Sunday, November 13, 2016

Extinction or Evolution? The Answer Isn't Always Clear

[Smithsonian] 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?
The answer had to do with invasive crayfish, which were likely introduced into the lake ecosystem by humans. Within three years of the crayfishes’ arrival, the two species had once again merged. “It seems like someone may have introduced the crayfish possibly as a food source,” says Seth Rudman, an evolutionary biologist at the University of British Columbia whose paper on the phenomenon came out in Current Biology earlier this year. “The crayfish physically altered the way the sticklebacks nest and breed, which increased the probability of mating” between the two species, he says.
You might be saying to yourself: Wait, that’s not how evolution works. Actually, it can be. What happened with the finger-length fishes is an example of “introgressive extinction,” otherwise known as reverse speciation. Regular speciation happens when members of one species are divided by changes in their habitat or behavior. The most well-known example is Darwin’s finches: Over time, finches on different, isolated islands diverged in beak size and other qualities until they became distinct species. Reverse speciation is when those distinct species come together again, until they become one species yet again. Read More