Which trait is primarily used to distinguish marine and freshwater stickleback populations in classic armor studies?

• A. Gill raker number
• B. Eye size
• C. Scale type
• D. Lateral plate number and armor pattern

Answer: (D)

In sticklebacks, armor plating is a highly visible, heritable feature that reflects the environment where the fish evolve. Marine populations face stronger predation from mobile fish, so natural selection favors heavy armor, resulting in many lateral plates and a complete armor pattern. Freshwater populations, with different predators and energy trade-offs, often evolve reduced armor—fewer plates or even plate loss—across many lineages. This direct link between environment and a clear, measurable trait made lateral plate number and armor pattern the classic and most reliable way to distinguish marine from freshwater sticklebacks. Other traits like gill raker number, eye size, or scale type can vary with diet or local conditions but don’t provide the same consistent, overarching separation between marine and freshwater groups.