How does predator regime influence stickleback armor evolution?

How does predator regime influence stickleback armor evolution?

• A. Predator-rich habitats favor heavier armor; predator-poor habitats favor reduced armor
• B. Predator regime has no effect on armor
• C. Armor evolves randomly regardless of predators
• D. Only predator-rich habitats increase pelvic spines

Answer: (A)

Predator pressure drives natural selection on stickleback armor. In habitats with many predators, individuals with heavier armor and longer spines are more likely to survive attacks, so these traits become more common over generations. In predator-poor environments, the armor costs energy to produce and can reduce mobility and foraging efficiency, so lighter armor is favored and becomes more prevalent. Over time, populations diverge in armor depending on local predation risks, which is why marine sticklebacks often have heavier plates while many freshwater populations show reduced armor. Pelvic spines are part of this armor strategy, but it isn’t an absolute rule that only predator-rich places increase spines; sometimes spines are reduced in environments with different predator communities or ecological pressures. The idea that predator regime has no effect or that armor evolves randomly doesn’t fit the observed pattern of trait variation tied to predation risk.

Predator pressure drives natural selection on stickleback armor. In habitats with many predators, individuals with heavier armor and longer spines are more likely to survive attacks, so these traits become more common over generations. In predator-poor environments, the armor costs energy to produce and can reduce mobility and foraging efficiency, so lighter armor is favored and becomes more prevalent. Over time, populations diverge in armor depending on local predation risks, which is why marine sticklebacks often have heavier plates while many freshwater populations show reduced armor.

Pelvic spines are part of this armor strategy, but it isn’t an absolute rule that only predator-rich places increase spines; sometimes spines are reduced in environments with different predator communities or ecological pressures. The idea that predator regime has no effect or that armor evolves randomly doesn’t fit the observed pattern of trait variation tied to predation risk.