Genetic drift

Genetic drift is change in the frequency of alleles in a population due purely to chance. In every generation, some individuals happen to leave more offspring than others, not because their genes are better but through luck, such as where they were when a flood struck or which sperm and egg happened to meet. Each new generation's gene pool is therefore a random sample of the last, and allele frequencies wander unpredictably.

Drift affects all populations but is strongest in small ones, where each individual makes up a larger share of the gene pool; losing one member of a group of ten removes a tenth of its genes. Over time drift tends to reduce genetic variation, as some alleles are lost entirely and others become fixed, meaning every individual carries them. Two special cases matter. In a bottleneck, a disaster kills most of a population and the survivors carry only part of the original diversity. In the founder effect, a small group starts a new population that inherits whatever alleles its founders happened to carry, which is why Huntington's disease is unusually common among Afrikaners in South Africa.

Unlike natural selection, drift is random and does not produce adaptations; it can even spread harmful alleles.