In stickleback adaptive radiation, which genetic source often fuels repeated divergence across populations?

• A. De novo mutations
• B. Gene flow from distant populations
• C. Standing genetic variation
• D. Polyploidization

Answer: (C)

Standing genetic variation is the fuel for repeated divergence because it uses alleles that already exist in the ancestral population. When sticklebacks move into new environments, like freshwater, natural selection can quickly act on this pre‑existing variation, producing similar adaptive changes in independent populations. This is why many freshwater stickleback populations repeatedly evolve the same traits, such as reduced armor, by tapping into the same alleles that were present at low frequency before isolation. If adaptation relied on new mutations, each population would have to wait for new beneficial changes to arise separately, making parallel evolution less likely and slower. Polyploidization isn’t a common mechanism in sticklebacks, and while gene flow can introduce variation, the pattern of rapid, repeated use of the same existing alleles best fits standing genetic variation.