Marine sticklebacks, for example, boast body armor: from head to tail, they are covered in rows of bony plates. Many freshwater sticklebacks have lost these. In marine sticklebacks, the pelvis is a complicated affair that comes complete with a pair of long spines. In some freshwater populations, individuals have a much reduced, lopsided pelvic structure. In others, they have just a remnant, a small, lopsided bone: the ghost of pelvis past...I think I will be an evolutionary biologist in my next life. But maybe I'll end up a stickleback.
What seems to have happened is that when sticklebacks invaded each lake, some of the invaders carried this rare version with them. In the ocean, being without body armor is deadly: it makes you vulnerable to predators. But lakes don’t have the same dangers as the ocean — and armor is heavy and makes you less agile. Thus, in these new environments, being without body armor conferred a significant advantage, and so in lake after lake, the rare variant of the gene swept through the population.
Let’s turn now to the ghostly pelvis. Pelvic loss is much less common than armor loss. But if you find sticklebacks that lack a pelvis, you can bet that they came from large, shallow lakes where the water is soft, there are no large fish that might act as predators, and the vegetation is dense. Soft water has little calcium, and you need calcium to make the pelvic spines. Shallow lakes that are thick with weeds are home to predators like dragonflies, which enjoy having a stickleback for breakfast. And whereas the spines are a defense against being eaten by other fish — trout, say, or pike — and can actually induce the predator to spit out the stickleback instead of trying to swallow it, insect predators catch sticklebacks by grabbing the spines...
Wednesday, January 30, 2008
Evolution in Action
A fascinating article in the Times by evolutionary biologist Olivia Judson about the evolution of the three-spine sticklebacks, a small ocean fish that got stuck in various northern-hemisphere lakes when the last glaciers receded 10-20 thousand years ago. It's a perfectly designed experiment in evolution as the lake fish took on an evolutionary trajectory different than their oceanic counterparts, and yet in the different lakes they evolved in similar but distinct ways: