What morphological trait differs most consistently between marine and freshwater threespine stickleback populations?

What morphological trait differs most consistently between marine and freshwater threespine stickleback populations?

• A. The number of lateral armor plates
• B. Body coloration
• C. Tail length
• D. Eye size

Answer: (A)

Armor plating on the sides is the trait that shows the clearest, most repeatable difference between marine and freshwater sticklebacks. Across many populations that have independently moved from the sea into freshwater, freshwater forms tend to have far fewer lateral plates, and often a reduced plate morph, compared with the heavily plated marine relatives. This consistent pattern is a hallmark of parallel evolution driven by the different ecological pressures in freshwater environments, such as different predators and energy tradeoffs. The genetic basis often involves changes in the regulatory region of the EDA gene, illustrating how a single genetic shift can produce a major, repeated morphological change in response to habitat. In contrast, body coloration, tail length, or eye size do not show as uniform a difference across populations, as they are more strongly influenced by local conditions and do not track habitat in the same consistent way.

Armor plating on the sides is the trait that shows the clearest, most repeatable difference between marine and freshwater sticklebacks. Across many populations that have independently moved from the sea into freshwater, freshwater forms tend to have far fewer lateral plates, and often a reduced plate morph, compared with the heavily plated marine relatives. This consistent pattern is a hallmark of parallel evolution driven by the different ecological pressures in freshwater environments, such as different predators and energy tradeoffs. The genetic basis often involves changes in the regulatory region of the EDA gene, illustrating how a single genetic shift can produce a major, repeated morphological change in response to habitat. In contrast, body coloration, tail length, or eye size do not show as uniform a difference across populations, as they are more strongly influenced by local conditions and do not track habitat in the same consistent way.