What evolutionary process explains repeated, independent reductions in lateral plates across freshwater sticklebacks?

• A. Genetic drift
• B. Bottleneck
• C. Parallel evolution
• D. Horizontal gene transfer

Answer: (C)

Parallel evolution explains this pattern: independent populations repeatedly evolve the same trait because they face similar environmental pressures. Freshwater habitats select for fewer lateral plates in sticklebacks, so multiple populations independently converge on the same reduced-plate phenotype. Often, the same genetic changes, such as in the Eda pathway, underlie these similar adaptations across different lineages. This differs from genetic drift, which acts randomly and targets lineages unpredictably, and from bottlenecks, which are about reduced diversity in a single population. Horizontal gene transfer isn’t a driver in these vertebrates. So the repeated, independent reductions are best explained by parallel evolution.