How did some stickleback populations come to live exclusively in fresh water?
the ancestors of freshwater sticklebacks are marine stickleback species that lived primarily in the ocean and migrated to freshwater to spawn.
sticklebacks developed traits that made them better adapted to fresh water and, as a result, they moved to a freshwater environment.
stickleback populations swam to freshwater lakes to spawn and stayed because there were no predators.
some stickleback populations became trapped in lakes that formed at the end of the last ice age.