Random changes in the gene frequencies of a population from generation to generation. This happens as a result of sampling error — some genotypes just happen to reproduce more than other genotypes, not because they are “better,” but just because they got lucky. This process causes gene frequencies in a population to drift around over time. Some genes may even “drift out” of a population (i.e., just by chance, some gene may reach a frequency of zero). In general, genetic drift has the effect of decreasing genetic variation within a population. For a more detailed explanation, see our resource on genetic drift in Evolution 101.
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