What is a genetic bottleneck and its typical effect on genetic variation?

• A. An increase in population size that increases diversity
• B. A sharp reduction in population size that decreases genetic diversity and increases genetic drift
• C. A scenario where mutation rate skyrockets causing diversity
• D. A stable population with no changes

Answer: (B)

A genetic bottleneck is a drastic drop in population size that reduces genetic variation because only a small subset of the original gene pool survives and reproduces. In such small populations, random sampling of alleles during reproduction causes allele frequencies to shift by chance, a process called genetic drift. With fewer alleles present, overall genetic diversity declines, and drift becomes a stronger force, so allele frequencies can change rapidly and diversity can be lost. This combination—rapid size reduction leading to loss of variation and heightened drift—captures the typical effect of a bottleneck. The other scenarios described (a population growing larger, a sudden spike in mutation rate, or no change) do not describe the bottleneck situation and its consequences.