How can habitat age influence the direction of stickleback adaptation?

• A. Habitat age has no impact on adaptation.
• B. Younger freshwater habitats may have stronger, more fixed adaptations.
• C. Older freshwater habitats may have stronger, more fixed adaptations.
• D. Older habitats always revert to marine phenotypes.

Answer: (C)

The main idea here is that the age of a habitat shapes how strongly sticklebacks can diverge from their marine ancestors. In older freshwater habitats, there have been many more generations for natural selection to act, allowing advantageous freshwater traits to become established and fixed. Over time, this leads to clearer, more pronounced differences from marine forms—traits like armor plate reduction or changes in body shape that fit the new environment stay consistently across individuals.

In younger freshwater habitats, there’s less time for selection to fix these traits, and ongoing gene flow from marine populations can keep variation higher. So the adaptations may still be evolving or remain variable rather than fully fixed.

That’s why older freshwater habitats are associated with stronger, more fixed adaptations. The other options contradict this pattern: habitat age does influence adaptation; younger habitats aren’t typically where the most fixed adaptations arise yet; and older habitats don’t inherently revert back to marine phenotypes.