Evolution after the development of sexual reproduction

My understanding of evolution is that genetic mutation occurs in individual members of a species, and they become a new species.

Isn't a definition of species a group of genetically similar organisms that are pretty much exclusive in breeding ability?

Wouldn't there have to be a male and female with the same genetic divergence at the same time to make a new species? What is the popularly accepted mechanism for the splitting into a new species in organisms that require sexual reproduction?

My understanding of evolution is that genetic mutation occurs in individual members of a species, and they become a new species.

Isn't a definition of species a group of genetically similar organisms that are pretty much exclusive in breeding ability?

Well, no.

You are correct in that the origin for any new species lies in mutations which occur in individuals and are then passed down through DNA.

You are misunderstanding that simply because a mutation exists in an individual that they become a new species. Take Lactase Persistence as a widely studied example in humans. The majority of the world is lactose intolerant after they stop drinking their mother's milk, but a single point mutation results in the ability to consume dairy products into adulthood. It does not make people who are lactose tolerant a new species, but it does open up a potential new nutrient source, which benefits the people who have it and thus it was passed down successfully over other variations.

In addition to that example, the definition of "Species" is NOT exactly defined. There are many variations, including (but not limited to):

1) A group of organisms that cannot interbreed in the wild.
2) A group of organisms with significantly different phenotypes (appearances).
3) A group of organisms with sufficient* genetic divergence.

*There is absolutely no scientific consensus on how much genetic divergence is necessary to create a new species. We share >95% of our genetics with Chimpanzees, so it's not much - it could be as little as a few key genes out of the entire genome.

So where you define the boundaries of species is up for grabs to some extent, but the most common is what you delineated - when a member of one group cannot interbreed with another group.

Wouldn't there have to be a male and female with the same genetic divergence at the same time to make a new species? What is the popularly accepted mechanism for the splitting into a new species in organisms that require sexual reproduction?

Nope. Assuming everything necessary for the gametes to produce fertile offspring is present (aka - the chromosomes aren't significantly altered and the gametes are compatible) then a new species emerges through successive generations.

One mother or father gives their mutation to their children, which pass it on to their children, which pass it on to their children, etc. If the mutation is significantly advantageous it will allow the children with the mutation to better survive in the environment, and over time it could become the dominant phenotype that could lead to the creation of a new species as further mutations build in the new group - further differing the generations of the mother/father from the original group the mother/father were born into. Good examples are the HAR - Human Accelerated Regions which are sets of genes that are pretty much the same throughout all other vertebrates, but startlingly different in humans. The HAR did not create humans, but it's thought the mutations contributed greatly to our intelligence - which allowed other mutations to be taken advantage of (like the humble opposable thumb) as our lifestyle changed due to the increased intelligence.

Not all mutations need to be advantageous, though. There's lots of fuzzy areas. If a mutation isn't beneficial, but also isn't that detrimental, it can also stick around in a population. These mutations might not lead to the creation of new species, but can certainly contribute to genetic divergence.

It should also be added that allopatric speciation is arguably the most common form of speciation (compared to sympatric speciation). This means that, often, the only thing needed for speciation is lack of gene flow, time and genetic drift. Add to that the different selection pressures at different locations. However, the different mechanisms of allopatric, sympatric and parapatric speciation are probably better seen as a continuum (Butlin et al. 2008). Generally, speciation is a gradual and incremental process, even though distinct mutations can sometimes surely be important. For a review on the genetics of speciation see Noor & Feder (2006)

Evolution of sexual reproduction

Sexual reproduction is an adaptive feature which is common to almost all multi-cellular organisms (and also some single-cellular organisms) with many being incapable of reproducing asexually. Prior to the advent of sexual reproduction, the adaptation process whereby genes would change from one generation to the next (genetic mutation) happened very slowly and randomly. Sex evolved as an extremely efficient mechanism for producing variation, and this had the major advantage of enabling organisms to adapt to changing environments. Sex did, however, come with a cost. In reproducing asexually, no time nor energy needs to be expended in choosing a mate. And if the environment has not changed, then there may be little reason for variation, as the organism may already be well adapted. Sex, however, has evolved as the most prolific means of species branching into the tree of life. Diversification into the phylogenetic tree happens much more rapidly via sexual reproduction than it does by way of asexual reproduction.

Evolution of sexual reproduction describes how sexually reproducing animals, plants, fungi and protists could have evolved from a common ancestor that was a single-celled eukaryotic species. [1] [2] [3] Sexual reproduction is widespread in the Eukarya, though a few eukaryotic species have secondarily lost the ability to reproduce sexually, such as Bdelloidea, and some plants and animals routinely reproduce asexually (by apomixis and parthenogenesis) without entirely having lost sex. The evolution of sex contains two related yet distinct themes: its origin and its maintenance.

The origin of sexual reproduction can be traced to early prokaryotes, around two billion years ago (Gya), when bacteria began exchanging genes via conjugation, transformation, and transduction. [4] Though these processes are distinct from true sexual reproduction, they share some basic similarities. In eukaryotes, true sex is thought to have arisen in the Last Eukaryotic Common Ancestor (LECA), possibly via several processes of varying success, and then to have persisted (compare to "LUCA"). [5]

Since hypotheses for the origin of sex are difficult to verify experimentally (outside of evolutionary computation), most current work has focused on the persistence of sexual reproduction over evolutionary time. The maintenance of sexual reproduction (specifically, of its dioecious form) by natural selection in a highly competitive world has long been one of the major mysteries of biology, since both other known mechanisms of reproduction – asexual reproduction and hermaphroditism – possess apparent advantages over it. Asexual reproduction can proceed by budding, fission, or spore formation and does not involve the union of gametes, which accordingly results in a much faster rate of reproduction compared to sexual reproduction, where 50% of offspring are males and unable to produce offspring themselves. In hermaphroditic reproduction, each of the two parent organisms required for the formation of a zygote can provide either the male or the female gamete, which leads to advantages in both size and genetic variance of a population.

Sexual reproduction therefore must offer significant fitness advantages because, despite the two-fold cost of sex (see below), it dominates among multicellular forms of life, implying that the fitness of offspring produced by sexual processes outweighs the costs. Sexual reproduction derives from recombination, where parent genotypes are reorganized and shared with the offspring. This stands in contrast to single-parent asexual replication, where the offspring is always identical to the parents (barring mutation). Recombination supplies two fault-tolerance mechanisms at the molecular level: recombinational DNA repair (promoted during meiosis because homologous chromosomes pair at that time) and complementation (also known as heterosis, hybrid vigor or masking of mutations).

Evolution of asexual and sexual reproduction in the aspergilli

Aspergillus nidulans has long-been used as a model organism to gain insights into the genetic basis of asexual and sexual developmental processes both in other members of the genus Aspergillus, and filamentous fungi in general. Paradigms have been established concerning the regulatory mechanisms of conidial development. However, recent studies have shown considerable genome divergence in the fungal kingdom, questioning the general applicability of findings from Aspergillus, and certain longstanding evolutionary theories have been questioned. The phylogenetic distribution of key regulatory elements of asexual reproduction in A. nidulans was investigated in a broad taxonomic range of fungi. This revealed that some proteins were well conserved in the Pezizomycotina (e.g. AbaA, FlbA, FluG, NsdD, MedA, and some velvet proteins), suggesting similar developmental roles. However, other elements (e.g. BrlA) had a more restricted distribution solely in the Eurotiomycetes, and it appears that the genetic control of sporulation seems to be more complex in the aspergilli than in some other taxonomic groups of the Pezizomycotina. The evolution of the velvet protein family is discussed based on the history of expansion and contraction events in the early divergent fungi. Heterologous expression of the A. nidulans abaA gene in Monascus ruber failed to induce development of complete conidiophores as seen in the aspergilli, but did result in increased conidial production. The absence of many components of the asexual developmental pathway from members of the Saccharomycotina supports the hypothesis that differences in the complexity of their spore formation is due in part to the increased diversity of the sporulation machinery evident in the Pezizomycotina. Investigations were also made into the evolution of sex and sexuality in the aspergilli. MAT loci were identified from the heterothallic Aspergillus (Emericella) heterothallicus and Aspergillus (Neosartorya) fennelliae and the homothallic Aspergillus pseudoglaucus (=Eurotium repens). A consistent architecture of the MAT locus was seen in these and other heterothallic aspergilli whereas much variation was seen in the arrangement of MAT loci in homothallic aspergilli. This suggested that it is most likely that the common ancestor of the aspergilli exhibited a heterothallic breeding system. Finally, the supposed prevalence of asexuality in the aspergilli was examined. Investigations were made using A. clavatus as a representative ‘asexual’ species. It was possible to induce a sexual cycle in A. clavatus given the correct MAT1-1 and MAT1-2 partners and environmental conditions, with recombination confirmed utilising molecular markers. This indicated that sexual reproduction might be possible in many supposedly asexual aspergilli and beyond, providing general insights into the nature of asexuality in fungi.

Event in Sexual Reproduction: 3 Events | Biology

All the events of sexual reproduction that take place before the fusion of gametes are included in this category. Two main pre-fertilisation events are gametogenesis (formation of gametes) and gamete transfer.

(i) Gametogenesis:

The process of formation of male and female gametes is called gametogenesis. Gametes are haploid cells. Organisms such as monerans, fungi, algae and bryophytes have haploid parental body. Such type of organisms produce gametes by mitotic division and if the parent body is diploid gametes are formed by meiosis.

When male and female gametes are similar appearance and it is not possible to differentiate them into male and female gametes, they are called homogametic or isogametes. If the male and female gametes are morphologically dissimilar they are called heterogametes. Among heterogametes, the male reproductive unit is called antherozoid or sperm and female reproductive unit is known as egg or ovum.

Cell Division during Gamete Formation:

Plants belonging to the pteridophyte, gymnosperm, angiosperm and most of the animals including human beings have diploid parental body. In such organisms, specialised cells are present, which take part in the production of gametes. These cells are called meiocytes.

At the time of gamete formation, meiocytes undergo meiotic division. As a result of meiotic division, the number of chromosomes in the daughter cells (i.e., in the gametes) reduce to half and thus from the diploid meiocytes thus, haploid gametes are formed.

(ii) Gamete Transfer:

In some algae and fungi, both male and female gametes are motile. In majority of organisms, male gametes are motile while, the female gametes are stationary. After gametes formation, male and female gametes should come in physical association, so that they can be fused with each other.

In algae, bryophytes and pteridophytes, for the transfer of male gametes water acts as the medium. During this transfer a large number of gametes fail to reach the female gametes. To fulfil this loss, the number of male gametes produced is several thousands times the number of female gametes.

In flowering planes, pollen grains carry male gametes. When anther bursts, a large number of pollen grains are released. These pollen grains with the help of an agent (e.g., wind, water, insects) are transferred to the stigma of the pistil.

The transfer of pollen grains from anther to the stigma is called pollination. On reaching the stigma, pollen grain germinates and a tube-like structure called pollen tube comes out of it. Pollen tube carries male gametes and grows downwards through the style until it reaches to ovule situated in the ovary. On reaching the ovule, pollen tube enters into it and then releases male gametes near the egg.

2. Fertilisation:

The process of fusion of male gametes with the female gametes is called fertilisation or syngamy. It results in the formation of a diploid zygote.

It is mainly of two types:

(i) External Fertilisation:

In majority of the aquatic organisms, e.g., algae, fish and in amphibians, the process of fertilisation takes place outside the body of the organism, i.e., in the water. This type of gametic fusion is called external fertilisation. To enhance the chances of fertilisation, the organisms exhibiting external fertilisation discharge a large number of gametes into the water, e.g., bony fishes, amphibians, etc.

(ii) Internal Fertilisation:

In most of the terrestrial organisms, e.g., fungi, higher animals and majority of plants such as bryophytes, pteridophytes, gymnosperms and angiosperms, the process of fertilisation takes place inside the body of the organism. This type of gametic fusion is called internal fertilisation.

In the organisms exhibiting internal fertilisation, non-motile egg is formed inside the female body and male gametes are motile. The number of ova produced are less, but a large number of male gametes are formed, as many of them fail to reach the ova.

3. Post-Fertilisation Events:

During the sexual reproduction, the events which take place after the formation of zygote are called post-fertilisation events.

Fertilisation leads to the formation of a diploid cell called zygote. It is the vital link that ensures the continuity of species between the organisms of one generation and of the next.

Development of zygote depends on:

(a) Type of life cycle of the organism.

(b) Environment it is exposed.

In the fungi and algae, the zygote before germination undergoes a resting period. A thick wall is developed around the zygote, which prevents it from desiccation and damage.

In organisms with haplontic life cycle, zygote divides by meiosis to form haploid spores, which grow into haploid individual.

(ii) Embryogenesis:

The process of development of embryo from the zygote is called embryogenesis,

Embryogenesis involves the following process:

(a) Cell division to increase in number of cells.

(b) Cell enlargement or growth to increase in mass/volume of living matter.

(c) Cell differentiation for the formation of different types of tissues.

Embryogenesis in Animals:

Based on whether the development of the zygote takes place outside or inside the body of the female parent, animals are divided into two categories

Those animals, in which development of zygote takes place outside the female parent, are called oviparous. They lay fertilised eggs covered with hard calcareous shell in a safe place in the environment, e.g., reptiles and birds. After a period of incubation, the young ones hatch out from the egg.

The animals in which the development of zygote takes place into a young one inside the body of the female parent, arc called viviparous. After attaining a certain stage of growth, the young ones are delivered out of the body of the female organism.

The chances of survival of young ones is greater in these because of proper embryonic care and protection.

Embryogenesis in Plants:

In all flowering plants, the zygote is formed inside the ovule. In most of the plants, with the formation of zygote, all the parts of the flower except the pistil wither and fall off. In some plants, such as tomato and brinjal, the sepals are persistent and remain attached to the developing fruit. In ovule, the zygote divides several times to form an embryo.

Meanwhile the wall of the ovule becomes hard and it develops into seed. With these developments, the wall of the ovary also starts to swell. As a result, the ovary develops into fruit. A thick wall that covers fruit is called pericarp. It is protective in function. When seeds mature, they are dispersed. Under favourable conditions, these seeds germinate to produce new plants.

Answer of Question of Reproduction & Development

Reproduction is a universal occurrence in all living organisms Both asexual and exual reproductions occur in the animal kingdom.

Asexual reproduction produce offspring whose genes all come from a single parent. ission, budding, and fragmentation with regeneration are mechanisms of asexual production in various invertebrates.

Sexual reproduction requires the fusion of male and female gametes to form a iploid zygote. The production of offspring with varying genotypes and phenotypes may nhance reproductive success in fluctuating environment:

Animals may reproduce exclusively sexually or asexually, or they may alternate etween the two, depending on environmental conditions. Variations on these two modes re made possible through parthenogenesis, hermaphroditism, and sequential ermaphroditism.

In sexual reproduction, gametes unite in the external environment (external rtilization) or within the body of female (internal fertilization).

Diverse reproductive systems have evolved in the animal kingdom. Human reproduction involves intricate anatomy and complex behavior.

Human male reproductive anatomy consists of internal organs and external enitalia, the scrotum and penis. The gonads, or testes reside in the cool environment of t e scrotum. They possess endocrine interstitial cells surrounding sperm—forming

miniferous tubules that successively lead into the epididymis, vas deferens, ejaculatory ct, and urethra, which exits at the tip of the penis. Accessory glands add secretions to t e semen.

Before a human male can mature and function sexually, special regulatory rmones (FSH, GnRH, inhibin, LH and testosterone) must function.

The reproductive roles of the human female are more complex than those of the ale. Not only do females produce eggs, but after fertilization, they also nourish, carry, d protect the developing embryo. They may also nourish the infant for a time after it is

The female reproductive system consists of two ovaries, two uterine tubes, a uterus, gine, and external genitalia. The mammary glands contained in the paired breasts oduce milk for the new born baby.

The human female is fertile for only a few days each month, and the pattern of rmone secretion is intricately related to the cyclical release of a secondary oocyte from t e ovary. Various hormones regulate the menstrual and ovarian cycles.

‘Pregnancy sets a new series of physiological events into motion that are directed to housing, protecting and nourishing the embryo.

The development of a human may be divided into prenatal and postnatal periods.

Human pregnancy can be divided into three trimesters. Organogenesis is completed by eight weeks

Placenta is the organ that sustains the embryo and fetus through out the pregnancy.

Birth, or parturition, results from strong, rhythmic uterine contractions that -bring about the three stages of labor dilation of the cervix, expulsion of the baby and delivery of the placenta. Positive feedback involving the hormones estrogen and oxytocin, and prostaglandins, regulate labor.

Lactation includes both milk secretions (production) by the mammary glands and milk release from the breasts.

Answers to the Questions

Q.1. Define asexual reproduction and describe forms of asexual reproduction in invertebrates.

Ans. Asexual reproduction is the creation of offspring whose genes all come from one parent without the fusion of gametes that is eggs or sperm. In most cases, asexual reproduction relies entirely on mitotic cell division. Offspring produced by asexual reproduction all have the same genotype (unless mutations occur) and are called clones.

Asexual, reproduction appears in many invertebrate phyla, such as cnidarians, bryozoans, annelids, echinoderms. and hemichordates. In animal phyla in which asexual reproduction occurs, most members also employ sexual reproduction. In these groups, asexual, reproduction ensures rapid increase in numbers when differentiation of the organism has not advanced to the point of forming gametes. The basic forms of asexual reproduction are fission, budding, gemmulation. fragmentation and parthenogenesis

Protists and some multicellular animals (cnidarians, annelids) may reproduce by fission. Fission (L.Fissio, the act of splitting) is the division of one cell, body, or body part into two. this process. the cell pinches in two by inward furrowing of the plasma membrane. Binary fission is common among bacteria and protozoa

binary fission the body of the parent divides by mitosis into two approximately equal parts, each of which grows into an individual similar to the parent. Binary fission may be lengthwise, as in flagellate protozoa, or transverse, as in ciliate protozoa. In multiple fission the nucleus divides repeatedly before division of the cytoplasm, producing many daughter cells simultaneously. Spore formation, called sporogony, is a form of multiple fission common among some parasitic protozoa, for example, malarial parasites. Fig. 7.1.

Budding (L.bud, a small protuberance) is an unequal division of an organism. The new individual arises as an outgrowth (bud) from the parent, develops organs like those of the parent and then detaches itself. If the buds remain attached to the parent, they form a colony. Budding occurs in several animal phyla and is especially prominent in cnidarians, tunicates and sponges. Fig. 7.2.


Gemmulation is the formation of a new individual from an aggregation of cells surrounded by a resistant capsule, called a gemmule. In many freshwater sponges, gemmules develop in the fall and survive the winter in the dried or frozen body of the parent. In the spring, the enclosed cells become active, emerge from the capsule, and grow into a new sponge. Fig. 7.3.


Fragmentation, is the breaking of the body into several pieces, some or all of which develop into complete adults. For an animal to reproduce this way, fragmentation must be accompanied by regeneration, i.e., the regrowth of lost body parts. Reproduction by fragmentation and regeneration occurs in many sponges, cnidarians, polychaete annelids, and tunicates.


Parthenogenesis (Gr.parthenos, virgin + genesis, production) is a spontaneous activation of a mature egg, followed by normal egg divisions and subsequent embryonic development. In fact, mature eggs of species that do not undergo parthenogenesis can sometimes be activated to develop without fertilization by pricking them with a needle, or by exposing them to high concentrations of calcium, or by altering their temperature. />

Parthenogenesis in invertebrates occurs in certain flatworms, rotifers, roundworms, insects, lobsters etc. Parthenogenesis has a role in the social organization of certain species of bees, wasps, and ants. Male honeybees, or drones, are produced parthenogenetically, whereas females, both sterile workers and reproductive females (queens), develop from fertilized eggs.

Q.2. What are some advantages of asexual reproduction in Invertebrates? What are some disadvantages?

Ans. Advantages of asexual


Asexual reproduction has several potential advantages. For instance, (1) it enables animals that live in isolation to produce offspring without locating mates. (2) It may also allow many offspring to be produced in a short period of time, which is ideal for colonizing a habitat rapidly. (3) Theoretically, asexual reproduction is most advantageous in stable, favorable environments because it perpetuates successful genotypes precisely.


Without the tremendous genetic variability bestowed by meiosis and sexual processes, however, a population of genetically identical animals stands a great increased chance of being devasted by a single disease or environmental insult, such as a long drought. A given line of asexually reproducing animals can cope with a changing environment only through the relatively rare spontaneous mutations (alterations , in genetic material) that prove to be beneficial. Paradoxically, however, most mutations are detrimental or lethal, and herein lies one of the greatest disadvantages of asexual reproduction.

Q.3. What is the difference between asexual and sexual reproduction.

Q.4. Define two alternatives to bisexual reproduction-hermaphroditism and parthenogenesis. What is the difference between ameiotic and meiotic parthenogenesis?

Ans. Hermaphroditism

Hermaphroditism (Gr.hermaphroditos an organism with the attributes of both sexes) occurs when an animal has both functional male and female reproductive systems. This dual sexuality is sometimes called the monoecious (Gr. monos, single + oikos, house) condition. Many sessile burrowing, or endoparasific invertebrate animales e g most flatworms, some hydroids and annelids, and all barnacles and pulmonates (snails) and a few vertebrates (some fishes), are hermaphroditic. Some hermaphrodites fertilize themselves, but most avoid self-fertilization by exchanging germ cells with another member of the some species. An advantage is that with every individual producing eggs, a hermaphroditic species could potentially produce twice as many offspring as could a dioecious species, in which half the individuals are nonproductive males.

Another variation of hermaphroditism, sequential hermaphroditism occurs when an animal is one sex during one phase of its life cycle and the opposite sex during another phase. Hermaphrodites are either protogynous or protandrous. In protandary, an animal is a male during its early life history and a female later in the life history. The reverse is true for protogynous animals. A change in the sex ratio of a population is one factor that can induce sequential hermaphroditism, which is common in oyster.


Parthenogenesis (“virgin origin”) is the development of an embryo from an unfertilized egg or one in which the male and female nuclei fail to unite following fertilization. Parthenogenesis may be ameiotic or meiotic.

Ameiotic parthenogenesis: In ameiofic parthenogenesis, no meiosis occurs, and the egg is formed by mitotic cell division. This “asexual” form of parthenogenesis is known to occur in some species of flatworms, rotifers, crustaceans, insects, and probably others. In these cases, the offspring are clones of the parents because, without meiosis, the parent’s chromosomal complement is passed intact to offspring.

Meiotic parthenogenesis: In meiotic parthenogenesis a haploid ovum is formed by meiosis, and it may or may not be activated by the influence of a male. For example, in some species of fishes, a female may be inseminated by a male of the same or related species, but the sperm serves only to activate the egg the male’s genome is.rejected before it can penetrate the egg. In several species of flatworms, rotifers, annelids, ‘mites, and insects, the haploid egg begins development spontaneously no males are required to stimulate activation of an ovum. The diploid condition is restored by chromosomal duplication.

The disadvantage of parthenogenesis is that if the environment should suddenly change, as it often does, parthenogenetic species have limited capacity to shift gene combinations to adapt to the new conditions.

Q.5. Define major patterns of fertilization and describe the environmental and

behavioral requirements of each pattern.

Ans. The mechanisms of fertilization the union of sperm and egg, play an important part in sexual reproduction. The two major patterns of fertilization that have evolved are external and internal fertilization.

External fertilization: In external fertilization, eggs are shed by the female and fertilized by the male in the environment. Because external fertilization requires an environment where an egg can develop without desiccation or heat stress, it occurs almost exclusively in moist habitats. Many aquatic invertebrates simply shed their eggs and sperm into the surroundings, and fertilization occurs without the parents actually making physical contact. Timing is crucial to ensure that mature sperms encounter ripe eggs. Environmental cues such as temperature or day length may cause all the individuals of a population to release gametes at once, or pheromones from one individual releasing gametes may trigger gamete release in others.

Most fishes and amphibians that use ekternal fertilization exhibit specific mating behaviors, resulting in one male fertilizing the eggs of one female. Courtship behavior is a mutual trigger for the release of gametes, with two effects: The probability of successful fertilization is increased, and the choice of mates may be somewhat selective.

Internal fertilization: It occurs when sperms are deposited in (or nearby) the female reproductive tract, and egg and sperm unite within her body. Internal fertilization requires cooperative behavior, leading to copulation. In some cases, uncharacteristic sexual behavior is eliminated by natural selection in a direct manner for example, female spiders will eat males if specific reproductive signals are not followed during mating. Internal fertilization also requires sophisticated reproductive systems. Copulatory organs for the delivery of sperm and receptacles for its storage and transport to the eggs must be present

Q.6. Define sexual reproduction. What are some advantages and disadvantages of sexual reproduction?

Ans. Sexual Reproduction:

Sexual reproduction in animals is the creation of offspring by the fusion of two haploid gametes (sperm and egg) to form a diploid zygote. Gametes are formed by meiosis, and sexual reproduction usually involves two parents, both contributing genes to the offspring. The offspring of sexual union are somewhat different from their parents and siblings-they have genetic diversity.

Advantages of Sexual Reproduction

New combinations of traits can arise more rapidly in sexual reproducing animals because of genetic recombinations. The resulting genetic diversity or variability increases the chances of the species surviving sudden environmental changes. Furthermore, variation is the foundation of evolution. In contrast to the way asexually reproducing populations tend to retain mutations sexually reproducing populations tend to eliminate deleterious and lethal mutations.

Many biologists believe that sexual reproduction, with its breakup and recombination of genomes keeps producing novel genotypes that in times of environmental changes may survive and reproduce, whereas most others die. Variability, advocates of this viewpoint argue, is sexual reproduction’s trump card.

Disadvantages of Sexual Reproduction

Sexual reproduction also has some disadvantages. For example, sexual reproduction is complicated, requires more time, and uses much more energy than asexual reproduction. Mating partners must come together and coordinate their activities to produce young.

Many biologists believe that an even more troublesome problem is the “cost of meiosis”. A female that reproduce asexually passes all of her genes to her offspring. But when she reproduces sexually the genome is divided during meiosis and only half her genes flow to the next generation. Another cost is wastage in production of males, many of which fail to reproduce and thus consume resources that could be applied to production of females. In addition, many of the gametes that are released are not fertilized, leading to a significant waste of metabolic effort.

Q.7. What reproductive strategy developed in each of the following to increase the chances of survival: (a) bony fishes, (b) amphibians, and (c) reptiles?

Ans. (a) Bony fishes

(1) Fishes are.well known for their high potential fecundity, with most species releasing thousands to millions of eggs and sperm annually. Males and females come together in great schools and release vast numbers of gametes into the water to drift with the current.

(2) Fish species have reproductive methods, structures, and an attendant physiology that have allowed them to adapt to a great variety of aquatic conditions. For example, unlike the minute, buoyant, transparent eggs of pelagic marine teleosts, those of many near shore bottom dwelling (benthic) species are larger, typically yolky, non buoyant, and adhesive. Some bury their eggs, many attach them to vegetation, some deposit them in nests, and some even incubate them in their mouths. Many benthic spawners guard their eggs.

Freshwater fishes that do provide some form of egg care, produce fewer, larger eggs that enjoy a better chance for survival.


The reproductive strategies in amphibians are much more diverse than those observed in other groups of vertebrates. In each of three living orders of amphibia (caecilians, salamanders, anurans) are trends toward terrestriality. The variety of these adaptations is especially noteworthy in anurans. These reproductive adaptations have been viewed as pioneering evolutionary experiments in the conquest of terrestrial environments by vertebrates. Noreworthy is the evolution of direct development of terrestrial eggs, ovoviviparity viviparity and parental care that have been important in successful invasion of mountainous environments by amphibians. Eggs are laid in large masses usually anchored to vegetation. Migration of frogs and toads . is correlated with their breeding habits. Males usually return to a pond or stream before females,

which they then attract by their calls. Some salamanders also have a strong homing instinct, returing each year to reproduce in the same pool, to which they are guided by olfactory cues.


The reptilian system includes shelled, desiccation — resistant eggs. These eggs had the three basic embryonic membranes that still characterize the mammalian embryo, as well as flat embryo that developed and underwent gastrulation atop a huge yolk mass.

Q.8. How do bird eggs and amphibian eggs differ?

Ans. The frog egg has no shell, and it dehydrates quickly in dry air. Fertilization is external in most species of amphibians, with the male grasping the female (amplexus), and spilling his sperm over the eggs as the female shed them. Amphibians generally lay their eggs in ponds or swamps or at least in moist environments. Some species lay vast numbers of eggs, and mortality is high. In contrast are species that display various types of parental care and that lay relatively few eggs. In bird’s egg, a shell is present which prevents desiccation of the egg, which can therefore be laid in a

dry place. Specialized membranes within the egg function in gas exchange, waste storage, and transfer of stored nutrients to the embryo. These are called extraembryonic membranes because they are not part of the body of the developing animal, although they develop from tissue layers that grow out from the embryo. The amniotic egg is named for one of these membranes, the amnion, which encloses a compartment of amniotic fluid that bathes the embryo and acts as a hydraulic shock absorber. The egg shells in birds are much thicker than those of reptiles. Thicker shells permit birds to sit on their eggs and warm isri them. This brooding, or incubation, hastens embryo development. Figure 7.4.

Q.9. Explain in detail how evolution in reproduction has taken place in

vertebrate classes.

Ans. Almost all vertebrates reproduce sexually, only a few lizards and fishes normally reproduce parthenogenetically. Sexual reproduction evolved among aquatic animals and then spread to the land as animals became terrestrial.

Most fishes favor a simple theme: they are dioecious, with external fertilization and external development of the eggs and embryos (oviparity). Soon after the egg of an oviparous species is laid and fertilized, it takes up water and the outer layer hardens. Cleavage follows, and the blastoderm forms, sitting astride a relatively enormous yolk mass. Soon the yolk mass is enclosed by the developing blastoderm, which then begins to assume a fish-like shape. The fish hatches as a larva carrying a semitransparent sac of yolk, which provides its food supply until the mouth and digestive tract have developed. After a period of growth the larva undergoes a metamorphosis, especially dramatic in many marine fishes. Body shape is refashioned, fin and color patterns change, and the animal becomes a juvenile bearing the unmistakable definitive body form of its species.

Unlike birds and mammals, which stop growing after reaching adult size, most fishes after attaining reproductive maturity continue to grow for as long as they live. This may be a selective advantage, since the larger the fish, the more gametes are produces and the greater its contribution to future generations. Thus, to ensure reproductive success in fishes:

  1. there is large number of eggs
  2. the fertilized egg develops rapidly, and
  3. the young achieve maturity within a short time.

Amphibians (frogs, salamanders, and the like) evolved from fish, and they too generally use external fertilization, they must therefore return to the water or to a very moist place on land to lay their eggs. Some salamanders have evolved a behavioral sequence in which the male releases a membranous packet (spermatophore) containing sperm that the female picks up with her cloaca. These amphibians have thus evolved a primitive type of internal fertilization, and some of them mate on land, but their eggs must still be laid in very moist places.

Depending on the species, either males or females may incubate eggs on their back (Pipe), in the mouth, or eggs and tadpoles even in the stomach (Rheobatrachtis females). Certain tropical tree frogs stir their egg masses into moist foamy nests that resist zirying. There are also itve-bearing amphibians (

eggs in the female reproductive tract, where embryos can develop without drying out. The developmental period is much longer in amphibians than in fishes, although the eggs do not contain appreciably more yolk. An evolutionary adaptation present in amphibians is the presence of two periods of development: larval and adult stages. The aquatic larval stage develops rapidly, and the animal spends much the Erne eating and growing. After reaching a sufficient size, the larval form undergoes a developmental transition called metamorphosis into the adult (often in terrestrial forms. Many amphibians exhibit complex and diverse social behavior, especially during their breeding seasons. Frogs are usually quiet creatures, but many species fill the air with their mating calls during the breeding season. Males may vocalize to defend breeding territory or to attract females. In some terrestrial species, migrations to specific breeding sites may involve vocal communication, celestial navigation, or chemical signaling. Reptiles

The reptiles, evolved from ancestral amphibians. They were the first vertebrates to be fully emancipated from the ancestral dependence on the aquatic environment for reproduction. The evolutionary adaptations found in reptiles are:

  1. They use internal fertilization, and lay eggs enclosed_ in tough membranes and shells. Since internal fertilization entails much less wastage of egg cells than external fertilization, only a few egg cells are released during each reproductive season.
  2. Many reptiles are oviparous. Others are ovoviviparous. Viviparity in reptiles is limited to squamates, and has evolved at least 100 separate times.
  3. The shelled egg and extraembryonic membrane is also first seen in reptiles.

These adaptations allowed reptiles to lay eggs in dry places without danger of desiccation. As the embryo develops, the extraembryonic chorion and amnion help protect it, the later by creating a fluid—filled sac for the embryo. The allantois permits gas exchange and stores excretory products. Complete development can occur within the shell. The young hatches as a lung — breathing juvenile. The appearance of the shelled egg widened the division between evolving amphibians and reptiles and, probably more than any other adaptation, contributed to the evolutionary establishment of reptiles. Fig. 7.4. Birds Birds have retained the important adaptations for life on land that evolved in the early reptiles. The evolutionary adaptations found in birds are:

  1. Males simply deposit semen against the cloaca for internal fertilization. Some water fowls and ostriches possess intromittent organs.
  2. All birds are oviparous, and the eggshells are much thicker than those of reptiles, and
  3. Extensive parental care and feeding of young are more common among birds than fishes, amphibians, or reptiles.

Fertilization takes place in the upper oviduct several hours before the layers of albumen, shell membranes, and shell are added. Sperm remain alive in the female oviduct for many days, after a single mating. The thicker shells of eggs permit birds to sit on their eggs and warm them. This brooding, or incubation, hastens embryo development. Most birds build some form of nest in which to rear their young. Newly hatched birds are of two types: precocial and altricial. Fig. 7.5. Precocial young, such as quail, fowl, .ducks, and most water birds, are covered with down when

hatched and can run or swim as soon as their plumage is dry. Altricial ones, on the other hand, are naked and helpless at birth and remain in the nest for a week or more. Young of both types require care from parents for some time after hatching. They must be fed, guarded, and protected against rain and sun. Parents of altricial species must carry food to their young almost constantly, for most young birds will eat more than their weight each day. This enormous food consumption explains the rapid growth of the young and their quick exit from the nest.

Because mammals are descendants of one of three lineages that originated with a common amniote ancestor, they inherited the amniotic egg. The earliest mammals were egg layers, and even today some mammals retain this primitive character the monotremes (duck — billed platypus and spiny anteater) lay large yolk eggs that closely resemble bird’s eggs. After hatching, the young suck milk from the fur of the mother around openings of the mammary glands. Thus in monotrems there is no gestation. All other mammals are viviparous, and mammalian viviparity was major evolutionary adaptation and it has taken two forms.

  1. The marsupials develop the ability to nourish their young in a puch after a short gestation inside the female.
  2. The placentals retain the young inside the female, where the mother norishes them by means of a placenta.

In marsupials (such as opossums and kangaroos), the embryos develop for a time within the mother’s uterus. But the embryo does not “take roof’ in the uterine wall, and consequently it receives little nourishment from the mother before birth. The young of marsupials are therefore born immature and are sheltered in a pouch in the mother’s abdominal wall and nourished with milk. Fig. 7.6.

All other mammals, composing 94% of class mammalia, are placental mammals. In placentals, the reproductive investment is in prolonged gestation, unlike marsupials in which the reproductive investment is in prolonged lactation. The embryo remains in the uterus, nourished by food supplied through a chorioallantoic type of placenta, an intimate connection between mother and young. Even after birth. mammals continue to nourish their young. Mammary glands are a unique mammalian adaptation that permits the female to nourish the young with milk that she produces Some mammals nurture their young until adulthood, when they are able to mate and fend for themselves Mammalian reproductive behavior also contributes to the transmission and evolution of culture that is the key to the evolution of the human species.

Q.10. What does the human male reproductive system consists of?

Ans. The reproductive role of the human male is to produce sperms and deliver them to the vagina of the female. This function requires the following structures:

  1. Two testes that produce sperms and the male sex hormone, testosterone.
  2. Accessory glands and tubes that furnish a fluid for carrying the sperms to the penis. This fluid, together with the sperm, is called semen.
  3. Accessory ducts that store and carry secretions from the testes and accessory glands to the penis.
  4. A penis that deposits semen in the vagina during sexual intercourse. Q.11. Describe in detail the production and transport of sperm in human male.

Ans. The male gonads or sex organs, are the testes — oval glandular structures that form in the dorsal portion of the abdominal cavity from the same embryonic tissue that gives rise to the ovaries in females.

In the human male the testes descend about the time of birth from their points of origin into the scrotal sac (scrotum), a pouch whose cavity is initially continuous with the abdominal cavity via a passageway called the inguinal canal. After the testes have descended through the inguinal canal into scrotum, the canal is slowly plugged by growth of connective tissue, so that the scrotal and abdominal cavities are no longer continuous. The temperature in a scrotum is about 2°C below that in the abdominal cavity. The lower temperature is necessary for active sperm production and survival.

Each testis contains over eight hundred tightly coiled seminiferous tubules, which produce thousands of sperms each second in healthy young men. The walls of the seminiferous tubules are lined with two types of cells.

1. Spermatogenic cells, which give rise to sperms, and

Sustentacular cells, which nourish the sperms as they form, also secrete a fluid (as well as the hormone inhibin) into the tubules to provide a liquid medium for the sperms. Between the seminiferous tubules are clusters of endocrine cells, called interstitial cells (Leydig cells), that produce testosterone. Inhibin and testosterone, both are androgens, the male sex hormones. Duct system

A system of tubes carries the sperm, that the testes produce, to the penis. The seminiferous tubules merge into a network of tiny tubules called the rete testis (L.rete, net), which merges into a coiled tube called the epididymis. The epididymis has three main functions:

  1. It stores sperm until they are mature and ready to be ejaculated.
  2. It contains smooth muscle that helps propel the sperm toward the penis by peristaltic contractions, and
  3. It serves as a duct system for sperm to pass from the testis to the ductus deferens.

The ductus deferens (formerly called the vas deferens or sperm duct) is the dilated continuation of the epididymis. Continuing upward after leaving the scrotum, the ductus deferens passes through the lower part of the abdominal wall via the inguinal canal. The ductus deferens then passes around the urinary bladder and enlarges to form the ampulla. The ampulla stores some sperm until they are ejaculated. Distal to the ampulla, the ductus deferens becomes the ejaculatory duct. The urethra is the final ein-tion of the reproductive duct system. Fig. 7.7. Accessory Glands Three sets of accessory glands add secretions to the semen, the fluid that is ejaculated:

  1. A pair of seminal vesicles contributes about 60% of the total volume of the semen. The fluid from the seminal vesicles is thick and clear, containing mucus, amino acids, vitamin C, prostaglandins, and large amount of fructose (sugar), which provides energy for the sperm and helps to neutralize the natural protective acidity of the vagina. (The pH of vagina is about 3 to 4, bui sperm motility is enhanced when it increases to about 6).
  2. The prostate gland is the largest of the semen secreting glands. It secretes its products directly into the urethra through several small ducts. Prostatic fluid is thin and milky, contains several enzymes, cholesterol, buffering salts and phospholipids.
  3. The bulbourethral glands are a pair of small glands along the urethra below the prostate. Before ejaculation they secrete a clear, alkaline and viscous fluid that lubricates the urethra to facilitate the ejaculation of semen. Bulbourethral fluid also carries some sperm released before ejaculation, which is one reason for the high failure rate of the withdrawal method of birth control.

The penis has two functions,

  1. It carries urine through the urethra to the outside during urination, and
  2. It transports semen through the urethra during ejaculation.
  3. The human penis is composed of three cylindrical strands of spongy erectile tissue derived from modified veins and capillaries: two corpora cavernosa and the corpus spongiosum. The corpus spongisum extends beyond the corpora cavernosa and becomes the expanded tip of the penis called the glans penis. The human glans is covered by a fold of skin called the foreskin, or prepuce, which may be removed by circumcision. Fig. 7.7

A mature human sperm consists of a head, midpiece, and tail. The head contains the haploid nucleus, which is mostly DNA. The acrosome, a cap over most of the head, contains an enzyme called acrosin that assists the sperm in penetrating the outer layer surrounding a secondary oocyte. The sperm tall contains an array of microtubules that bend to • produce whip like movements. The spiral mitochondria in the midpiece supply the ATP necessary for these movements.

Q.12. What are the major constituents of semen? What are the functions of seminal fluids?

Ans. Semen is a mixture of seminal fluid and sperm cells. The seminal fluid is secreted by three sets of glands: the seminal vesicles secrete water, fructose, prostaglandins, and vitamin C the prostate, secrete water enzymes, cholesterol, buffering salts, and phosphohpids and the bulbouretheral glands secrete a clear, alkaline fluid. The average human ejaculation produces 3 to 4m1 of semen and contains 300 to 400 million sperms.

Gene family evolution

To identify gene families likely to be associated with Candida pathogenicity and virulence, we used a phylogenomic approach across seven Candida and nine Saccharomyces genomes (Supplementary Information, section S10). Among 9,209 gene families, we identified 21 that are significantly enriched in the more common pathogens (Table 2). These include families encoding lipases, oligopeptide transporters and adhesins, which are all known to be associated with pathogenicity 8 , as well as poorly characterized families not previously associated with pathogenesis.

Three cell wall families are enriched in the pathogens: those encoding Hyr/Iff proteins (ref. 16), Als adhesins 17 and Pga30-like proteins (Table 2 and Supplementary text S11). The Als family (family 17 in Supplementary Tables 22 and 23) in C. albicans is associated with virulence, and in particular with adhesion to host surfaces 18 , invasion of host cells 19 and iron acquisition 20 . All these families are absent from the Saccharomyces clade species and are particularly enriched in the more pathogenic species (Supplementary Table 18). All three families are highly enriched for gene duplications (Supplementary Table 19), including tandem clusters of two to six genes, and show high mutation rates (fastest 5% of families) (Supplementary Information, section S10d). This variable repertoire of cell wall proteins is likely to be of profound importance to the niche adaptations and relative virulence of these organisms.

Als 17 and Hyr/Iff genes frequently contain intragenic tandem repeats, which modulate adhesion and biofilm formation in S. cerevisiae 21 (Fig. 4 and Supplementary Figs 19 and 20). The sequence of intragenic tandem repeats is conserved at the protein level across species (Supplementary Figs 19 and 20). Two proteins contain both an Als domain and repeats characteristic of the Hyr/Iff family.

The amino-terminal Als domain (red) and Hyr/Iff domain (green) are shown as ovals. Intragenic tandem repeats (ITRs see Supplementary Information, section S11) are shown as rectangles, coloured to represent similar amino-acid sequences.

Candida clade pathogens show expansions of extracellular enzyme and transmembrane transporter families (Table 2 and Supplementary Table 22). These families are either not found in Saccharomyces (including amino-acid permeases, lipases and superoxide dismutases) or are present in S. cerevisiae but significantly expanded in pathogens (including phospholipase B, ferric reductases, sphingomyelin phosphodiesterases and GPI-anchored yapsin proteases, which have been linked to virulence in C. glabrata 22 ). Several groups of cell-surface transporters are also enriched (including oligopeptide transporters, amino-acid permeases and the major facilitator superfamily). Overall, these family expansions illustrate the importance of extracellular activities in virulence and pathogenicity. Genes involved in stress response are also variable between species (Supplementary Information, section S12).

C. albicans also showed species-specific expansion of some families, including two associated with filamentous growth, a leucine-rich repeat family and the Fgr6-1 family (Table 2). Candida albicans forms hyphae whereas C. tropicalis and C. parapsilosis produce only pseudohyphae, so these families may contribute to differences in hyphal growth.

We identified 64 families showing positive selection in the highly pathogenic Candida species (Supplementary Table 32). These are highly enriched for cell wall, hyphal, pseudohyphal, filamentous growth and biofilm functions (Supplementary Information, section S13). Six of the families have been previously associated with pathogenesis, including that of ERG3, a C-5 sterol desaturase essential for ergosterol biosynthesis, for which mutations can cause drug resistance 23 .

Evolution after the development of sexual reproduction - Biology


13. Evolution and Natural Selection

13.2. The Development of Evolutionary Thought

For centuries, people believed that the various species of plants and animals were unchanged from the time of their creation. Although today we know this is not true, we can understand why people may have thought it was true. Because they knew nothing about DNA, meiosis, genetics, or population genetics, they did not have the tools to examine the genetic nature of species. Furthermore, the process of evolution is so slow that the results of evolution are usually not recognized during a human lifetime. It is even difficult for modern scientists to recognize this slow change in many kinds of organisms.

Early Thinking About Evolution

In the mid-1700s, Georges-Louis Buffon, a French naturalist, wondered if animals underwent change (evolved) over time. After all, if animals didn’t change, they would stay the same, and it was becoming clear from the study of fossils that changes had occurred. However, Buffon didn’t come up with any suggestions on how such changes might come about. In 1809, Jean-Baptiste de Lamarck, a student of Buffon’s, suggested a process by which evolution might occur. He proposed that acquired characteristics were transmitted to offspring. Acquired characteristics are traits gained during an organism’s life and not determined genetically. For example, he proposed that giraffes originally had short necks but because they constantly stretched their necks to get food, their necks got slightly longer (figure 13.2). When these giraffes reproduced, their offspring acquired their parents’ longer necks. Because the offspring also stretched to eat, the third generation ended up with even longer necks. And so Lamarck’s story was thought to explain why the giraffes we see today have long necks. Although we now know Lamarck’s theory was wrong (because acquired characteristics are not inherited), it stimulated further thought as to how evolution might occur. From the mid-1700s to the mid-1800s, lively arguments continued about the possibility of evolutionary change. Some, like Lamarck and others, thought that change did take place many others said that it was not even possible. It was the thinking of two English scientists that finally provided a mechanism for explaining how evolution occurs.

FIGURE 13.2. The Contrasting Ideas of Lamarck and the Darwin-Wallace Theory

(a) Lamarck thought that acquired characteristics could be passed on to the next generation. Therefore, he postulated that, as giraffes stretched their necks to get food, their necks got slightly longer. This characteristic was passed on to the next generation, which would have longer necks. (b) The Darwin-Wallace theory states that there is variation within the population and that those with longer necks are more likely to survive and reproduce, passing on their genes for long necks to the next generation.

The Theory of Natural Selection

In 1858, Charles Darwin and Alfred Wallace suggested the theory of natural selection as a mechanism for evolution. The theory of natural selection is the idea that some individuals whose genetic combinations favor life in their surroundings are more likely to survive, reproduce, and pass on their genes to the next generation than are individuals who have unfavorable genetic combinations. This theory was clearly set forth in 1859 by Darwin in his book On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life (How Science Works 13.1).

The theory of natural selection is based on the following assumptions about the nature of living things:

1. All organisms produce more offspring than can survive.

2. No two organisms are exactly alike.

3. Among organisms, there is a constant struggle for survival.

4. Individuals that possess favorable characteristics for their environment have a higher rate of survival and produce more offspring.

5. Favorable characteristics become more common in the species, and unfavorable characteristics are lost.

Using these assumptions, the Darwin-Wallace theory of evolution by natural selection offers a different explanation for the development of long necks in giraffes (figure 13.2b):

1. In each generation, more giraffes would be born than the food supply could support.

2. In each generation, some giraffes would inherit longer necks, and some would inherit shorter necks.

3. All giraffes would compete for the same food sources.

4. Giraffes with longer necks would obtain more food, have a higher survival rate, and produce more offspring.

5. As a result, succeeding generations would show an increase in the number of individuals with longer necks.

Modern Interpretations of Natural Selection

The logic of the Darwin-Wallace theory of evolution by natural selection seems simple and obvious today, but at the time Darwin and Wallace proposed their theory, the processes of meiosis and fertilization were poorly understood, and the concept of the gene was only beginning to be discussed. Nearly 50 years after Darwin and Wallace suggested their theory, the rediscovery of the work a monk, Gregor Mendel (see chapter 10) provided an explanation for how characteristics could be transmitted from one generation to the next. Mendel’s concepts of the gene explained how traits could be passed from one generation to the next. It also provided the first step in understanding mutations, gene flow, and the significance of reproductive isolation. All of these ideas are interwoven into the modern concept of evolution. If we update the five basic ideas from the thinking of Darwin and Wallace, they might look something like the following:

1. An organism’s ability to overreproduce results in surplus organisms.

2. Because of mutation, new, genetically determined traits enter the gene pool. Because of sexual reproduction, involving meiosis and fertilization, new genetic combinations are present in every generation. These processes are so powerful that each individual in a sexually reproducing population is genetically unique. The genetic information present is expressed in the phenotype of the organism.

3. Resources, such as food, soil nutrients, water, mates, and nest materials, are in short supply, so some individuals do without. Other environmental factors, such as disease organisms, predators, and helpful partnerships with other species, also affect survival. All the specific environmental factors that affect survival by favoring certain characteristics are called selecting agents.

4. Selecting agents favor individuals with the best combination of alleles—that is, those individuals are more likely to survive and reproduce, passing on more of their genes to the next generation. An organism is selected against if it has fewer offspring than other individuals that have a more favorable combination of alleles. The organism does not need to die to be selected against.

5. Therefore, alleles or allele combinations that produce characteristics favorable to survival become more common in the population and, on the average, the members of the species will be better adapted to their environment.

Evolution results when there are changes in allele frequency in a population. Recall that individual organisms cannot evolve—only populations can. Although evolution is a population process, the mechanisms that bring it about operate at the level of the individual.

Recall that a theory is a well-established generalization supported by many kinds of evidence. The theory of natural selection was first proposed by Charles Darwin and Alfred Wallace. Since the time it was first proposed, the theory of natural selection has been subjected to countless tests yet remains the core concept for explaining how evolution occurs.

The Voyage of HMS Beagle, 1831-1836

Probably the most significant event in Charles Darwin's life was his opportunity to sail on the British survey ship Beagle.

Surveys were common at that time they helped refine maps and chart hazards to shipping. Darwin was 22 years old and probably would not have had the opportunity, had his uncle not persuaded Darwin's father to allow him to take the voyage. Darwin was to be a gentleman naturalist and companion to the ship's captain, Robert Fitzroy. When the official naturalist left the ship and returned to England, Darwin replaced him and became the official naturalist for the voyage. The appointment was not a paid position.

The voyage of the Beagle lasted nearly 5 years. During the trip, the ship visited South America, the Galapagos Islands, Australia, and many Pacific Islands. The Beagle's entire route is shown on the accompanying map. Darwin suffered greatly from seasickness and, perhaps because of it, made extensive journeys by mule and on foot some distance inland from wherever the Beagle happened to be at anchor. These inland trips gave Darwin the opportunity to make many of his observations. His experience was unique for a man so young and very difficult to duplicate because of the slow methods of travel used at that time.

Charles Darwin set forth on a sailing vessel similar to this, the HMS Beagle, in 1831 at the age of 22.

Although many people had seen the places that Darwin visited, never before had a student of nature collected volumes of information on them. Also, most other people who had visited these faraway places were military men or adventurers who did not recognize the significance of what they saw. Darwin's notebooks included information on plants, animals, rocks, geography, climate, and the native peoples he encountered. The natural history notes he took during the voyage served as a vast storehouse of information, which he used in his writings for the rest of his life. Because Darwin was wealthy, he did not need to work to earn a living and could devote a good deal of his time to the further study of natural history and the analysis of his notes. He was a semiinvalid during much of his later life. Many people think his ill health was caused by a tropical disease he contracted during the voyage of the Beagle. As a result of his experiences, he wrote several volumes detailing the events of the voyage, which were first published in 1839 in conjunction with other information related to the voyage. His volumes were revised several times and eventually were entitled The Voyage of the Beagle. He also wrote books on barnacles, the formation of coral reefs, how volcanoes might have been involved in reef formation, and finally On the Origin of Species. This last book, written 23 years after his return from the voyage, changed biological thinking for all time.

2. Why has Lamarck’s theory been rejected?

3. List five assumptions about the nature of living things that support the concept of evolution by natural selection.

15.1 History of Evolutionary Thought

The proposal that one type of organism could descend from another type goes back to some of the first pre-Socratic Greek philosophers, such as Anaximander and Empedocles. Such proposals survived into Roman times.

In contrast to these materialistic views, Aristotelianism considered all natural things as actualisations of fixed natural possibilities, known as forms. This was part of a medieval teleological understanding of nature in which all things have an intended role to play in a divine cosmic order. Variations of this idea became the standard understanding of the Middle Ages and were integrated into Christian learning, but Aristotle did not demand that real types of organisms always correspond one-for-one with exact metaphysical forms and specifically gave examples of how new types of living things could come to be.

In the 17th century, the new method of modern science rejected the Aristotelian approach. It sought explanations of natural phenomena in terms of physical laws that were the same for all visible things and that did not require the existence of any fixed natural categories or divine cosmic order. However, this new approach was slow to take root in the biological sciences, the last bastion of the concept of fixed natural types. John Ray applied one of the previously more general terms for fixed natural types, “species”, to plant and animal types, but he strictly identified each type of living thing as a species and proposed that each species could be defined by the features that perpetuated themselves generation after generation. The biological classification introduced by Carl Linnaeus in 1735 explicitly recognised the hierarchical nature of species relationships, but still viewed species as fixed according to a divine plan.

Other naturalists of this time speculated on the evolutionary change of species over time according to natural laws. In 1751, Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species. Georges-Louis Leclerc, Comte de Buffon suggested that species could degenerate into different organisms, and Erasmus Darwin proposed that all warm-blooded animals could have descended from a single microorganism (or “filament”). The first full-fledged evolutionary scheme was Jean-Baptiste Lamarck’s “transmutation” theory of 1809, which envisaged spontaneous generation continually producing simple forms of life that developed greater complexity in parallel lineages with an inherent progressive tendency, and postulated that on a local level, these lineages adapted to the environment by inheriting changes caused by their use or disuse in parents. (The latter process was later called Lamarckism.) These ideas were condemned by established naturalists as speculation lacking empirical support. In particular, Georges Cuvier insisted that species were unrelated and fixed, their similarities reflecting divine design for functional needs. In the meantime, John Ray’s ideas of benevolent design had been developed by William Paley into the Natural Theology or Evidences of the Existence and Attributes of the Deity (1802), which proposed complex adaptations as evidence of divine design and which was admired by Charles Darwin.

The crucial break from the concept of constant typological classes or types in biology came with the theory of evolution through natural selection, which was formulated by Charles Darwin in terms of variable populations. Darwin used the expression “descent with modification” rather than “evolution”. Partly influenced by An Essay on the Principle of Population (1798) by Thomas Robert Malthus, Darwin noted that population growth would lead to a “struggle for existence” in which favourable variations prevailed as others perished. In each generation, many offspring fail to survive to an age of reproduction because of limited resources. This could explain the diversity of plants and animals from a common ancestry through the working of natural laws in the same way for all types of organism. Darwin developed his theory of “natural selection” from 1838 onwards and was writing up his “big book” on the subject when Alfred Russel Wallace sent him a version of virtually the same theory in 1858. Their separate papers were presented together at an 1858 meeting of the Linnean Society of London. At the end of 1859, Darwin’s publication of his “abstract” as On the Origin of Species explained natural selection in detail and in a way that led to an increasingly wide acceptance of Darwin’s concepts of evolution at the expense of alternative theories. Thomas Henry Huxley applied Darwin’s ideas to humans, using paleontology and comparative anatomy to provide strong evidence that humans and apes shared a common ancestry. Some were disturbed by this since it implied that humans did not have a special place in the universe.

The mechanisms of reproductive heritability and the origin of new traits remained a mystery. Towards this end, Darwin developed his provisional theory of pangenesis. In 1865, Gregor Mendel reported that traits were inherited in a predictable manner through the independent assortment and segregation of elements (later known as genes). Mendel’s laws of inheritance eventually supplanted most of Darwin’s pangenesis theory. August Weismann made the important distinction between germ cells that give rise to gametes (such as sperm and egg cells) and the somatic cells of the body, demonstrating that heredity passes through the germ line only. Hugo de Vries connected Darwin’s pangenesis theory to Weismann’s germ/soma cell distinction and proposed that Darwin’s pangenes were concentrated in the cell nucleus and when expressed they could move into the cytoplasm to change the cell’s structure. De Vries was also one of the researchers who made Mendel’s work well known, believing that Mendelian traits corresponded to the transfer of heritable variations along the germline. To explain how new variants originate, de Vries developed a mutation theory that led to a temporary rift between those who accepted Darwinian evolution and biometricians who allied with de Vries. In the 1930s, pioneers in the field of population genetics, such as Ronald Fisher, Sewall Wright and J. B. S. Haldane set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin’s theory, genetic mutations, and Mendelian inheritance was thus reconciled.

  • levels of selection
  • evolution of individuality
  • evolution of development
  • origins of sexual reproduction
  • maintenance of sexual reproduction
  • genomic conflict and genomic imprinting
  • coevolution
  • sexual selection
  • sexual conflict
  • sex allocation
  • sex in hermaphrodites
  • cultural evolution

The Scripts will usually be uploaded here by Tuesday evening before each Wednesday lecture. And the Mandatory Reading is intended to be read after the lecture, to help digest the lecture, and questions about the reading can be asked and discussed at the start of follwing week's lecture.

Week Date Lecture (Wednesdays from 14:15-16:00) Script Mandatory Reading Suggested Reading
1 16.9. Brief Introduction to the Course &
Levels of Selection
Levels.pdf Reeve&Keller1999.pdf none
2 23.9. Evolution of Individuality Individuality.pdf fromBuss1987.pdf Buss1987.pdf
3 30.9. Evolution of Development Development.pdf Homework Cardona&al2005.pdf
4 7.10. Origin of Sexual Reproduction Origins.pdf MaynardSmith&Szathmary1995.pdf Hickey&Rose1988.pdf
5 14.10. Maintenance of Sexual Reproduction Maintenance.pdf West&al1999.pdf MaynardSmith&Szathmary1999.pdf
6 21.10. Genomic Conflict Genomic_Conflict.pdf fromBurt&Trivers2006.pdf Haig1996.pdf
7 28.10. Coevolution Coevolution.pdf none Janzen&al2010.pdf
8 4.11. Sexual Selection Sexual_Selection.pdf fromBirkhead2000.pdf
complete Birkhead 2000
9 11.11. Sexual Conflict Sexual_Conflict.pdf Homework Chapman&al2003.pdf
10 18.11. Sex Allocation Sex_Allocation.pdf Queller2006.pdf Munday&al2006.pdf
Fig wasp movie (87.6MB)
11 25.11. Sex in Simultaneous Hermaphrodites Sex_Hermaphrodites.pdf none Michiels1998.pdf
12 2.12. Cultural Evolution Cultural_Evolution.pdf none none
13 9.12. Dangerous Models Dangerous_Models.pdf none Tawfik2010.pdf
14 16.12. Examination

General Principles

Over the past fifty years, the conceptual framework of post-copulatory sexual selection has been well developed. When a female is polyandrous, there is a potential for the sperm of several males to compete for fertilizations of her eggs (sperm competition), and for the female to exert some choice over which sperm will fertilize her eggs (cryptic female choice). Parker 1970, a review of sperm competition in insects, stimulated the development of sperm competition theory. Eberhard 1996, a book on cryptic female choice, underscores the diversity of mechanisms that can allow polyandrous females to choose sperm from a particular male to fertilize their eggs and launched the field of cryptic female choice. Much discussion and experimentation has centered on how to disentangle sperm competition from cryptic female choice. Simmons, et al. 1996 provides a rare example where cryptic female choice can be eliminated as a factor influencing paternity, and therefore the role of sperm competition is clear. Birkhead and Pizzari 2002 provides a comprehensive examination of both processes. Ball and Parker 2003 uses game theory to model the actions of multiple mechanisms of post-copulatory selection: sperm competition, female use of sperms and sexual conflict. Birkhead and Kappeler 2004 compares post-copulatory sexual selection in birds and primates to explore commonalities and differences related to diverse life histories and mating systems. Eberhard 2009 examines how post-copulatory sexual selection has progressed over the past few decades and points out that several mechanisms of sperm competition and cryptic female choice were studied independently of sexual selection theory until recently. Birkhead 2010 dives into the history of the post-copulatory sexual selection and identifies major turning points in the field. Birkhead 2000 presents the main principles of post-copulatory sexual selection in a book written for the general public.

Ball, Michael A., and Geoff A. Parker. 2003. Sperm competition games: Sperm selection by females. Journal of Theoretical Biology 224.1: 27–42.

Game theory modeling of sperm allocation and potential evolutionary stable strategies that incorporate sperm competition, cryptic female choice, and sexual conflict. Two models are described in depth that focus on favored and unfavored males, in addition to the associated costs for each sex (e.g., reduced fecundity, reduced mating rates).

Birkhead, Tim. 2000. Promiscuity: An evolutionary history of sperm competition. Cambridge, MA: Harvard Univ. Press.

The topic of post-copulatory sexual selection is presented in a clear and well-organized manner that is easily accessible to the general public.

Birkhead, Tim R. 2010. How stupid to not have thought of that: Post-copulatory sexual selection. Journal of Zoology 281.2: 78–93.

The theory of post-copulatory sexual selection is reviewed, with historical and contemporary accounts of key pioneers in biology (e.g., Darwin, Parker, Eberhard), their major contributions relative to post-copulatory sexual selection, and important scientific events.

Birkhead, Tim R., and Peter M. Kappeler. 2004. Post-copulatory sexual selection in birds and primates. In Sexual selection in primates: New and comparative perspectives. Edited by Peter M. Kappeler and Carel P. van Schaik, 151–174. Cambridge, UK: Cambridge Univ. Press.

The mechanisms and consequences of post-copulatory sexual selection are reviewed in birds and primates, two groups that differ dramatically in their reproductive strategies and mating systems. Behavioral and morphological adaptations are compared. Convergent evolution is attributed to shaping similarities in post-copulatory sexual selection in these groups.

Birkhead, Tim R., and Tommaso Pizzari. 2002. Evolution of sex: Postcopulatory sexual selection. Nature Reviews Genetics 3.4: 262–273.

The theory and predictions of sperm competition and cryptic female choice (directional and nondirectional) are reviewed, with examples of how these mechanisms of post-copulatory sexual selection are mediated across a broad range of taxa, and how they often result in sexual conflict. The relationship between pre- and post-copulatory selection, the integration between different post-copulatory mechanisms, and the rate of evolutionary change are discussed as major unresolved issues. Suggested areas of future research include the use of genetic and molecular tools to examine the genetic basis of post-copulatory traits.

Eberhard, William G. 1996. Female control: Sexual selection by cryptic female choice. Princeton, NJ: Princeton Univ. Press.

This seminal book was a catalyst for interest in cryptic female choice that uses a wide range of taxa (especially insects) to illustrate different mechanisms by which females can bias paternity during and after copulation. The importance of cryptic female choice in the process of sexual selection is underscored, with a multitude of examples to demonstrate the likely prevalence of the mechanism.

Eberhard, William G. 2009. Postcopulatory sexual selection: Darwin’s omission and its consequences. Proceedings of the National Academy of Sciences of the United States of America 106.S1: 10025–10032.

Post-copulatory sexual selection (sperm competition, cryptic female choice, and sexually antagonistic coevolution) is reviewed. Unusual and rapidly divergent phenomena (e.g., the elaborate structure of male genitalia), the physiological effects of seminal fluids on female reproductive processes, the occurrence of courtship behavior after initiation of copulation, and the complexity of interactions between gametes in plants are highlighted.

Parker, Geoff A. 1970. Sperm competition and its evolutionary consequences in the insects. Biological Reviews 45.4: 525–567.

This seminal paper launched the development of the field of post-copulatory sexual selection, particularly sperm competition. Parker predicted that when females mate with different males, each male would benefit by simultaneously eliminating rival sperm from previous female matings, as well as from evolving traits to protect their own sperm from such elimination by rivals. An examination of the outcome of multiple mating in several insects supported the importance of sperm competition.

Simmons, Leigh W., Paula Stockley, Rebecca L. Jackson, and Geoff A. Parker. 1996. Sperm competition or sperm selection: No evidence for female influence over paternity in yellow dung flies Scatophaga stercoraria. Behavioral Ecology and Sociobiology 38:199–206.

It can be challenging to disentangle the roles of sperm competition and cryptic female choice, but an experimental approach in yellow dung flies demonstrated that despite an apparent advantage of large size on paternity, controlling for body size on sperm transfer and displacement eliminated the large male advantage. Therefore, traits under sperm competition influenced paternity more than an apparent female preference for large body size. The importance of controlling for male effects before drawing conclusions about cryptic female choice is underscored.

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