Why is including both marine and freshwater populations important when studying stickleback evolution?

• A. To capture the full spectrum of adaptive variation and identify patterns of parallel evolution.
• B. To compare marine and freshwater populations to track gene flow.
• C. Because including only one habitat yields simpler results.
• D. To minimize the number of populations studied.

Answer: (D)

Studying both marine and freshwater sticklebacks reveals the full spectrum of adaptive changes these fish can undergo and shows whether similar solutions arise in independent populations. When marine ancestors repeatedly colonize freshwater environments, they often evolve comparable traits—such as changes in armor, body shape, or feeding—an example of parallel evolution. By comparing both habitats, we can map which traits are repeatedly favored and uncover the genetic paths that underlie those repeated changes. This approach also helps separate patterns driven by shared ancestry from those produced by repeated adaptation, and it clarifies how gene flow between habitats might influence or limit evolutionary outcomes. Looking at only one habitat would miss many of these recurring adaptations and would make it harder to generalize about how sticklebacks evolve in response to different environments.