In freshwater stickleback populations, one common evolutionary change is pelvic reduction.

• A. Pelvic reduction
• B. Increase of dorsal spines
• C. No armor loss
• D. Fin ray changes

Answer: (A)

Adaptation to a new environment often favors traits that reduce unnecessary costs while still providing protection. In freshwater sticklebacks, many populations repeatedly evolve reduced pelvic armor as they adapt away from their marine ancestors. This pelvic reduction involves the pelvic girdle and spines, a change that is seen across diverse freshwater populations and reflects a broad pattern of parallel evolution. The benefits come from the fact that armor is metabolically costly to build and can hinder swimming efficiency or reproduction in certain freshwater settings where predators and ecological pressures differ from the marine environment. A genetic basis, such as regulatory changes affecting pelvic development (like Pitx1), helps explain why this trait appears repeatedly in independent populations.

The other options don’t align with this well-documented pattern. Increasing dorsal spines would add armor rather than reduce it, which isn’t the common freshwater trend. Claiming no armor loss contradicts the well-supported observation. Fin ray changes do occur in sticklebacks but are not as consistently and prominently associated with freshwater adaptation as pelvic reduction.