Allele Frequencies in a Fragmented Population
A population of 500 beetles lives in a forest fragment. A dominant allele A confers dark color; aa is light. Sampling shows 180 dark and 320 light beetles. The population is assumed to be in Hardy–Weinberg equilibrium for this locus. A road is built, splitting the fragment; after 10 generations, the two subpopulations (250 each) are sampled. Subpopulation 1 has 25% light beetles; Subpopulation 2 has 64% light beetles. No migration occurs between them, and mutation is negligible.
1. Calculate the allele frequencies in the original population and check whether the dark phenotype count is consistent with Hardy–Weinberg.
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Light (aa) frequency q² = 320/500 = 0.64, so q = 0.8, p = 0.2. Expected dark = p² + 2pq = 0.04 + 0.32 = 0.36 → 0.36 × 500 = 180. Observed dark = 180. Consistent with HWE. Allele frequencies: A = 0.2, a = 0.8.
2. Calculate p and q in each subpopulation after fragmentation and determine whether allele frequencies changed.
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Subpop 1: q² = 0.25 → q = 0.5, p = 0.5. Subpop 2: q² = 0.64 → q = 0.8, p = 0.2. Original: p = 0.2, q = 0.8. Subpop 2 matches the original; Subpop 1 has shifted markedly (p rose from 0.2 to 0.5). So allele frequencies changed in Subpop 1 but not Subpop 2, indicating that fragmentation plus genetic drift (small population, 250 individuals) altered frequencies in Subpop 1.
3. Explain the mechanism most likely responsible and predict the long-term consequence for the two subpopulations.
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With no migration and small effective population sizes, genetic drift is the likely cause: random sampling of alleles across generations can shift frequencies, especially in the smaller fragment. Subpop 1's shift toward higher A frequency is consistent with drift (or selection, but drift is the simplest given no migration and small N). Long term, the two subpopulations diverge genetically; reduced gene flow limits homogenization, and if drift continues, alleles may become fixed differently in each fragment, reducing overall genetic diversity and potentially increasing extinction risk in the smaller fragment.