What is genetic drift and how can it affect allele frequencies in small populations?

• A. Random fluctuations in allele frequencies due to chance; effects are stronger in small populations and can lead to fixation or loss of alleles.
• B. Systematic selection of advantageous alleles.
• C. Gene flow from migration only.
• D. Mutation is the sole driver.

Answer: (A)

Genetic drift is about random changes in how common different alleles are from one generation to the next, caused by chance rather than by how advantageous an allele is. In small populations, these random sampling effects are large, so allele frequencies can shift a lot simply by luck. Over time, this can lead to one allele becoming fixed (everyone carries it) or being lost from the population, which also reduces genetic variation.

This isn’t about natural selection, where advantageous traits spread because they improve survival or reproduction. Nor is it only about migration bringing new alleles in or out; drift can happen with or without movement between populations, and it particularly stands out in small groups. It’s not driven solely by mutation—the new alleles mutation creates matter, but drift describes how existing alleles change in frequency due to randomness.