Why do some stickleback populations lack pelvic spines?

• A. In lakes where there are no predatory fish, there is no advantage to having pelvic spines
• B. Spines are always advantageous in any environment
• C. Spines disappear due to random drift
• D. Spines form only in male fish

Answer: (A)

When a trait no longer helps with survival, natural selection can favor its reduction or loss. Pelvic spines in sticklebacks are useful deterrents against certain predators, but in lakes without predatory fish there’s no survival advantage to having them. Carrying spines can incur costs—energy to grow and maintain them, potential hindrance to swimming or maneuverability, and other trade-offs—so individuals without spines can have higher fitness in that environment. Over generations, the population shifts toward lacking pelvic spines, illustrating how local ecological conditions drive trait loss.

Spines aren’t universally advantageous in every environment, and random drift isn’t the primary force here, especially given the repeated patterns seen in lakes with different predation pressures. Spines are not restricted to one sex, so they aren’t formed only in males either.