Stickleback adaptive radiation shows major-effect loci contributing to phenotypic divergence.

• A. True
• B. False
• C. Only in freshwater populations
• D. It depends on the trait

Answer: (A)

The main idea is that a few genes with strong effects can drive the big differences we see between stickleback populations after they colonize new environments. In sticklebacks, many traits that separate marine and freshwater forms are controlled by major-effect loci, so a single regulatory change can shift an entire phenotype quickly. For example, the Eda gene controls how many lateral plates the fish have; freshwater forms often carry a version that reduces plate number, a change favored by the lighter armor in less predator-rich, inland habitats. Another key locus, near Pitx1, influences pelvic spine development, with freshwater populations frequently showing reduced spines. Because these changes come from regulatory shifts rather than sweeping changes across many genes, they can produce large, coordinated phenotypic shifts across populations and do so repeatedly in different lakes. So, major-effect loci contributing to phenotypic divergence is indeed what we observe in stickleback adaptive radiations.