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→Genetic drift
===Genetic drift===
===Genetic drift===
Genetic drift is the change in allele frequency from one generation to the next that occurs because alleles in the offspring generation are a [[sampling (statistics)|random sample]] of those in the parent generation, and are thus subject to [[sampling error]]. As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward in a [[random walk]]. This drift halts when an allele eventually becomes [[Fixation (population genetics)|fixed]], either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone, and two separate populations that began with the same genetic structure can drift apart by random fluctuation into two divergent populations with different sets of alleles. The time for an allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.[http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=9178020]
Genetic drift is the change in allele frequency from one generation to the next that occurs because alleles in the offspring generation are a [[sampling (statistics)|random sample]] of those in the parent generation, and are thus subject to [[sampling error]]. As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward in a [[random walk]]. This drift halts when an allele eventually becomes [[Fixation (population genetics)|fixed]], either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone, and two separate populations that began with the same genetic structure can drift apart by random fluctuation into two divergent populations with different sets of alleles. The time for an allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.[http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=9178020]