By NEW YORK TIMES
Added: Thu, 02 Oct 2008 23:00:00 UTC

Thanks to Catalin for the link.

http://www.nytimes.com/2008/10/07/science/07obfish.html?_r=1&oref=slogin

Seeing Red and Blue Can Divide a Species — of Fish
By HENRY FOUNTAIN

Geographic isolation is a driving force in evolution. If there's a mountain, a body of water or other physical barrier between two populations of a species, then chances are good they'll diverge over time.

But members of a species can become isolated from each other in other, nonphysical, ways — through the way they sense the world, for instance. Evidence for this kind of speciation has been incomplete, however.

Now, Ole Seehausen of the University of Bern and the Swiss Federal Institute for Aquatic Science and Technology and colleagues report strong ecological, genetic and molecular evidence for speciation among cichlid fish in Lake Victoria in Africa based on how they perceive color. The researchers looked at two related species; in both, the females choose mates based on their coloration. In one species, found in deeper parts of the lake, the males have red features. In the other, found in shallower waters, the males are blue. What's more, in some parts of the lake the two aren't really separate species, but rather are intermixed.

Lake Victoria's water is cloudy due to organic material in it. That material filters the light, removing the blue wavelengths so that as depth increases the light shifts toward the red end of the spectrum. "We wondered if perhaps the split of the original species into these two is driven by adaptations in their visual system due to light at different water depths," Dr. Seehausen said.

Their new research, published in Nature, shows that that's what has happened. They found that in the species with red males, over time, in a deeper, red-shifted environment, the gene responsible for color perception had changed to perceive red better. That change made it more likely for females to choose red males.

They also found that in some extremely cloudy parts of the lake, the red-shift was so sharp and sudden that the water was essentially uniform in terms of the wavelength of light. In those areas, Dr. Seehausen said, there is really only one, intermediate species. "All the genes are there, but they are not differentiating along the depth axis, because there is really only one light environment," he said.

TAGGED: BIOLOGY, EVOLUTION