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Natural selection—probably the most well-known evolutionary mechanism—increases the prevalence of traits that enhance survival and reproduction. However, evolution does not merely propagate favorable traits, nor does it always benefit populations.

Life is not fair. A deer grazing contentedly in a field can have her meal cut tragically short by a bolt of lightning. If the doomed doe is one of only three in the population, 1/3 of the population’s gene pool is lost. Random events like this can indelibly affect a population, sometimes for generations. This evolutionary mechanism is called genetic drift.

Genetic drift is a shift in population allele frequencies due to chance events. Alleles are variations of a gene, and their frequency is the portion, or percentage, of the population with that allele. Genetic drift can alter the frequencies of advantageous, neutral, and harmful alleles alike.

Genetic drift does not dramatically impact sufficiently large populations; this is because it does not occur in isolation, but alongside other evolutionary mechanisms, like natural selection. In large populations, many individuals can be lost, and the remaining gene pool is still diverse enough for natural selection to act.

However, genetic drift can sharply reduce genetic diversity in small populations, creating a sampling error. A sampling error occurs when a sample is not representative of the population from which it is derived. When part of a population is eliminated, the remaining members may represent only a fraction of the original population’s genetic diversity. Larger samples are typically more representative, which is why scientists maximize sample size for their experiments.

Two extreme examples of genetic drift are the bottleneck effect—caused by catastrophic events, like natural disasters—and the founder effect, a result of colonization. In both cases, smaller populations derived from larger ones create a sampling error that leads to evolution, sometimes from less-than-favorable traits.

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