Why are freshwater sticklebacks often used as a model for rapid adaptation?

• A. Because they have stable habitats and no variation
• B. Because they only adapt slowly
• C. Because they show random morphological changes
• D. Because they repeatedly and rapidly adapt to freshwater habitats with predictable morphological changes, such as plate reduction and spine changes

Answer: (D)

Freshwater sticklebacks are studied because they repeatedly colonize new freshwater ponds and respond to the new environment with rapid, repeatable changes. In many independent populations, selection consistently favors similar traits—most notably a reduction in bony armor plates and changes in spines—so the same morphological directions appear across different lakes. This combination of speed and parallel evolution provides a clear window into how natural selection can shape form in a short time, and genetic work has shown that major shifts can arise from comparatively small, influential genetic changes. Because the pattern is predictable, fast, and genetically tractable, freshwater sticklebacks have become a go-to model for understanding rapid adaptation.