Genetic drift: chance changes in populations — Biology, 14–17 years
Genetic drift changes how common gene versions are because of chance. It is strongest in small populations, where a few births, deaths, or migrants can have a large effect.
Idea
Genetic drift is a change in a population caused by chance, not by which trait works best. Each generation carries only a sample of the previous generation’s gene versions. In a small population, that sample can be very uneven, so one version may become common or disappear.
Why it matters
Natural selection is not the only process that changes populations. A storm, an accident, or simply unusual reproductive luck can alter gene frequencies even when traits have no survival advantage. This idea explains why small, isolated populations often lose genetic diversity and may become vulnerable to new changes.
Worked example
A small island has 10 beetles: 6 carry gene version A and 4 carry version B. A bird accidentally eats three beetles, all carrying A. The survivors are 3 with A and 4 with B, so A has fallen from 60% to about 43%. Nothing made B better; chance changed the sample.
Common trap
People often describe every change in gene frequency as natural selection. That seems reasonable because selection is a familiar explanation for evolution. Drift is different: a gene version can spread or disappear even when it has no effect on survival, simply because the individuals reproducing were a chance group.
Outside school
Conservation scientists use genetic drift to understand endangered species. If only a few animals remain, chance can remove gene versions that might be useful later, so protecting a larger, connected population matters. The same reasoning helps explain why isolated island populations can differ from mainland relatives.
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