Why is genetic drift more pronounced in small populations?

• A. Because larger populations have more rapid allele frequency changes due to sampling.
• B. Because natural selection is always stronger in small populations.
• C. Because mutation rates are higher in small populations.
• D. Because random sampling can cause larger shifts in allele frequencies when only a few individuals contribute to the next generation.

Answer: (D)

Genetic drift is the random fluctuation of allele frequencies from one generation to the next because of chance sampling of gametes. In small populations, only a tiny subset of individuals contributes to the next generation, so those random sampling effects are amplified. A few alleles can be overrepresented by luck or lost entirely simply because so few copies are available to be passed on. That leads to bigger shifts in allele frequencies and quicker fixation or loss of alleles purely by chance. In large populations, many more individuals contribute, so random fluctuations tend to cancel out, and allele frequencies stay more stable over time. The magnitude of drift effectively scales with 1 divided by population size, so it's stronger when populations are small.

For context, this isn’t about natural selection becoming stronger in small populations—selection depends on fitness differences, and drift can overwhelm it when the population is tiny. Mutation rates per generation are about the same regardless of population size, so they don’t explain why drift is more pronounced in small groups.