Which statement best describes armor differences among stickleback populations?

Which statement best describes armor differences among stickleback populations?

• A. Armor is identical across all populations
• B. Freshwater populations have heavier armor than marine
• C. Armor differences: Marine and sea-run populations have heavy armor, while freshwater populations have reduced armor
• D. Armor only exists in marine populations

Answer: (C)

Armor in sticklebacks is an adaptive feature that varies with habitat. Marine environments, and sea-run populations that spend substantial time in saltwater, often face predators and ecological conditions that favor heavy armor, so these fish carry more plates. Freshwater populations, by contrast, encounter different predator communities and incur energetic costs to maintain plating; in many lakes the selective pressure is toward reduced armor, so they evolve fewer plates. This pattern—heavy armor in marine and sea-run forms and reduced armor in freshwater forms—reflects how local ecological pressures shape a heritable trait. The other statements don’t fit because armor is not the same across all populations, freshwater isn’t heavier, and armor isn’t limited to marine populations.

Armor in sticklebacks is an adaptive feature that varies with habitat. Marine environments, and sea-run populations that spend substantial time in saltwater, often face predators and ecological conditions that favor heavy armor, so these fish carry more plates. Freshwater populations, by contrast, encounter different predator communities and incur energetic costs to maintain plating; in many lakes the selective pressure is toward reduced armor, so they evolve fewer plates. This pattern—heavy armor in marine and sea-run forms and reduced armor in freshwater forms—reflects how local ecological pressures shape a heritable trait. The other statements don’t fit because armor is not the same across all populations, freshwater isn’t heavier, and armor isn’t limited to marine populations.