producing plants. They are distinguished from gymnosperms by having characteristics such as flowers, endosperm within their seeds, and production of fruits that contain the seeds.

          Fungi

          Fungi are eukaryotic organisms that include microorganisms such as yeasts and molds, as well as the more familiar mushrooms. A characteristic that places fungi in a different kingdom from plants, bacteria, and some protists is chitin in their cell walls. Fungi, like animals, are heterotrophs they acquire their food by absorbing dissolved molecules, typically by secreting digestive enzymes into their environment. Fungi do not photosynthesize. Growth is their means of mobility, except for spores (a few of which are flagellated), which may travel through the air or water. Fungi are the principal decomposers in ecological systems. These and other differences place fungi in a single group of related organisms, named the Eumycota (true fungi or Eumycetes), which share a common ancestor (from a monophyletic group). This fungal group is distinct from the structurally similar myxomycetes (slime molds) and oomycetes (water molds).

          Most fungi are inconspicuous because of the small size of their structures, and their cryptic lifestyles in soil or on dead matter. Fungi include symbionts of plants, animals, or other fungi and also parasites. They may become noticeable when fruiting, either as mushrooms or as molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange in the environment.

          The fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, and morphologies ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of Kingdom Fungi, which has been estimated at 2.2 million to 3.8 million species. [164] Of these, only about 148,000 have been described, [165] with over 8,000 species known to be detrimental to plants and at least 300 that can be pathogenic to humans. [166]

          Animal diversity

          Animals are multicellular eukaryotic organisms that form the kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. They have complex interactions with each other and their environments, forming intricate food webs.

          Sponges, the members of the phylum Porifera, are a basal Metazoa (animal) clade as a sister of the Diploblasts. [167] [168] [169] [170] [171] They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells.

          97%) of animal species are invertebrates, [172] which are animals that neither possess nor develop a vertebral column (commonly known as a backbone or spine), derived from the notochord. This includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include arthropods (insects, arachnids, crustaceans, and myriapods), mollusks (chitons, snail, bivalves, squids, and octopuses), annelid (earthworms and leeches), and cnidarians (hydras, jellyfishes, sea anemones, and corals). Many invertebrate taxa have a greater number and variety of species than the entire subphylum of Vertebrata. [173]

          In contrast, vertebrates comprise all species of animals within the subphylum Vertebrata (chordates with backbones). Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described. [174] Vertebrates include such groups as jawless fishes, jawed vertebrates such as cartilaginous fishes (sharks, rays, and ratfish), bony fishes, tetrapods such as amphibians, reptiles, birds and mammals.

          Viruses

          Viruses are submicroscopic infectious agents that replicate inside the living cells of organisms. [175] Viruses infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. [176] [177] More than 6,000 virus species have been described in detail. [178] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. [179] [180]

          When infected, a host cell is forced to rapidly produce thousands of identical copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, or virions, consisting of the genetic material (DNA or RNA), a protein coat called capsid, and in some cases an outside envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope, as they are one-hundredth the size of most bacteria.

          The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. [181] Because viruses possess some but not all characteristics of life, they have been described as "organisms at the edge of life", [182] and as self-replicators. [183]

          Viruses can spread in many ways. One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids and viruses in animals can be carried by blood-sucking insects. Influenza viruses are spread by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water. Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection.

          Plant form and function

          Plant body

          The plant body is made up of organs that can be organized into two major organ systems: a root system and a shoot system. [184] The root system anchors the plants into place. The roots themselves absorb water and minerals and store photosynthetic products. The shoot system is composed of stem, leaves, and flowers. The stems hold and orient the leaves to the sun, which allow the leaves to conduct photosynthesis. The flowers are shoots that have been modified for reproduction. Shoots are composed of phytomers, which are functional units that consist of a node carrying one or more leaves, internode, and one or more buds.

          A plant body has two basic patterns (apical–basal and radial axes) that been established during embryogenesis. [184] Cells and tissues are arranged along the apical-basal axis from root to shoot whereas the three tissue systems (dermal, ground, and vascular) that make up a plant's body are arranged concentrically around its radial axis. [184] The dermal tissue system forms the epidermis (or outer covering) of a plant, which is usually a single cell layer that consists of cells that have differentiated into three specialized structures: stomata for gas exchange in leaves, trichomes (or leaf hair) for protection against insects and solar radiation, and root hairs for increased surface areas and absorption of water and nutrients. The ground tissue makes up virtually all the tissue that lies between the dermal and vascular tissues in the shoots and roots. It consists of three cell types: Parenchyma, collenchyma, and sclerenchyma cells. Finally, the vascular tissues are made up of two constituent tissues: xylem and phloem. The xylem is made up two of conducting cells called tracheids and vessel elements whereas the phloem is characterized by the presence of sieve tube elements and companion cells. [184]

          Plant nutrition and transport

          Like all other organisms, plants are primarily made up of water and other molecules containing elements that are essential to life. [185] The absence of specific nutrients (or essential elements), many of which have been identified in hydroponic experiments, can disrupt plant growth and reproduction. The majority of plants are able to obtain these nutrients from solutions that surrounds their roots in the soil. [185] Continuous leaching and harvesting of crops can deplete the soil of its nutrients, which can be restored with the use of fertilizers. Carnivorous plants such as Venus flytraps are able to obtain nutrients by digesting other arthropods whereas parasitic plants such as mistletoes can parasitize other plants for water and nutrients.

          Plants need water to conduct photosynthesis, transport solutes between organs, cool their leaves by evaporation, and maintain internal pressures that support their bodies. [185] Water is able to diffuse in and out of plant cells by osmosis. The direction of water movement across a semipermeable membrane is determined by the water potential across that membrane. [185] Water is able to diffuse across a root cell's membrane through aquaporins whereas solutes are transported across by the membrane by ion channels and pumps. In vascular plants, water and solutes are able to enter the xylem, a vascular tissue, by way of an apoplast and symplast. Once in the xylem, the water and minerals are distributed upward by transpiration from the soil to the aerial parts of the plant. [162] [185] In contrast, the phloem, another vascular tissue, distributes carbohydrates (e.g., sucrose) and other solutes such as hormones by translocation from a source (e.g., mature leaf or root) in which they were produced to a sink (e.g., root, flower, or developing fruit) in which they will be used and stored. [185] Sources and sinks can switch roles, depending on the amount of carbohydrates accumulated or mobilized for the nourishment of other organs.

          Plant development

          Plant development is regulated by environmental cues and the plant's own receptors, hormones, and genome. [186] Morever, they have several characteristics that allow them to obtain resources for growth and reproduction such as meristems, post-embryonic organ formation, and differential growth.

          Development begins with a seed, which is an embryonic plant enclosed in a protective outer covering. Most plant seeds are usually dormant, a condition in which the seed's normal activity is suspended. [186] Seed dormancy may last may last weeks, months, years, and even centuries. Dormancy is broken once conditions are favorable for growth, and the seed will begin to sprout, a process called germination. Imbibition is the first step in germination, whereby water is absorbed by the seed. Once water is absorbed, the seed undergoes metabolic changes whereby enzymes are activated and RNA and proteins are synthesized. Once the seed germinates, it obtains carbohydrates, amino acids, and small lipids that serve as building blocks for its development. These monomers are obtained from the hydrolysis of starch, proteins, and lipids that are stored in either the cotyledons or endosperm. Germination is completed once embryonic roots called radicle have emerged from the seed coat. At this point, the developing plant is called a seedling and its growth is regulated by its own photoreceptor proteins and hormones. [186]

          Unlike animals in which growth is determinate, i.e., ceases when the adult state is reached, plant growth is indeterminate as it is an open-ended process that could potentially be lifelong. [184] Plants grow in two ways: primary and secondary. In primary growth, the shoots and roots are formed and lengthened. The apical meristem produces the primary plant body, which can be found in all seed plants. During secondary growth, the thickness of the plant increases as the lateral meristem produces the secondary plant body, which can be found in woody eudicots such as trees and shrubs. Monocots do not go through secondary growth. [184] The plant body is generated by a hierarchy of meristems. The apical meristems in the root and shoot systems give rise to primary meristems (protoderm, ground meristem, and procambium), which in turn, give rise to the three tissue systems (dermal, ground, and vascular).

          Plant reproduction

          Most angiosperms (or flowering plants) engage in sexual reproduction. [187] Their flowers are organs that facilitate reproduction, usually by providing a mechanism for the union of sperm with eggs. Flowers may facilitate two types of pollination: self-pollination and cross-pollination. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination happened in flowers where the stamen and carpel mature at the same time, and are positioned so that the pollen can land on the flower’s stigma. This pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators. [188]

          Plant responses

          Like animals, plants produce hormones in one part of its body to signal cells in another part to respond. The ripening of fruit and loss of leaves in the winter are controlled in part by the production of the gas ethylene by the plant. Stress from water loss, changes in air chemistry, or crowding by other plants can lead to changes in the way a plant functions. These changes may be affected by genetic, chemical, and physical factors.

          To function and survive, plants produce a wide array of chemical compounds not found in other organisms. Because they cannot move, plants must also defend themselves chemically from herbivores, pathogens and competition from other plants. They do this by producing toxins and foul-tasting or smelling chemicals. Other compounds defend plants against disease, permit survival during drought, and prepare plants for dormancy, while other compounds are used to attract pollinators or herbivores to spread ripe seeds.

          Many plant organs contain different types of photoreceptor proteins, each of which reacts very specifically to certain wavelengths of light. [189] The photoreceptor proteins relay information such as whether it is day or night, duration of the day, intensity of light available, and the source of light. Shoots generally grow towards light, while roots grow away from it, responses known as phototropism and skototropism, respectively. They are brought about by light-sensitive pigments like phototropins and phytochromes and the plant hormone auxin. [190] Many flowering plants bloom at the appropriate time because of light-sensitive compounds that respond to the length of the night, a phenomenon known as photoperiodism.

          In addition to light, plants can respond to other types of stimuli. For instance, plants can sense the direction of gravity to orient themselves correctly. They can respond to mechanical stimulation. [191]

          Animal form and function

          Principles

          The cells in each animal body are bathed in interstitial fluid, which make up the cell's environment. This fluid and all its characteristics (e.g., temperature, ionic composition) can be described as the animal's internal environment, which is in contrast to the external environment that encompasses the animal's outside world. [192] Animals can be classified as either regulators or conformers. Animals such as mammals and birds are regulators as they are able to maintain a constant internal environment such as body temperature despite their environments changing. These animals are also described as homeotherms as they exhibit thermoregulation by keeping their internal body temperature constant. In contrast, animals such as fishes and frogs are conformers as they adapt their internal environment (e.g., body temperature) to match their external environments. These animals are also described as poikilotherms or ectotherms as they allow their body temperatures to match their external environments. In terms of energy, regulation is more costly than conformity as an animal expands more energy to maintain a constant internal environment such as increasing its basal metabolic rate, which is the rate of energy consumption. [192] Similarly, homeothermy is more costly than poikilothermy. Homeostasis is the stability of an animal's internal environment, which is maintained by negative feedback loops. [192] [193]

          The body size of terrestrial animals vary across different species but their use of energy does not scale linearly according to their size. [192] Mice, for example, are able to consume three times more food than rabbits in proportion to their weights as the basal metabolic rate per unit weight in mice is greater than in rabbits. [192] Physical activity can also increase an animal's metabolic rate. When an animal runs, its metabolic rate increases linearly with speed. [192] However, the relationship is non-linear in animals that swim or fly. When a fish swims faster, it encounters greater water resistance and so its metabolic rates increases exponential. [192] Alternatively, the relationship of flight speeds and metabolic rates is U-shaped in birds. [192] At low flight speeds, a bird must maintain a high metabolic rates to remain airborne. As it speeds up its flight, its metabolic rate decreases with the aid of air rapidly flows over its wings. However, as it increases in its speed even further, its high metabolic rates rises again due to the increased effort associated with rapid flight speeds. Basal metabolic rates can be measured based on an animal's rate of heat production.

          Water and salt balance

          An animal's body fluids have three properties: osmotic pressure, ionic composition, and volume. [194] Osmotic pressures determine the direction of the diffusion of water (or osmosis), which moves from a region where osmotic pressure (total solute concentration) is low to a region where osmotic pressure (total solute concentration) is high. Aquatic animals are diverse with respect to their body fluid compositions and their environments. For example, most invertebrate animals in the ocean have body fluids that are isosmotic with seawater. In contrast, ocean bony fishes have body fluids that are hyposmotic to seawater. Finally, freshwater animals have body fluids that are hyperosmotic to fresh water. Typical ions that can be found in an animal's body fluids are sodium, potassium, calcium, and chloride. The volume of body fluids can be regulated by excretion. Vertebrate animals have kidneys, which are excretory organs made up of tiny tubular structures called nephrons, which make urine from blood plasma. The kidneys' primary function is to regulate the composition and volume of blood plasma by selectively removing material from the blood plasma itself. The ability of xeric animals such as kangaroo rats to minimize water loss by producing urine that is 10-20 times concentrated than their blood plasma allows them to adapt in desert environments that receive very little precipitation. [194]

          Nutrition and digestion

          Animals are heterotrophs as they feed on other living organisms to obtain energy and organic compounds. [195] They are able to obtain food in three major ways such as targeting visible food objects, collecting tiny food particles, or depending on microbes for critical food needs. The amount of energy stored in food can be quantified based on the amount of heat (measured in calories or kilojoules) emitted when the food is burnt in the presence of oxygen. If an animal were to consume food that contains an excess amount of chemical energy, it will store most of that energy in the form of lipids for future use and some of that energy as glycogen for more immediate use (e.g., meeting the brain's energy needs). [195] The molecules in food are chemical building blocks that are needed for growth and development. These molecules include nutrients such as carbohydrates, fats, and proteins. Vitamins and minerals (e.g., calcium, magnesium, sodium, and phosphorus) are also essential. The digestive system, which typically consist of a tubular tract that extends from the mouth to the anus, is involved in the breakdown (or digestion) of food into small molecules as it travels down peristaltically through the gut lumen shortly after it has been ingested. These small food molecules are then absorbed into the blood from the lumen, where they are then distributed to the rest of the body as building blocks (e.g., amino acids) or sources of energy (e.g., glucose). [195]

          In addition to their digestive tracts, vertebrate animals have accessory glands such as a liver and pancreas as part of their digestive systems. [195] The processing of food in these animals begins in the foregut, which includes the mouth, esophagus, and stomach. Mechanical digestion of food starts in the mouth with the esophagus serving as a passageway for food to reach the stomach, where it is stored and disintegrated (by the stomach's acid) for further processing. Upon leaving the stomach, food enters into the midgut, which is the first part of the intestine (or small intestine in mammals) and is the principal site of digestion and absorption. Food that does not get absorbed are stored as indigestible waste (or feces) in the hindgut, which is the second part of the intestine (or large intestine in mammals). The hindgut then completes the reabsorption of needed water and salt prior to eliminating the feces from the rectum. [195]

          Breathing

          The respiratory system consists of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals the respiratory surface is internalized as linings of the lungs. [196] Gas exchange in the lungs occurs in millions of small air sacs in mammals and reptiles these are called alveoli, and in birds they are known as atria. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood. [197] These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds the bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.

          Circulation

          A circulatory system usually consists of a muscular pump such as a heart, a fluid (blood), and system of blood vessels that deliver it. [198] [199] Its principal function is to transport blood and other substances to and from cell (biology)s and tissues. There are two types of circulatory systems: open and closed. In open circulatory systems, blood exits blood vessels as it circulates throughout the body whereas in closed circulatory system, blood is contained within the blood vessels as it circulates. Open circulatory systems can be observed in invertebrate animals such as arthropods (e.g., insects, spiders, and lobsters) whereas closed circulatory systems can be found in vertebrate animals such as fishes, amphibians, and mammals. Circulation in animals occur between two types of tissues: systemic tissues and breathing (or pulmonary) organs. [198] Systemic tissues are all the tissues and organs that make up an animal's body other than its breathing organs. Systemic tissues take up oxygen but adds carbon dioxide to the blood whereas a breathing organs takes up carbon dioxide but add oxygen to the blood. [200] In birds and mammals, the systemic and pulmonary systems are connected in series.

          In the circulatory system, blood is important because it is the means by which oxygen, carbon dioxide, nutrients, hormones, agents of immune system, heat, wastes, and other commodities are transported. [198] In annelids such as earthworms and leeches, blood is propelled by peristaltic waves of contractions of the heart muscles that make up the blood vessels. Other animals such as crustaceans (e.g., crayfish and lobsters), have more than one heart to propel blood throughout their bodies. Vertebrate hearts are multichambered and are able to pump blood when their ventricles contract at each cardiac cycle, which propels blood through the blood vessels. [198] Although vertebrate hearts are myogenic, their rate of contraction (or heart rate) can be modulated by neural input from the body's autonomic nervous system.

          Muscle and movement

          In vertebrates, the muscular system consists of skeletal, smooth and cardiac muscles. It permits movement of the body, maintains posture and circulates blood throughout the body. [201] Together with the skeletal system, it forms the musculoskeletal system, which is responsible for the movement of vertebrate animals. [202] Skeletal muscle contractions are neurogenic as they require synaptic input from motor neurons. A single motor neuron is able to innervate multiple muscle fibers, thereby causing the fibers to contract at the same time. Once innervated, the protein filaments within each skeletal muscle fiber slide past each other to produce a contraction, which is explained by the sliding filament theory. The contraction produced can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials. Unlike skeletal muscles, contractions of smooth and cardiac muscles are myogenic as they are initiated by the smooth or heart muscle cells themselves instead of a motor neuron. Nevertheless, the strength of their contractions can be modulated by input from the autonomic nervous system. The mechanisms of contraction are similar in all three muscle tissues.

          In invertebrates such as earthworms and leeches, circular and longitudinal muscles cells form the body wall of these animals and are responsible for their movement. [203] In an earthworm that is moving through a soil, for example, contractions of circular and longitudinal muscles occur reciprocally while the coelomic fluid serves as a hydroskeleton by maintaining turgidity of the earthworm. [204] Other animals such as mollusks, and nematodes, possess obliquely striated muscles, which contain bands of thick and thin filaments that are arranged helically rather than transversely, like in vertebrate skeletal or cardiac muscles. [205] Advanced insects such as wasps, flies, bees, and beetles possess asynchronous muscles that constitute the flight muscles in these animals. [205] These flight muscles are often called fibrillar muscles because they contain myofibrils that are thick and conspicuous. [206]

          Nervous system

          The nervous system is a network of cells that processes sensory information and generates behaviors. At the cellular level, the nervous system is defined by the presence of neurons, which are cells specialized to handle information. [208] They can transmit or receive information at sites of contacts called synapses. [208] More specifically, neurons can conduct nerve impulses (or action potentials) that travel along their thin fibers called axons, which can then be transmitted directly to a neighboring cell through electrical synapses or cause chemicals called neurotransmitters to be released at chemical synapses. According to the sodium theory, these action potentials can be generated by the increased permeability of the neuron's cell membrane to sodium ions. [209] Cells such as neurons or muscle cells may be excited or inhibited upon receiving a signal from another neuron. The connections between neurons can form neural pathways, neural circuits, and larger networks that generate an organism's perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glia or glial cells, which provide structural and metabolic support.

          Nervous systems are found in most multicellular animals, but vary greatly in complexity. [210] In vertebrates, the nervous system consists of the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which consists of nerves that connect the CNS to every other part of the body. Nerves that transmit signals from the CNS are called motor nerves or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory nerves or afferent nerves. Spinal nerves are mixed nerves that serve both functions. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit directly from the brain are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

          Many animals have sense organs that can detect their environment. These sense organs contain sensory receptors, which are sensory neurons that convert stimuli into electrical signals. [211] Mechanoreceptors, for example, which can be found in skin, muscle, and hearing organs, generate action potentials in response to changes in pressures. [211] [212] Photoreceptor cells such as rods and cones, which are part of the vertebrate retina, can respond to specific wavelengths of light. [211] [212] Chemoreceptors detect chemicals in the mouth (taste) or in the air (smell). [212]

          Hormonal control

          Hormones are signaling molecules transported in the blood to distant organs to regulate their function. [213] [214] Hormones are secreted by internal glands that are part of an animal's endocrine system. In vertebrates, the hypothalamus is the neural control center for all endocrine systems. In humans specifically, the major endocrine glands are the thyroid gland and the adrenal glands. Many other organs that are part of other body systems have secondary endocrine functions, including bone, kidneys, liver, heart and gonads. For example, kidneys secrete the endocrine hormone erythropoietin. Hormones can be amino acid complexes, steroids, eicosanoids, leukotrienes, or prostaglandins. [215] The endocrine system can be contrasted to both exocrine glands, which secrete hormones to the outside of the body, and paracrine signaling between cells over a relatively short distance. Endocrine glands have no ducts, are vascular, and commonly have intracellular vacuoles or granules that store their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.

          Animal reproduction

          Animals can reproduce in one of two ways: asexual and sexual. Nearly all animals engage in some form of sexual reproduction. [216] They produce haploid gametes by meiosis. The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. [217] These fuse to form zygotes, [218] which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge. [219] In most other groups, the blastula undergoes more complicated rearrangement. [220] It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. [221] In most cases, a third germ layer, the mesoderm, also develops between them. [222] These germ layers then differentiate to form tissues and organs. [223] Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation budding, such as in Hydra and other cnidarians or parthenogenesis, where fertile eggs are produced without mating, such as in aphids. [224] [225]

          Animal development

          Animal development begins with the formation of a zygote that results from the fusion of a sperm and egg during fertilization. [226] The zygote undergoes a rapid multiple rounds of mitotic cell period of cell divisions called cleavage, which forms a ball of similar cells called a blastula. Gastrulation occurs, whereby morphogenetic movements convert the cell mass into a three germ layers that comprise the ectoderm, mesoderm and endoderm.

          The end of gastrulation signals the beginning of organogenesis, whereby the three germ layers form the internal organs of the organism. [227] The cells of each of the three germ layers undergo differentiation, a process where less-specialized cells become more-specialized through the expression of a specific set of genes. Cellular differentiation is influenced by extracellular signals such as growth factors that are exchanged to adjacent cells, which is called juxtracrine signaling, or to neighboring cells over short distances, which is called paracrine signaling. [228] [229] Intracellular signals consist of a cell signaling itself (autocrine signaling), also play a role in organ formation. These signaling pathways allows for cell rearrangement and ensures that organs form at specific sites within the organism. [227] [230]

          Immune system

          The immune system is a network of biological processes that detects and responds to a wide variety of pathogens. Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use molecules and cells to perform their functions.

          Nearly all organisms have some kind of immune system. Bacteria have a rudimentary immune system in the form of enzymes that protect against virus infections. Other basic immune mechanisms evolved in ancient plants and animals and remain in their modern descendants. These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system. Jawed vertebrates, including humans, have even more sophisticated defense mechanisms, including the ability to adapt to recognize pathogens more efficiently. Adaptive (or acquired) immunity creates an immunological memory leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination.

          Animal behavior

          Behaviors play a central a role in animals' interaction with each other and with their environment. [231] They are able to use their muscles to approach one another, vocalize, seek shelter, and migrate. An animal's nervous system activates and coordinates its behaviors. Fixed action patterns, for instance, are genetically determined and stereotyped behaviors that occur without learning. [231] [232] These behaviors are under the control of the nervous system and can be quite elaborate. [231] Examples include the pecking of kelp gull chicks at the red dot on their mother's beak. Other behaviors that have emerged as a result of natural selection include foraging, mating, and altruism. [233] In addition to evolved behavior, animals have evolved the ability to learn by modifying their behaviors as a result of early individual experiences. [231]

          Ecology

          Ecosystems

          Ecology is the study of the distribution and abundance of living organisms, the interaction between them and their environment. [234] The community of living (biotic) organisms in conjunction with the nonliving (abiotic) components (e.g., water, light, radiation, temperature, humidity, atmosphere, acidity, and soil) of their environment is called an ecosystem. [235] [236] [237] These biotic and abiotic components are linked together through nutrient cycles and energy flows. [238] Energy from the sun enters the system through photosynthesis and is incorporated into plant tissue. By feeding on plants and on one another, animals play an important role in the movement of matter and energy through the system. They also influence the quantity of plant and microbial biomass present. By breaking down dead organic matter, decomposers release carbon back to the atmosphere and facilitate nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes. [239]

          The Earth's physical environment is shaped by solar energy and topography. [237] The amount of solar energy input varies in space and time due to the spherical shape of the Earth and its axial tilt. Variation in solar energy input drives weather and climate patterns. Weather is the day-to-day temperature and precipitation activity, whereas climate is the long-term average of weather, typically averaged over a period of 30 years. [240] [241] Variation in topography also produces environmental heterogeneity. On the windward side of a mountain, for example, air rises and cools, with water changing from gaseous to liquid or solid form, resulting in precipitation such as rain or snow. [237] As a result, wet environments allow for lush vegetation to grow. In contrast, conditions tend to be dry on the leeward side of a mountain due to the lack of precipitation as air descends and warms, and moisture remains as water vapor in the atmosphere. Temperature and precipitation are the main factors that shape terrestrial biomes.

          Populations

          A population is the number of organisms of the same species that occupy an area and reproduce from generation to generation. [242] [243] [244] [245] [246] Its abundance can be measured using population density, which is the number of individuals per unit area (e.g., land or tree) or volume (e.g., sea or air). [242] Given that it is usually impractical to count every individual within a large population to determine its size, population size can be estimated by multiplying population density by the area or volume. Population growth during short-term intervals can be determined using the population growth rate equation, which takes into consideration birth, death, and immigration rates. In the longer term, the exponential growth of a population tends to slow down as it reaches its carrying capacity, which can be modeled using the logistic equation. [243] The carrying capacity of an environment is the maximum population size of a species that can be sustained by that specific environment, given the food, habitat, water, and other resources that are available. [247] The carrying capacity of a population can be affected by changing environmental conditions such as changes in the availability resources and the cost of maintaining them. In human populations, new technologies such as the Green revolution have helped increase the Earth's carrying capacity for humans over time, which has stymied the attempted predictions of impending population decline, the famous of which was by Thomas Malthus in the 18th century. [242]

          Communities

          A community is a group of populations of two or more different species occupying the same geographical area at the same time. A biological interaction is the effect that a pair of organisms living together in a community have on each other. They can be either of the same species (intraspecific interactions), or of different species (interspecific interactions). These effects may be short-term, like pollination and predation, or long-term both often strongly influence the evolution of the species involved. A long-term interaction is called a symbiosis. Symbioses range from mutualism, beneficial to both partners, to competition, harmful to both partners. [249]

          Every species participates as a consumer, resource, or both in consumer–resource interactions, which form the core of food chains or food webs. [250] There are different trophic levels within any food web, with the lowest level being the primary producers (or autotrophs) such as plants and algae that convert energy and inorganic material into organic compounds, which can then be used by the rest of the community. [54] [251] [252] At the next level are the heterotrophs, which are the species that obtain energy by breaking apart organic compounds from other organisms. [250] Heterotrophs that consume plants are primary consumers (or herbivores) whereas heterotrophs that consume herbivores are secondary consumers (or carnivores). And those that eat secondary consumers are tertiary consumers and so on. Omnivorous heterotrophs are able to consume at multiple levels. Finally, there are decomposers that feed on the waste products or dead bodies of organisms. [250]

          On average, the total amount of energy incorporated into the biomass of a trophic level per unit of time is about one-tenth of the energy of the trophic level that it consumes. Waste and dead material used by decomposers as well as heat lost from metabolism make up the other ninety percent of energy that is not consumed by the next trophic level. [253]

          Biosphere

          In the global ecosystem (or biosphere), matter exist as different interacting compartments, which can be biotic or abiotic as well as accessible or inaccessible, depending on their forms and locations. [255] For example, matter from terrestrial autotrophs are both biotic and accessible to other living organisms whereas the matter in rocks and minerals are abiotic and inaccessible to living organisms. A biogeochemical cycle is a pathway by which specific elements of matter are turned over or moved through the biotic (biosphere) and the abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth. There are biogeochemical cycles for nitrogen, carbon, and water. In some cycles there are reservoirs where a substance remains or is sequestered for a long period of time.

          Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale. [256] The largest driver of warming is the emission of greenhouse gases, of which more than 90% are carbon dioxide and methane. [257] Fossil fuel burning (coal, oil, and natural gas) for energy consumption is the main source of these emissions, with additional contributions from agriculture, deforestation, and manufacturing. [258] Temperature rise is accelerated or tempered by climate feedbacks, such as loss of sunlight-reflecting snow and ice cover, increased water vapor (a greenhouse gas itself), and changes to land and ocean carbon sinks.

          Conservation

          Conservation biology is the study of the conservation of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction and the erosion of biotic interactions. [259] [260] [261] It is concerned with factors that influence the maintenance, loss, and restoration of biodiversity and the science of sustaining evolutionary processes that engender genetic, population, species, and ecosystem diversity. [262] [263] [264] [265] The concern stems from estimates suggesting that up to 50% of all species on the planet will disappear within the next 50 years, [266] which has contributed to poverty, starvation, and will reset the course of evolution on this planet. [267] [268] Biodiversity affects the functioning of ecosystems, which provide a variety of services upon which people depend.

          Conservation biologists research and educate on the trends of biodiversity loss, species extinctions, and the negative effect these are having on our capabilities to sustain the well-being of human society. Organizations and citizens are responding to the current biodiversity crisis through conservation action plans that direct research, monitoring, and education programs that engage concerns at local through global scales. [269] [262] [263] [264]


          Interdisciplinary Fields

          Ecology also plays important roles in many inter-disciplinary fields:

          • ecological design and ecological engineering
          • ecological economics
          • festive ecology
          • human ecology and ecological anthropology
          • social ecology, ecological health and environmental psychology

          Ecology has also inspired (and lent its name to) other non-biological disciplines such as

          Finally, ecology is used to describe several philosophies or ideologies, such as


          1.1F: Branches and Subdisciplines of Biology - Biology

          In this section, you will explore the following questions:

          • What are the characteristics shared by the natural sciences?
          • What are the steps of the scientific method?

          Connection for AP ® courses

          Biology is the science that studies living organisms and their interactions with one another and with their environment. The process of science attempts to describe and understand the nature of the universe by rational means. Science has many fields those fields related to the physical world, including biology, are considered natural sciences. All of the natural sciences follow the laws of chemistry and physics. For example, when studying biology, you must remember living organisms obey the laws of thermodynamics while using free energy and matter from the environment to carry out life processes that are explored in later chapters, such as metabolism and reproduction.

          Two types of logical reasoning are used in science: inductive reasoning and deductive reasoning. Inductive reasoning uses particular results to produce general scientific principles. Deductive reasoning uses logical thinking to predict results by applying scientific principles or practices. The scientific method is a step-by-step process that consists of: making observations, defining a problem, posing hypotheses, testing these hypotheses by designing and conducting investigations, and drawing conclusions from data and results. Scientists then communicate their results to the scientific community. Scientific theories are subject to revision as new information is collected.

          The content presented in this section supports the Learning Objectives outlined in Big Idea 2 of the AP ® Biology Curriculum Framework. The Learning Objectives merge Essential Knowledge content with one or more of the seven Science Practices. These objectives provide a transparent foundation for the AP ® Biology course, along with inquiry-based laboratory experiences, instructional activities, and AP ® Exam questions.

          Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
          Enduring Understanding 2.A Growth, reproduction and maintenance of living systems require free energy and matter.
          Essential Knowledge 2.A.1 All living systems require constant input of free energy.
          Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models
          Learning Objectives 2.3 The student is able to predict how changes in free energy availability affect organisms, populations and ecosystems.

          What is biology? In simple terms, biology is the study of living organisms and their interactions with one another and their environments. This is a very broad definition because the scope of biology is vast. Biologists may study anything from the microscopic or submicroscopic view of a cell to ecosystems and the whole living planet (Figure 1.2). Listening to the daily news, you will quickly realize how many aspects of biology are discussed every day. For example, recent news topics include Escherichia coli (Figure 1.3) outbreaks in spinach and Salmonella contamination in peanut butter. On a global scale, many researchers are committed to finding ways to protect the planet, solve environmental issues, and reduce the effects of climate change. All of these diverse endeavors are related to different facets of the discipline of biology.

          The Process of Science

          Biology is a science, but what exactly is science? What does the study of biology share with other scientific disciplines? Science (from the Latin scientia, meaning “knowledge”) can be defined as knowledge that covers general truths or the operation of general laws, especially when acquired and tested by the scientific method. It becomes clear from this definition that the application of the scientific method plays a major role in science. The scientific method is a method of research with defined steps that include experiments and careful observation.

          The steps of the scientific method will be examined in detail later, but one of the most important aspects of this method is the testing of hypotheses by means of repeatable experiments. A hypothesis is a suggested explanation for an event, which can be tested. Although using the scientific method is inherent to science, it is inadequate in determining what science is. This is because it is relatively easy to apply the scientific method to disciplines such as physics and chemistry, but when it comes to disciplines like archaeology, psychology, and geology, the scientific method becomes less applicable as it becomes more difficult to repeat experiments.

          These areas of study are still sciences, however. Consider archaeology—even though one cannot perform repeatable experiments, hypotheses may still be supported. For instance, an archaeologist can hypothesize that an ancient culture existed based on finding a piece of pottery. Further hypotheses could be made about various characteristics of this culture, and these hypotheses may be found to be correct or false through continued support or contradictions from other findings. A hypothesis may become a verified theory. A theory is a tested and confirmed explanation for observations or phenomena. Science may be better defined as fields of study that attempt to comprehend the nature of the universe.

          Natural Sciences

          What would you expect to see in a museum of natural sciences? Frogs? Plants? Dinosaur skeletons? Exhibits about how the brain functions? A planetarium? Gems and minerals? Or, maybe all of the above? Science includes such diverse fields as astronomy, biology, computer sciences, geology, logic, physics, chemistry, and mathematics (Figure 1.4). However, those fields of science related to the physical world and its phenomena and processes are considered natural sciences . Thus, a museum of natural sciences might contain any of the items listed above.

          There is no complete agreement when it comes to defining what the natural sciences include, however. For some experts, the natural sciences are astronomy, biology, chemistry, earth science, and physics. Other scholars choose to divide natural sciences into life sciences , which study living things and include biology, and physical sciences , which study nonliving matter and include astronomy, geology, physics, and chemistry. Some disciplines such as biophysics and biochemistry build on both life and physical sciences and are interdisciplinary. Natural sciences are sometimes referred to as “hard science” because they rely on the use of quantitative data social sciences that study society and human behavior are more likely to use qualitative assessments to drive investigations and findings.

          Not surprisingly, the natural science of biology has many branches or subdisciplines. Cell biologists study cell structure and function, while biologists who study anatomy investigate the structure of an entire organism. Those biologists studying physiology, however, focus on the internal functioning of an organism. Some areas of biology focus on only particular types of living things. For example, botanists explore plants, while zoologists specialize in animals.

          Scientific Reasoning

          One thing is common to all forms of science: an ultimate goal “to know.” Curiosity and inquiry are the driving forces for the development of science. Scientists seek to understand the world and the way it operates. To do this, they use two methods of logical thinking: inductive reasoning and deductive reasoning.

          Inductive reasoning is a form of logical thinking that uses related observations to arrive at a general conclusion. This type of reasoning is common in descriptive science. A life scientist such as a biologist makes observations and records them. These data can be qualitative or quantitative, and the raw data can be supplemented with drawings, pictures, photos, or videos. From many observations, the scientist can infer conclusions (inductions) based on evidence. Inductive reasoning involves formulating generalizations inferred from careful observation and the analysis of a large amount of data. Brain studies provide an example. In this type of research, many live brains are observed while people are doing a specific activity, such as viewing images of food. The part of the brain that “lights up” during this activity is then predicted to be the part controlling the response to the selected stimulus, in this case, images of food. The “lighting up” of the various areas of the brain is caused by excess absorption of radioactive sugar derivatives by active areas of the brain. The resultant increase in radioactivity is observed by a scanner. Then, researchers can stimulate that part of the brain to see if similar responses result.

          Deductive reasoning or deduction is the type of logic used in hypothesis-based science. In deductive reason, the pattern of thinking moves in the opposite direction as compared to inductive reasoning. Deductive reasoning is a form of logical thinking that uses a general principle or law to forecast specific results. From those general principles, a scientist can extrapolate and predict the specific results that would be valid as long as the general principles are valid. Studies in climate change can illustrate this type of reasoning. For example, scientists may predict that if the climate becomes warmer in a particular region, then the distribution of plants and animals should change. These predictions have been made and tested, and many such changes have been found, such as the modification of arable areas for agriculture, with change based on temperature averages.

          Both types of logical thinking are related to the two main pathways of scientific study: descriptive science and hypothesis-based science. Descriptive (or discovery) science , which is usually inductive, aims to observe, explore, and discover, while hypothesis-based science , which is usually deductive, begins with a specific question or problem and a potential answer or solution that can be tested. The boundary between these two forms of study is often blurred, and most scientific endeavors combine both approaches. The fuzzy boundary becomes apparent when thinking about how easily observation can lead to specific questions. For example, a gentleman in the 1940s observed that the burr seeds that stuck to his clothes and his dog’s fur had a tiny hook structure. On closer inspection, he discovered that the burrs’ gripping device was more reliable than a zipper. He eventually developed a company and produced the hook-and-loop fastener often used on lace-less sneakers and athletic braces. Descriptive science and hypothesis-based science are in continuous dialogue.

          The Scientific Method

          Biologists study the living world by posing questions about it and seeking science-based responses. This approach is common to other sciences as well and is often referred to as the scientific method. The scientific method was used even in ancient times, but it was first documented by England’s Sir Francis Bacon (1561–1626) (Figure 1.5), who set up inductive methods for scientific inquiry. The scientific method is not exclusively used by biologists but can be applied to almost all fields of study as a logical, rational problem-solving method.

          The scientific process typically starts with an observation (often a problem to be solved) that leads to a question. Let’s think about a simple problem that starts with an observation and apply the scientific method to solve the problem. One Monday morning, a student arrives at class and quickly discovers that the classroom is too warm. That is an observation that also describes a problem: the classroom is too warm. The student then asks a question: “Why is the classroom so warm?”

          Proposing a Hypothesis

          Recall that a hypothesis is a suggested explanation that can be tested. To solve a problem, several hypotheses may be proposed. For example, one hypothesis might be, “The classroom is warm because no one turned on the air conditioning.” But there could be other responses to the question, and therefore other hypotheses may be proposed. A second hypothesis might be, “The classroom is warm because there is a power failure, and so the air conditioning doesn’t work.”

          Once a hypothesis has been selected, the student can make a prediction. A prediction is similar to a hypothesis but it typically has the format “If . . . then . . . .” For example, the prediction for the first hypothesis might be, “If the student turns on the air conditioning, thenthe classroom will no longer be too warm.”

          Testing a Hypothesis

          A valid hypothesis must be testable. It should also be falsifiable , meaning that it can be disproven by experimental results. Importantly, science does not claim to “prove” anything because scientific understandings are always subject to modification with further information. This step—openness to disproving ideas—is what distinguishes sciences from non-sciences. The presence of the supernatural, for instance, is neither testable nor falsifiable. To test a hypothesis, a researcher will conduct one or more experiments designed to eliminate one or more of the hypotheses. Each experiment will have one or more variables and one or more controls. A variable is any part of the experiment that can vary or change during the experiment. The control group contains every feature of the experimental group except it is not given the manipulation that is hypothesized about. Therefore, if the results of the experimental group differ from the control group, the difference must be due to the hypothesized manipulation, rather than some outside factor. Look for the variables and controls in the examples that follow. To test the first hypothesis, the student would find out if the air conditioning is on. If the air conditioning is turned on but does not work, there should be another reason, and this hypothesis should be rejected. To test the second hypothesis, the student could check if the lights in the classroom are functional. If so, there is no power failure and this hypothesis should be rejected. Each hypothesis should be tested by carrying out appropriate experiments. Be aware that rejecting one hypothesis does not determine whether or not the other hypotheses can be accepted it simply eliminates one hypothesis that is not valid (see this figure). Using the scientific method, the hypotheses that are inconsistent with experimental data are rejected.

          While this “warm classroom” example is based on observational results, other hypotheses and experiments might have clearer controls. For instance, a student might attend class on Monday and realize she had difficulty concentrating on the lecture. One observation to explain this occurrence might be, “When I eat breakfast before class, I am better able to pay attention.” The student could then design an experiment with a control to test this hypothesis.

          In hypothesis-based science, specific results are predicted from a general premise. This type of reasoning is called deductive reasoning: deduction proceeds from the general to the particular. But the reverse of the process is also possible: sometimes, scientists reach a general conclusion from a number of specific observations. This type of reasoning is called inductive reasoning, and it proceeds from the particular to the general. Inductive and deductive reasoning are often used in tandem to advance scientific knowledge (see this figure). In recent years a new approach of testing hypotheses has developed as a result of an exponential growth of data deposited in various databases. Using computer algorithms and statistical analyses of data in databases, a new field of so-called "data research" (also referred to as "in silico" research) provides new methods of data analyses and their interpretation. This will increase the demand for specialists in both biology and computer science, a promising career opportunity.

          SCIENCE PRACTICE CONNECTION FOR AP® COURSES

          THINK ABOUT IT

          Almost all plants use water, carbon dioxide, and energy from the sun to make sugars. Think about what would happen to plants that don’t have sunlight as an energy source or sufficient water. What would happen to organisms that depend on those plants for their own survival?

          Make a prediction about what would happen to the organisms living in a rain forest if 50% of its trees were destroyed. How would you test your prediction?

          VISUAL CONNECTION

          In the example below, the scientific method is used to solve an everyday problem. Order the scientific method steps (numbered items) with the process of solving the everyday problem (lettered items). Based on the results of the experiment, is the hypothesis correct? If it is incorrect, propose some alternative hypotheses.

          Scientific Method Everyday process
          1 Observation A There is something wrong with the electrical outlet.
          2 Question B If something is wrong with the outlet, my coffeemaker also won’t work when plugged into it.
          3 Hypothesis (answer) C My toaster doesn’t toast my bread.
          4 Prediction D I plug my coffee maker into the outlet.
          5 Experiment E My coffeemaker works.
          6 Result F What is preventing my toaster from working?
          1. The original hypothesis is correct. There is something wrong with the electrical outlet and therefore the toaster doesn’t work.
          2. The original hypothesis is incorrect. Alternative hypothesis includes that toaster wasn’t turned on.
          3. The original hypothesis is correct. The coffee maker and the toaster do not work when plugged into the outlet.
          4. The original hypothesis is incorrect. Alternative hypotheses includes that both coffee maker and toaster were broken.

          VISUAL CONNECTION

          1. All flying birds and insects have wings. Birds and insects flap their wings as they move through the air. Therefore, wings enable flight.
          2. Insects generally survive mild winters better than harsh ones. Therefore, insect pests will become more problematic if global temperatures increase.
          3. Chromosomes, the carriers of DNA, separate into daughter cells during cell division. Therefore, DNA is the genetic material.
          4. Animals as diverse as insects and wolves all exhibit social behavior. Therefore, social behavior must have an evolutionary advantage for humans.
          1. 1- Inductive, 2- Deductive, 3- Deductive, 4- Inductive
          2. 1- Deductive, 2- Inductive, 3- Deductive, 4- Inductive
          3. 1- Inductive, 2- Deductive, 3- Inductive, 4- Deductive
          4. 1- Inductive, 2-Inductive, 3- Inductive, 4- Deductive

          The scientific method may seem too rigid and structured. It is important to keep in mind that, although scientists often follow this sequence, there is flexibility. Sometimes an experiment leads to conclusions that favor a change in approach often, an experiment brings entirely new scientific questions to the puzzle. Many times, science does not operate in a linear fashion instead, scientists continually draw inferences and make generalizations, finding patterns as their research proceeds. Scientific reasoning is more complex than the scientific method alone suggests. Notice, too, that the scientific method can be applied to solving problems that aren’t necessarily scientific in nature.

          Two Types of Science: Basic Science and Applied Science

          The scientific community has been debating for the last few decades about the value of different types of science. Is it valuable to pursue science for the sake of simply gaining knowledge, or does scientific knowledge only have worth if we can apply it to solving a specific problem or to bettering our lives? This question focuses on the differences between two types of science: basic science and applied science.

          Basic science or “pure” science seeks to expand knowledge regardless of the short-term application of that knowledge. It is not focused on developing a product or a service of immediate public or commercial value. The immediate goal of basic science is knowledge for knowledge’s sake, though this does not mean that, in the end, it may not result in a practical application.

          In contrast, applied science or “technology,” aims to use science to solve real-world problems, making it possible, for example, to improve a crop yield, find a cure for a particular disease, or save animals threatened by a natural disaster (Figure 1.8). In applied science, the problem is usually defined for the researcher.

          Some individuals may perceive applied science as “useful” and basic science as “useless.” A question these people might pose to a scientist advocating knowledge acquisition would be, “What for?” A careful look at the history of science, however, reveals that basic knowledge has resulted in many remarkable applications of great value. Many scientists think that a basic understanding of science is necessary before an application is developed therefore, applied science relies on the results generated through basic science. Other scientists think that it is time to move on from basic science and instead to find solutions to actual problems. Both approaches are valid. It is true that there are problems that demand immediate attention however, few solutions would be found without the help of the wide knowledge foundation generated through basic science.

          One example of how basic and applied science can work together to solve practical problems occurred after the discovery of DNA structure led to an understanding of the molecular mechanisms governing DNA replication. Strands of DNA, unique in every human, are found in our cells, where they provide the instructions necessary for life. During DNA replication, DNA makes new copies of itself, shortly before a cell divides. Understanding the mechanisms of DNA replication enabled scientists to develop laboratory techniques that are now used to identify genetic diseases. Without basic science, it is unlikely that applied science would exist.

          Another example of the link between basic and applied research is the Human Genome Project, a study in which each human chromosome was analyzed and mapped to determine the precise sequence of DNA subunits and the exact location of each gene. (The gene is the basic unit of heredity an individual’s complete collection of genes is his or her genome.) Other less complex organisms have also been studied as part of this project in order to gain a better understanding of human chromosomes. The Human Genome Project (Figure 1.9) relied on basic research carried out with simple organisms and, later, with the human genome. An important end goal eventually became using the data for applied research, seeking cures and early diagnoses for genetically related diseases.

          While research efforts in both basic science and applied science are usually carefully planned, it is important to note that some discoveries are made by serendipity , that is, by means of a fortunate accident or a lucky surprise. Penicillin was discovered when biologist Alexander Fleming accidentally left a petri dish of Staphylococcus bacteria open. An unwanted mold grew on the dish, killing the bacteria. The mold turned out to be Penicillium, and a new antibiotic was discovered. Even in the highly organized world of science, luck—when combined with an observant, curious mind—can lead to unexpected breakthroughs.

          Reporting Scientific Work

          Whether scientific research is basic science or applied science, scientists must share their findings in order for other researchers to expand and build upon their discoveries. Collaboration with other scientists—when planning, conducting, and analyzing results—are all important for scientific research. For this reason, important aspects of a scientist’s work are communicating with peers and disseminating results to peers. Scientists can share results by presenting them at a scientific meeting or conference, but this approach can reach only the select few who are present. Instead, most scientists present their results in peer-reviewed manuscripts that are published in scientific journals. Peer-reviewed manuscripts are scientific papers that are reviewed by a scientist’s colleagues, or peers. These colleagues are qualified individuals, often experts in the same research area, who judge whether or not the scientist’s work is suitable for publication. The process of peer review helps to ensure that the research described in a scientific paper or grant proposal is original, significant, logical, and thorough. Grant proposals, which are requests for research funding, are also subject to peer review. Scientists publish their work so other scientists can reproduce their experiments under similar or different conditions to expand on the findings. The experimental results must be consistent with the findings of other scientists.

          A scientific paper is very different from creative writing. Although creativity is required to design experiments, there are fixed guidelines when it comes to presenting scientific results. First, scientific writing must be brief, concise, and accurate. A scientific paper needs to be succinct but detailed enough to allow peers to reproduce the experiments.

          The scientific paper consists of several specific sections—introduction, materials and methods, results, and discussion. This structure is sometimes called the “IMRaD” format. There are usually acknowledgment and reference sections as well as an abstract (a concise summary) at the beginning of the paper. There might be additional sections depending on the type of paper and the journal where it will be published for example, some review papers require an outline.

          The introduction starts with brief, but broad, background information about what is known in the field. A good introduction also gives the rationale of the work it justifies the work carried out and also briefly mentions the end of the paper, where the hypothesis or research question driving the research will be presented. The introduction refers to the published scientific work of others and therefore requires citations following the style of the journal. Using the work or ideas of others without proper citation is considered plagiarism .

          The materials and methods section includes a complete and accurate description of the substances used, and the method and techniques used by the researchers to gather data. The description should be thorough enough to allow another researcher to repeat the experiment and obtain similar results, but it does not have to be verbose. This section will also include information on how measurements were made and what types of calculations and statistical analyses were used to examine raw data. Although the materials and methods section gives an accurate description of the experiments, it does not discuss them.

          Some journals require a results section followed by a discussion section, but it is more common to combine both. If the journal does not allow the combination of both sections, the results section simply narrates the findings without any further interpretation. The results are presented by means of tables or graphs, but no duplicate information should be presented. In the discussion section, the researcher will interpret the results, describe how variables may be related, and attempt to explain the observations. It is indispensable to conduct an extensive literature search to put the results in the context of previously published scientific research. Therefore, proper citations are included in this section as well.

          Finally, the conclusion section summarizes the importance of the experimental findings. While the scientific paper almost certainly answered one or more scientific questions that were stated, any good research should lead to more questions. Therefore, a well-done scientific paper leaves doors open for the researcher and others to continue and expand on the findings.

          Review articles do not follow the IMRAD format because they do not present original scientific findings, or primary literature instead, they summarize and comment on findings that were published as primary literature and typically include extensive reference sections.


          Paleontology

          Researchers work on excavating dinosaur fossils at a site in Castellón, Spain.

          Biology is the culmination of the achievements of the natural sciences from their inception to today. Excitingly, it is the cradle of emerging sciences such as the biology of brain activity, genetic engineering of custom organisms, and the biology of evolution that uses the laboratory tools of molecular biology to retrace the earliest stages of life on earth. A scan of news headlines—whether reporting on immunizations, a newly discovered species, sports doping, or a genetically-modified food—demonstrates the way biology is active in and important to our everyday world.