One fish, two fish, crayfish—new fish?
Though it might sound like the plotline of a Dr. Seuss book, that’s what actually happened to the threespine stickleback fishes of Canada’s Enos Lake. For thousands of years, two distinct species of these spiny silver sea creatures—known as the benthic sticklebacks and the limnetic sticklebacks, both descended from a single species—lived in peaceful coexistence. The former stayed near the floor, where they fed on bottom-dwellers; the latter swam up near the sun, eating insects at the surface. Their habitats and behaviors were so different that they rarely met, and never interbred. And all was well.
But then something strange happened: The two species of fish once again became one. How?