How Does Genetic Drift Affect Populations?
Learn how genetic drift changes allele frequencies by chance, why its effects are strongest in small populations, and how it differs from natural selection.
Genetic drift changes the frequency of alleles in a population by chance. It has a stronger effect in small populations, where an allele may become common or disappear simply because of which individuals survive and reproduce. Over generations, this can reduce genetic diversity.
Why population size matters
An allele is a version of a gene. Its frequency is the proportion of copies of that gene in a population that are that version. If a population is large, the chance loss of a few individuals usually has little effect on allele frequencies. In a small population, the same loss can make a substantial difference.
For example, imagine a small population in which an allele is carried by only two individuals. If neither reproduces, the allele could disappear from the population, even if it is not harmful. Alternatively, chance could cause an allele to spread until it is the only version left. This is called fixation.
Two small populations can start with similar allele frequencies but end up with different frequencies because different individuals happen to reproduce. An allele lost from one population might remain in the other. Genetic drift can therefore change populations without necessarily making their members better adapted to their environments.
Drift versus natural selection
Genetic drift does not favour an allele because it is useful. Its effect is random with respect to whether an allele helps an organism survive or reproduce. Natural selection, by contrast, can increase the frequency of an advantageous allele because its carriers tend to have greater reproductive success.
For an exam answer, link the steps clearly: chance differences in survival or reproduction change allele frequencies; the effect is greater in small populations; alleles may be lost, reducing genetic diversity. The common mistake is to say that an allele spreads because the population needs it. Genetic drift has no goal.
