Which mechanism causes allele frequencies to change by chance, potentially reducing variation in small populations?

• A. Genetic drift
• B. Gene flow
• C. Natural selection
• D. Mutation

Answer: (A)

Genetic drift is the random fluctuation of allele frequencies from one generation to the next due to chance sampling of alleles during reproduction. In small populations, this sampling error is larger, so some alleles can be passed on more by luck and others can be lost entirely. Over time, these random changes can lead to the fixation of certain alleles and the loss of others, reducing genetic variation within the population. This contrast with other mechanisms: gene flow changes frequencies through the introduction or removal of alleles by migration, natural selection changes frequencies based on differential reproductive success, and mutation introduces new alleles but typically at a slower, gradual rate. Examples like bottlenecks or founder effects illustrate how drift can dramatically reduce variation when a small group establishes or a population size sharply decreases.