Genetic drift

Genetic drift is a contributing factor in biological evolution, in which traits which do not affect reproductive fitness change in a population over time. Whereas natural selection causes traits to become more prevalent when they contribute to fitness, or eliminates those which harm it, genetic drift is a somewhat random process which affects traits that are more neutral.

Drift versus selection

Genetic drift and natural selection rarely occur in isolation of each other; both forces are always at play in a population. However, the degree to which alleles are affected by drift and selection varies according to circumstance.

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In a large population, where genetic drift occurs very slowly, even weak selection on an allele will push its frequency upwards or downwards (depending on whether the allele is beneficial or harmful). However, if the population is very small, drift will predominate. In this case, weak selective effects may not be seen at all as the small changes in frequency they would produce are overshadowed by drift.

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