What evidence supports repeated, independent evolution of a trait in sticklebacks?

• A. Observing the same trait change (e.g., plate loss) in multiple, geographically isolated freshwater populations with similar genetic changes.
• B. Plate changes occur only in one population.
• C. No genetic changes accompany plate loss.
• D. Plate loss occurs randomly with no genetic pattern.

Answer: (A)

The main idea is that repeated, independent evolution is shown when the same trait emerges in several geographically isolated populations, and the trait arises through similar genetic changes. In sticklebacks, plate loss appearing in many separate freshwater populations indicates the same adaptive problem—living in freshwater with different predators and resources—has repeatedly led to the same solution. When genetic studies reveal similar changes in the same genes or genetic pathways across these separate populations, it shows parallel evolution: different groups arrive at the same phenotype through comparable genetic routes, rather than inheriting the trait from a single ancestral mutation.

If plate changes were found only in one population, that wouldn’t demonstrate repeated independent evolution across multiple populations. If there were no genetic changes accompanying plate loss, the trait would likely be due to non-genetic factors rather than adaptation via evolution. If plate loss occurred randomly with no genetic pattern, that wouldn’t reflect a repeatable, selective process shaping the trait in similar ways across populations.