Hardy-Weinberg in a fragmented population
A beetle species lives in two habitat patches. Patch A has 900 individuals and patch B has 100. In patch A, the frequency of the recessive allele a is 0.2. In patch B, the frequency of a is 0.6. A road is built between the patches, reducing migration to near zero. Before the road, migration was 10% per generation from B to A. After 20 generations of isolation, patch B has only 60 individuals and shows reduced heterozygosity. A student claims the populations will now evolve identically because selection is the same in both patches.
1. Evaluate the student's claim.
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The claim is wrong. Even with identical selection, the populations differ in allele frequency (a = 0.2 vs 0.6) and in size. Patch B is small (60 individuals), so genetic drift is strong and can change allele frequencies rapidly and unpredictably, and it can fix or lose alleles. Patch A is large (900), so drift is weak and allele frequencies change slowly. Reduced migration means gene flow no longer homogenises the patches. Therefore the populations will diverge, not evolve identically. The student confuses identical selection with identical evolutionary outcomes; drift, initial frequency, and population size also matter.
2. Explain why patch B shows reduced heterozygosity and predict the effect on future variation.
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Reduced heterozygosity is expected in a small isolated population because genetic drift removes alleles by chance, and inbreeding increases homozygosity. With only 60 individuals, the effective population size is small, so the rate of loss of heterozygosity per generation is roughly 1/(2Ne). Isolation also prevents immigration of new alleles from patch A. Over time, patch B will lose genetic variation, reducing its ability to respond to environmental change. If the population remains small, inbreeding depression may reduce fitness. The prediction is continued decline in heterozygosity and increased extinction risk unless migration is restored or the population grows.