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Genetic drift is the process of change in the gene frequencies of a population due to chance events. It refers to the random fluctuations in the frequency of the appearance of a gene in a small isolated population, presumably owing to chance rather than natural selection.
Genetic drift is a random statistical effect and can occur only in small, isolated populations in which the gene pool is small enough that chance events can change its makeup substantially. It is a stochastic effect that arises from the role of random sampling in the production of offspring.
The individual changes are most often small or gradual and genetic drift is most often slow. But in the long run it can constitute a large part of the changes in a gene pool. Genetic drift also often cause alleles to disappear completely. The changes due to genetic drift are not driven by environmental or adaptive pressures, and may be beneficial, neutral, or detrimental to reproductive success.
The effect of genetic drift is larger in small populations, and smaller in large populations. Accidents in small populations can change the frequency of some genes in a population, even if they give no survival advantage. Thus genetic drift provides diversity without any adaptations.
Genetic drift can result in genetic traits being lost from a population or becoming widespread in a population without respect to the survival or reproductive value of the alleles involved.
Genetic drift—along with natural selection, mutation, and migration—is one of the basic mechanisms of evolution. Genetic drift affects the genetic makeup of the population but, unlike natural selection, through an entirely random process. So although genetic drift is a mechanism of evolution, it doesn’t work to produce adaptations.
Genetic drift reduces the amount of genetic variation in a population. And with less genetic variation, there is less for natural selection to work with. All populations experience drift, but the smaller the population is, the sooner drift will have a drastic effect. This may be a big problem for endangered species that have low population sizes.