Questions: Genetic Drift

Question 1

Which of the following best describes the mechanism of genetic drift?

• A. Natural selection favoring adaptive traits in small populations
• B. Random sampling error in allele transmission between generations
• C. Directed mutations accumulating in isolated populations
• D. Gene flow introducing new alleles from neighboring populations

Answer: (B)

Genetic drift is a statistical sampling phenomenon — each generation, by chance, some alleles are over- or under-represented in offspring relative to their frequency in the parents. It is not driven by fitness (that is natural selection), does not create new variants (that is mutation), and is not caused by migration (that is gene flow).

Question 2

Genetic drift will generally push a population toward better-adapted alleles over time, because random chance tends to eliminate harmful variants.

• T. True
• F. False

Answer: False

Drift is fitness-blind — it operates on allele frequencies by chance alone, regardless of whether an allele is beneficial, neutral, or harmful. A deleterious allele can increase in frequency or even reach fixation purely by chance, especially in small populations. Evolution through drift does not trend toward improvement.

Question 3

Why does genetic drift have a stronger effect on small populations than on large ones?

Model answer: In small populations, each individual represents a larger fraction of the gene pool, so chance events in reproduction cause larger swings in allele frequency. In large populations, random fluctuations tend to cancel out, keeping allele frequencies closer to expected values — the same reason a coin flipped 10 times can easily give 8 heads, but 10,000 flips reliably give near 50%.

This is the law of large numbers applied to population genetics. Genetic drift is sampling error, and sampling error decreases as sample size increases. The bottleneck effect and founder effect are powerful precisely because they drastically reduce effective population size, magnifying drift.