Genetic Drift

Genetic drift is the change in the frequency of an existing allele within a population that occurs purely by chance rather than through any directional pressure such as natural selection. It arises from the random sampling of which alleles get passed to the next generation, an effect that is strongest in small populations, where chance can shift allele frequencies far more dramatically per generation than in large ones. Because sampling is random, drift can drive an allele to complete fixation or to total loss regardless of whether it helps or harms the organisms that carry it, and over time it tends to reduce a population's genetic variation.