Why are the stickleback populations in Frog Lake more similar to marine and sea-run populations than to Bear Paw Lake?

• A. Frog Lake sticklebacks have complete pelvises like marine populations, while Bear Paw populations have reduced or absent pelvises.
• B. Frog Lake is geographically closer to the ocean than Bear Paw.
• C. Bear Paw has more vegetation and hiding places than Frog Lake.
• D. Frog Lake experiences greater predation by large fish than Bear Paw.

Answer: (A)

Pelvic morphology in sticklebacks reflects the environment they live in. Marine and sea-run populations retain a complete pelvic girdle because that trait is advantageous in their aquatic settings and represents the ancestral state. Freshwater lake populations, on the other hand, commonly evolve reduced or absent pelvises as an adaptation to different predators and energy costs in inland waters. Seeing that Frog Lake sticklebacks have complete pelvises like marine populations, they resemble marine/sea-run groups in this trait. Bear Paw Lake sticklebacks, with reduced or absent pelvises, show the freshwater adaptation pattern. So the Frog Lake populations are more similar to marine populations for this characteristic, whereas Bear Paw differs due to the freshwater selective pressures. The other factors listed—geography, vegetation, or predation pressure unrelated to pelvis morphology—do not directly explain the pelvic trait the question focuses on.