Fish Fins Point to Genes’ Role in Evolution

Biologists have found that one small change in a fish gene makes a significant difference in the size of its fins -- a rare textbook example of how abrupt evolutionary changes can occur.

The scientists, whose report was published in the current issue of Nature, studied different types of threespine stickleback fish. Marine populations of sticklebacks have large, spiky fins on their undersides to protect them from the gaping jaws of predators. But in certain freshwater lakes, those pelvic fins have been totally or partially lost over the course of evolution, possibly because they were no longer required for protection and were costly for a creature to produce.

By comparing the genes of such small-finned fish in a Canadian lake with those of full-finned marine fish, a team led by scientists at Stanford University discovered that a slight change in a gene known as Pitx1 was responsible for the bulk of the fin size difference.

The gene is known to be important in embryo development, including the development of the pelvic fin. Because of a small genetic change in the small-finned fish, the Pitx1 gene is no longer active in the part of the fish larvae that becomes the fin.

Such alterations in Pitx1 appear to have occurred more than once during the course of evolution -- not only in Canadian fish but also in small-finned populations thousands of miles away in Iceland and Scotland.