Which statement best summarizes the genetic architecture of stickleback adaptive divergence?

• A. It is driven exclusively by de novo mutations
• B. It shows no genetic basis
• C. It is often driven by a few major-effect loci and standing variation
• D. It is driven solely by gene flow

Answer: (C)

Adaptive divergence in sticklebacks is often driven by a small number of genes that have large effects, using alleles that are already present in the population (standing variation) and become advantageous in the freshwater environment. This setup lets populations diverge quickly and repeatedly because beneficial variants are readily available in the gene pool, so selection can act on them without waiting for new mutations. New mutations can occur, but they don’t have to be the main story for the pattern we see. Gene flow tends to blur differences between populations, so it doesn’t alone explain rapid, repeated divergence either. So the best summary is that a few major-effect loci, together with standing variation, underlie much of stickleback adaptive divergence.