Fish in Lake Victoria evolving around pollution

"The split of one species into two was initiated by adaptation of the sensory system, in this case the eyes, to the local environment," said Seehausen, an evolutionary biologist at the Swiss Federal Institute of Aquatic Science and Technology.

The cichlid fish are an important model for evolutionary biologists because no other group of vertebrates has split into so many species - about 2,000 - so quickly, Seehausen said in a telephone interview.

Scientists also generally believe that originating a new species requires geographical isolation -- such as two continents drifting apart. The fact that the two different cichlid fish species live side by side is puzzling, he said.

"These fish meet each other all the time and really live in the same spot," Seehausen said. "We knew there were two different species but we didn't know how that came about."

The findings have implications for conservation efforts because they suggest that pollution that changes the light in the water would lead the two species to collapse and merge into a single one, Seehausen said.

The findings, and the introduction of a predatory fish, "help explain the very rapid loss of cichlid species in Lake Victoria over the past 30 years," Seehausen said, adding that the number of species there had fallen by half from 500.

The researchers looked at two species, marked by their red or blue colours, found off five islands throughout the lake.  They determined through lab experiments that certain genetic mutations helped some fish adapt their vision at deeper levels to see the colour red, and others in shallower water to recognize shades of blue.

That gave blue males a mating advantage in shallower water and red ones an edge in deeper parts of the lake because they were more attractive to female fish.

Evolution by vision could explain why Lake Victoria is home to hundreds of species of cichlids, which are popular aquarium fish. However, as urban growth pumps more sediment and algae-feeding fertilisers into Lake Victoria unchecked, many species are vanishing.

"Species diversity in this lake has imploded in the last 30 years," Seehausen says. "It is the largest human-witnessed mass extinction of vertebrates."  Rather than die out, species facing radically changed optical environments interbreed out of existence, he says.