Questions: Population Bottlenecks: Drift, Inbreeding, and Recovery

Question 1

A population of 10,000 individuals is reduced to 50 for a single generation due to a disease outbreak, then recovers to 8,000 over the following decades. Compared to a population that was never bottlenecked, how much genetic diversity has been lost?

• A. Very little — the population recovered to near its original size, so diversity should be largely restored
• B. A small amount — only the rarest alleles would be lost in a single-generation bottleneck
• C. Substantial diversity — the effective population size during the bottleneck (Ne ≈ 50) drives severe allele loss regardless of recovery
• D. All diversity — a single-generation bottleneck through 50 individuals eliminates nearly all variation

Answer: (C)

It is the effective population size *during* the bottleneck, not before or after, that determines diversity loss. With Ne ≈ 50 for one generation, the founder sample carries only a random subset of the original variation: rare alleles (present in only a few individuals) are almost certainly lost, and common alleles may shift dramatically in frequency. Even after recovering to 8,000, that diversity cannot be recovered quickly — it requires new mutations accumulating over thousands of generations. Option A is the classic misconception: population size recovery does not restore genetic diversity.

Question 2

Two bottlenecked populations both recover to 10,000 individuals. Population X passed through a bottleneck of 20 individuals for 5 generations; Population Y passed through a bottleneck of 200 individuals for 1 generation. Which is expected to retain more genetic diversity?

• A. Population X — the longer bottleneck allowed more time for new mutations to accumulate
• B. Population Y — the larger bottleneck size and shorter duration mean less allele loss due to drift
• C. They will retain equal diversity — census size at recovery is what matters
• D. Population X — smaller populations evolve faster, generating more variation

Answer: (B)

Population Y experienced less severe drift: higher Ne (200 vs 20) and fewer generations of small size means the random allele losses from sampling were less severe. The harmonic mean of Ne across generations determines cumulative diversity loss, and even a single generation of very small Ne can be devastating. Population X's smaller Ne (20) means stronger drift each generation, and 5 generations of this compounded the losses. Option C is the error: it is Ne *during* the bottleneck, not after recovery, that determines diversity retained.

Question 3

A species that was bottlenecked but has since recovered to its pre-bottleneck census size will typically have restored most of its pre-bottleneck genetic diversity within a few generations.

• T. True
• F. False

Answer: False

Recovery of genetic diversity is not driven by population size recovery — it depends on the slow accumulation of new mutations, which occurs at a rate of roughly 10⁻⁸ per base pair per generation in most organisms. Restoring diversity across thousands of loci takes thousands of generations even in large populations. Northern elephant seals recovered to over 100,000 individuals from fewer than 30 in the 1890s, yet they still show dramatically reduced genetic variation compared to non-bottlenecked southern elephant seals over a century later. Population size recovery restores demographic viability but not genetic diversity.

Question 4

During a population bottleneck, rare alleles are disproportionately likely to be lost compared to common alleles.

• T. True
• F. False

Answer: True

A bottleneck samples a small number of individuals from the larger population. Rare alleles — those present in only a handful of individuals — are statistically unlikely to be represented in that small sample. Common alleles, by contrast, are present in many individuals and are much more likely to appear in any random subset. This is why bottlenecks consistently erode allelic diversity (the number of alleles) more severely than heterozygosity (the frequency of the most common alleles), and why rare-allele loss is one of the first genetic signatures of a past bottleneck.

Question 5

Why do biologists argue that preventing population bottlenecks is far more effective for conservation than trying to restore genetic diversity after one has occurred?

Model answer: Recovery of genetic diversity after a bottleneck depends almost entirely on new mutations — a process that occurs at roughly 10⁻⁸ per base pair per generation and requires thousands of generations to meaningfully restore diversity across the genome. Once alleles are lost from a population, they cannot be recovered except through mutation (or immigration from another population). In contrast, maintaining a large effective population size preserves existing variation indefinitely because drift is weak and allele losses are rare. The asymmetry is stark: diversity is lost in a single catastrophic generation but can only be restored over geological timescales.

This asymmetry explains why conservation geneticists prioritize habitat preservation and maintaining connectivity between populations above all other interventions. Interventions after a bottleneck — captive breeding, translocation — can help, but they primarily restore census size, not genetic diversity. The cheetah example illustrates the permanence of bottleneck effects: 10,000 years after their severe bottleneck, cheetahs still show near-zero immunological diversity, and no amount of population growth has changed this.