What is the effect of habitat age on the trajectory of freshwater stickleback adaptation?

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

Answer: (B)

Habitat age determines how long selection has to shape the population. In older freshwater habitats, sticklebacks have many more generations to accumulate advantageous changes and fix them in the genome. This leads to stronger, more uniform adaptations that are highly stabilized, such as reduced armor plating in many lake populations. In younger habitats, adaptation is still in progress, more variable, and there’s often more influence from ongoing gene flow with the marine ancestor, so the changes haven’t had time to become fully fixed. The idea is that time allows alleles that boost fitness in freshwater to become common and steady, making older habitats show more pronounced and fixed adaptations. The other options don’t fit because they imply no effect, the opposite strength, or an unwarranted reversion, which isn’t supported by how populations accumulate and stabilize adaptive changes over generations.