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RED FISH, BLUE FISHNeighboring groups of cichlid fish in Africa’s Lake Victoria appear to be splitting into two species. Females whose vision has adapted to red-lit areas prefer red-colored males, while females in blue-lit areas prefer blue-colored males. This could be the first example of speciation due to changes in sense organs.Inke van der Sluijs; Ole Seehausen

Some cichlid fish see red better while others only have eyes for blue. This difference in vision, observed in fish in an African lake, could be pushing red-bodied cichlids to branch off from their blue-bodied brethren and to form a new species.

If so, it would be the first time that scientists have caught evolution in the act of creating a new species because of changes in sense organs. For one species to diverge into two, some barrier must prevent two groups of individuals from interbreeding. Physical separation of two groups and changes to reproductive organs are two of the wedges that scientists have shown can drive the formation of new species, and evolutionary biologists are always keen to discover new mechanisms.

“Speciation can occur even without physical isolation when individuals are adapted to a particular environment by [their] sensory system,” says Norihiro Okada, an evolutionary biologist at Tokyo Institute of Technology.

Okada and his colleagues had previously shown that cichlid fish in Lake Victoria’s shallow waters are bathed in bluer light, while the turbid water of the lake predominantly lets redder light filter down to fish living in deeper water. The researchers showed that the fish’s eyes have adapted to this difference so that fish that live in deeper water have a pigment in their eyes that is more sensitive to red light, while shallow-water fish have a pigment that’s sensitive to blue.

By looking at the DNA of fish from both groups, Okada’s team showed that each has accumulated genetic changes not shared by the other, which suggests that the two groups aren’t interbreeding, Okada and his colleagues report in the Oct. 2 Nature. They also showed in experimental studies that female fish from the red-light group preferred red-colored males, and vice versa. Taken together, the results suggest that changes in the fish’s vision could be starting to split the fish into two species.

“It’s pretty spectacular,” comments Michael J. Ryan, an expert in the evolution of behavior at the University of Texas at Austin. “When they encounter members of neighboring populations, they don’t recognize them as potential mates.”

Speciation is only occurring at locations in the lake where blue light is filtered out slowly, leaving a large blue-lit area for the blue-colored fish. In more turbid areas, the blue-light zone is much smaller, so the two groups of fish intermingle and interbreed. The researchers point out that pollution of the lake by human activity is making the water more turbid, which could increase interbreeding of related species. This genetic mixing could reduce the number of cichlid species in the lake, the scientists suggest.

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