Why can freshwater sticklebacks evolve armor reduction rapidly in a few thousand years?

• A. They exploit standing genetic variation and face different selective pressures that favor reduced armor, enabling rapid adaptation.
• B. They mutate armor genes at high rate
• C. They rarely encounter predators
• D. They are isolated from marine populations

Answer: (A)

Adaptive change can be rapid when a population already carries genetic variants that fit the new environment. In freshwater sticklebacks, alleles that reduce armor plates exist as standing genetic variation in ancestral marine populations. When these fish move into freshwater habitats, the selective pressures shift: armor is costly to produce and the freshwater predator and ecological context often favor lighter bodies. Individuals with alleles for reduced armor have higher fitness, so those alleles increase in frequency quickly. Because the relevant genetic changes can involve a few genes with large effects, the response to selection is fast, yielding noticeable armor reduction within a few thousand years. De novo mutations don’t have to drive the change, and while isolation can influence gene flow, the rapid shift mainly stems from readily available variation that selection can act on.