Population Genetics After a Bottleneck
A population of 500 lizards lives on an island. A hurricane reduces the population to 40 breeding adults, all descended from a small subset of the original group. Twenty years later, researchers sample 200 lizards and find that the frequency of a recessive allele causing reduced camouflage is 0.35, compared with 0.10 before the hurricane. The island has no immigration. A student claims natural selection must have favored the reduced-camouflage allele because its frequency rose. Another student says the change is proof the population is evolving adaptively.
1. Explain how the bottleneck could change allele frequencies without natural selection favoring the allele.
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When the population crashes to 40 breeders, the survivors carry only a sample of the original alleles. By chance, the recessive allele may be overrepresented among survivors simply because the founding group was small-a founder effect following a bottleneck. Genetic drift in a small population can then shift allele frequencies rapidly across generations, independent of fitness. The rise from 0.10 to 0.35 is consistent with drift rather than selection, especially if the reduced-camouflage phenotype is not obviously advantageous. The judgement is to recognize that a frequency change is evolution, but not necessarily adaptive evolution; drift is a mechanism that can produce the same direction of change without improving survival.
2. What evidence would help distinguish drift from selection in this population?
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Selection would predict a consistent fitness disadvantage for the other phenotype-for example, higher predation on well-camouflaged lizards or differential survival measured directly. Drift would predict random fluctuation across generations and possibly different allele frequencies in isolated subpopulations, with no consistent fitness difference. Comparing allele frequencies before and after the bottleneck, measuring survival by phenotype, and checking whether the change is directionally consistent with predation would help. A control population unaffected by the hurricane would also show whether the shift is unusual. The reasoning is to test the mechanism, not just observe that the frequency changed.