A population of ground beetles lived on a continuous prairie. After a highway and wide gravel median were built, beetles on opposite sides rarely crossed. Ten years later, allele frequencies at multiple loci differ between sides. In lab trials, beetles from the same side mate readily, but cross-side pairings show frequent courtship failure even when placed together. No differences in survival are detected when larvae from both sides are raised under identical conditions. Which process most directly led to the observed divergence and reduced mating between the two populations?
- Polyploidy producing instant reproductive isolation within a single generation
- Habitat fragmentation reducing gene flow, allowing divergence and behavioral isolation (correct answer)
- Individuals adapting to the gravel median and then choosing new mates
- Seasonal differences in flowering time preventing gamete transfer
- Hybrid offspring sterility caused by chromosomal nondisjunction in meiosis
Explanation: This question tests your ability to analyze speciation mechanisms, specifically how geographic barriers lead to reproductive isolation. The highway created allopatric separation, reducing gene flow between beetle populations on opposite sides, which allowed genetic divergence through drift and/or local selection pressures. Over time, this divergence led to behavioral differences that now prevent successful mating even when beetles are brought together—a prezygotic reproductive barrier. Choice C incorrectly suggests individual adaptation and mate choice, implying a Lamarckian view where individuals change and then choose mates, rather than population-level evolutionary change. When analyzing speciation scenarios, identify whether populations are physically separated (allopatric) or together (sympatric), then trace how reduced gene flow enables divergence and eventual reproductive barriers.