LONDON (Reuters) -- Some colorful cichlid fish in Africa's Lake Victoria formed a new species by adapting their vision, showing that geographical isolation is not essential for divergence, researchers said Wednesday.

The fish evolved to improve their ability to see food and predators at different depths, and this also affected the way they saw colors and attracted mates, said Ole Seehausen, who led the study published in the journal Nature.
""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 in Kastanienbaum.
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 colors, 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 color 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.
""In short, what you see determines what you get, and with whom you get it on,"" Mark Kirkpatrick of the University of Texas, Austin and Trevor Price of the University of Chicago, wrote in a Nature commentary.