Gendrift is the random change in the genotype of a particular allele within a population. Especially in small populations, gene drift is important because alleles can disappear from the gene pool of the population relatively quickly. But the exact opposite is also possible, namely that certain genes suddenly appear extremely frequently in a population, for example after natural disasters, when only a few individuals have survived and many of them carry a previously rare allele in itself.
Example for gendrift - original effect
In the basic effect, a new population is established by a few individuals. Within this new founder population (P2), the allele frequency (allele frequency) differs from the now isolated (there is no more gene flow between P1 and P2) starting population (P1). This results in significantly lower gene variability because some of the alleles that appear in P1 are not present in any of the individuals of P2. Therefore, the gene pool of P2 consists of a completely different allele composition than P1.
Necessary condition of the initial effect is isolation from the initial population, for example by geographical isolation (see Allopatric speciation)
Example for Gendrift - bottleneck effect
The bottleneck effect describes the strong reduction of genetic variability in connection with the randomized change of allelic frequencies. The bottleneck effect is usually based on an event, such as a natural disaster in which many individuals are killed, sudden geographic isolation by plate tectonics, but also e.g. the drifting of individuals on a still unpopulated island. In all cases, the random selection of individuals for the bottleneck effect is crucial, regardless of their adaptation to environmental factors.