Distinguish standing genetic variation from new mutations in the context of stickleback adaptation.

• A. Standing variation refers to pre-existing alleles in a population that selection can act on immediately; new mutations arise later and may contribute more slowly.
• B. Standing variation arises after adaptation begins
• C. New mutations are always present from birth
• D. Standing variation cannot contribute to rapid adaptation

Answer: (A)

The main idea here is how evolution uses existing genetic diversity versus new changes. Standing genetic variation means alleles that a population already carries before a change in environment. In sticklebacks, when populations move from the sea into freshwater, selection can immediately act on these pre-existing alleles—for traits like armor plates or spine reduction—allowing rapid adaptation across populations. New mutations, on the other hand, arise after the environmental shift and must appear and spread through the population, which typically takes more time. They can add novel variation and contribute to adaptation, but they don’t provide an immediate substrate for selection the way standing variation does. So standing variation explains rapid, initial responses to new conditions, while new mutations provide additional, slower-to-accumulate genetic changes.