The genetic architecture of stickleback divergence commonly involves major-effect loci that contribute to large phenotypic differences.

• A. False
• B. Rarely
• C. True
• D. Not at all

Answer: (C)

Adaptation in sticklebacks is often driven by a small number of large-effect genetic changes, which lets populations diverge rapidly in phenotype even when there is gene flow between them. This pattern shows up repeatedly across marine and freshwater stickleback populations. A classic example is the Eda locus, where a regulatory change leads to fewer armor plates in freshwater fish, producing a striking and consistent difference. Another well-documented major locus is Pitx1, where a regulatory deletion reduces pelvic girdle development, contributing to body-plan differences between ecotypes. Because these single genetic changes can produce substantial phenotypic shifts, major-effect loci are commonly involved in stickleback divergence. While many traits also have contributions from multiple small-effect loci, the observed trend supports the statement as true.