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Genetic Drift
No external factors are involved in genetic drift, which is a chance occurrence. However, a population of infinite size cannot occur Hardy-Weinberg Equilibrium so genetic drift is possible.
The size of the population is inversely proportionate to the gene frequency changes due to genetic drift and thus, a larger number of individuals reproducing results in small genetic drift effects. This relationship is similar in populations though the actual number of individuals is not the important factor here. "Effective" population size refers to the number of individuals producing offspring.
The effects of genetic drift
are only pronounced over several generations, which
consist of thousands of reproducing individuals and if the other
process of evolution are missing, fixed populations would occur.
Genetic drift causes an allele to have a precise frequency (e.g. an
allele with frequency of 0.9 would definitely have a 90% chance of
being present in the next generation).
An experiment conducted with eight individual fruit flies resulted
in the discovery of the fact that as generations past, the allele frequencies will
gradually decrease as is seen in this example (where the initial
frequency was 0.5 and the final frequency was 0.23.)
Two types of genetic drift commonly observed include the founder effect and the bottleneck. The founder effect occurs when allele frequencies in a group of migrants differ from their population of origin. These individuals may develop distinct different traits if isolated. This results in species in neighbouring territories being more heterogeneous than species in adjacent territories.
Another type of special
genetic drift is the bottleneck, which occurs when a dramatic
decrease in population size occurs. A number of reasons including
climatic, ecological changes and natural disasters could be
responsible. A recovery of the original population size is possible
though the frequencies of the alleles have been considerably
altered, possibly affecting future generations of the species. An
ideal condition for the occurrence of bottleneck is the large
organism sizes due to the fact that typically such populations
consist of fewer individuals. Persistent bottlenecks, however, may
result in a decrease in the genetic variation thus altering the
chance of future evolution of that species.