Which type of genetic variation is emphasized as enabling rapid and repeated phenotypic divergence in sticklebacks?

• A. Standing genetic variation
• B. Copy number variation
• C. Somatic mutations
• D. Epigenetic marks

Answer: (A)

Standing genetic variation means alleles are already present in a population before a new environmental pressure arrives. In sticklebacks, freshwater populations repeatedly evolve similar traits, like reduced armor plates, by selecting from these pre-existing variants found in the ancestral marine gene pool. Because the advantageous alleles are already there, they can rise in frequency quickly and in parallel across independent populations facing similar freshwater conditions. The other options describe different forms of genetic or epigenetic change that either aren’t inherited in the same way, aren’t the main drivers of these rapid, repeated changes, or don’t provide the same quick, heritable material for selection to act on.