London, October 2 : Studying cichlids living off the shores of five Lake Victoria islands, a research team has found that polluted waters are blinding female fish that have eyes for their man, and thereby causing them to interbreed.
Among several closely related species of cichlids living in Lake Victoria, males come in either red or blue. Brighter males tend to get the girls.
However, the new study indicates that both sexes have evolved to preferentially see only one colour, creating new species of fish in the process.
"Reds and blues live in the exact same spot. Colour is very important in mate choice," New Scientist magazine quoted Ole Seehausen, an evolutionary biologist at the University of Bern in Switzerland and the Swiss Federal Institute of Aquatic Science and Technology in Kastanienbaum, as saying.
While blue pigments shine brightest in shallow waters, red pigments further down. Cichlids usually heed this direction, with red species living at shallower depths than blue species.
For understanding what happens to fish in the transition between red and blue zones, the researchers studied cichlids living off the shores of five Lake Victoria islands.
Seehausen has revealed that the waters near some lakes were cloudier than others due to sediment.
In clear waters, according to the researcher, the colour that appears brighter shifts from red to blue gradually with depth, and red and blue fish stick to their zones, cementing their genetic differences.
However, in murky waters, the transition from red to blue happens much quicker and blue and red fish sometimes interbreed, destroying species differences.
Seehausen further said that the genes for light-detecting proteins called opsins had changed faster than the rest of the fish's genes, a sign of evolution in action.
While red fish evolved red-sensing opsins, blue fish developed blue-sensing proteins.
During the tests conducted in lab tanks, the researchers observed that females with red-sensing eyes went for red males, while blue-eyed females followed suit.
Hybrid females, just like those in the murky waters, showed no preference at all.
The researchers believe that evolution by vision can help discern why Lake Victoria is home to hundreds of species of cichlids, which are popular aquarium fish.
They, however, insist that many fish species are vanishing due to an unchecked pumping of sediment and algae-feeding fertilisers into Lake Victoria.
"Species diversity in this lake has imploded in the last 30 years. It is the largest human-witnessed mass extinction of vertebrates," Seehausen says.
He points out that though fish species do not die out, they face extinction due to interbreeding resulting from their changing optical environments.
Mark Kirkpatrick, an evolutionary biologist at the University of Texas in Austin, feels that guppies and perch might also be facing the same fate as cichlids.
"It's not just restricted to fishes," he says.
A research article describing the study has been published in the journal Nature.