Genetic drift

Genetic drift is the random change in allele frequencies across generations caused by chance differences in which individuals reproduce. Its effects are strongest in small populations, where it can eliminate alleles and reduce genetic variation.

Wright–Fisher model: A population-genetic model in which each generation is sampled randomly from the previous generation’s gene pool. It formalizes drift as random sampling of alleles between generations.
Founder effect: A shift in allele frequencies when a new population is established by a small number of individuals. The founders’ chance allele sample can differ sharply from the source population.
Natural selection: A process in which heritable traits that improve reproductive success become more common across generations. Selection changes frequencies nonrandomly with respect to reproductive success, unlike drift.
Pingelapese achromatopsia: A rare inherited condition causing near-total color blindness, unusually common on the Micronesian atoll of Pingelap. A population bottleneck and subsequent isolation helped raise the frequency of its causal allele.
Effective population size: The size of an idealized population experiencing the same strength of genetic drift as a real population. It predicts drift more accurately than a population’s head count.
Inbreeding depression: Reduced survival or reproduction caused by mating between relatives and increased expression of harmful recessive alleles. Drift can raise harmful alleles to high frequency in small populations, increasing genetic risks.
Gene flow: The movement of alleles between populations through migration and reproduction. Migration changes frequencies by importing or exporting alleles rather than by sampling chance alone.
Cheetah genetic diversity: The low genetic variation found in cheetahs, associated with historical population contractions. Cheetahs illustrate how severe declines can leave populations with reduced genetic variation.
Allele frequency: The proportion of copies of a gene in a population that carry a particular allele. Drift is measured as random change in this quantity.
Genetic load: The reduction in population fitness caused by harmful genetic variants. Random frequency shifts can increase the contribution of deleterious alleles to this burden.

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