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The evolution of theories of evolution

When a segment of a population splits off genetically and evolves into a new species, it typically happens because the group gets separated from the rest of its kind. But now, researchers say they have demonstrated that evolution from one species to another doesn’t always require geographical isolation between groups of the same species. Instead, evolution also can occur through a kind of sensory isolation among next-door neighbors with no physical barrier between them.

That’s the word from an international team of scientists led by Ole Seehausen, a zoologist at the University of Bern in Switzerland. Their test subjects were cichlid fish in Africa’s Lake Victoria.

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When female cichlids look for a mate, they go for the best-dressed fish. So, red cichlids want the brightest red mates and blue cichlid swoon over bright blue partners. But the dominant wavelength, or color, of ambient light in the water changes with depth due to pollution levels and the amount of suspended sediment. Near the surface the light tends more toward blue and deep down it leans more toward red.

Using genetic techniques, the team found that the eyes in females at each of these depths had become more sensitive to the dominant color of ambient light, and so would see the males with that color as the best dressed.

The team found that in places where the light dramatically shifts in color from blue to red, or changed very gradually, there was far more interbreeding among cichlids of all colors, largely because everyone was in very close proximity. But where the color change fell in between those two extremes, the team found a more pronounced segregation, with more blue-hued male cichlids near the surface and more red-toned males deeper down. These males showed other genetic changes that suggested they were becoming a new species.

The conclusion the team drew: Changes in females’ color sensitivity with time and water quality – through its effect on mate selection – was driving populations at each depth into becoming new species. They add that their results suggest that pollution may be a key player in the loss of biodiversity among cichlids in the lake as populations that once interbred are separated by these diverging sensory cues. The results appear in a recent issue of the journal Nature.

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