A short-term event that drastically reduces population size, leading to loss of genetic variation.

• A. Founder effect
• B. Gene flow
• C. Genetic drift
• D. Genetic bottleneck

Answer: (D)

A bottleneck is a drastic, often sudden drop in population size due to a catastrophe or severe event. When only a small group survives, the new gene pool may not reflect the original genetic diversity, so many alleles can be lost by chance. This leaves the population with reduced genetic variation and makes future evolution more driven by random genetic drift. The other options describe different ideas: a founder effect involves establishing a new population from a few individuals, not a collapse of an existing one; genetic drift is the random change in allele frequencies that happens in small populations, not the event itself; gene flow is the movement of alleles between populations, which tends to increase variation rather than reduce it.