What is the role of standing genetic variation in rapid stickleback adaptation to freshwater?

• A. No role; new mutations are needed
• B. It delays adaptation
• C. Pre-existing genetic variation enables rapid adaptation without waiting for new mutations
• D. It is detrimental

Answer: (C)

When the environment shifts suddenly, evolution can act almost immediately if the population already carries alleles that fit the new conditions. This is standing genetic variation: variants that are already in the population and may be neutral or even slightly deleterious under the old environment but become advantageous when conditions change. In sticklebacks moving from marine to freshwater, alleles that affect traits like armor plating or pelvic structure can be already present at low frequencies. When freshwater conditions favor reduced armor or different pelvic morphology, selection can quickly increase the frequency of those pre-existing alleles, producing rapid, repeated adaptive changes without waiting for new mutations to arise. New mutations can still appear, but they’re not required for the initial rapid shift, because the existing genetic variation provides the necessary raw material for adaptation.