Questions: Genetic Drift and Random Change in Small Populations

Question 1

A population of 10,000 individuals is reduced to 20 by a catastrophic event, then recovers to 10,000 over the next century. Which statement best describes the genetic outcome?

• A. The population recovers its original genetic diversity quickly as numbers rebound
• B. The bottleneck causes permanent loss of alleles that existed only in the individuals who died
• C. Natural selection acts more strongly during the bottleneck because the population is small
• D. Genetic drift has no lasting effect because the final population size is the same as the original

Answer: (B)

Alleles carried only by individuals who did not survive are permanently lost — no amount of subsequent population growth can recreate them. Genetic diversity is determined by which alleles passed through the bottleneck, not by the recovery in numbers. Selection actually becomes less effective (not more) in small populations because drift dominates.

Question 2

In a large population, a neutral mutation (no fitness effect) will almost certainly be lost to drift before it spreads.

• T. True
• F. False

Answer: True

In any finite population, a new neutral mutation starts at a frequency of 1/(2N). The probability that it fixes (reaches 100%) is equal to its starting frequency — 1/(2N) — which is very small in a large population. The vast majority of neutral mutations are lost by chance. This is why effective population size strongly shapes how much neutral variation persists.

Question 3

How does a founder effect differ from a population bottleneck, and what do they share in common?

Model answer: A founder effect occurs when a small group colonizes a new habitat, establishing a population from just a few individuals. A bottleneck is a drastic reduction in an existing population's size. Both result in a population rebuilt from a small sample of the original gene pool, causing loss of alleles, reduced heterozygosity, and accelerated genetic drift.

The key shared mechanism is sampling error: a small subset of individuals cannot carry all the alleles present in the original population. The difference is context — founder events involve geographic dispersal and isolation, while bottlenecks involve mortality or other reductions in place. Both leave lasting genetic signatures in the surviving lineage.