Geographic isolation is a driving force in evolution. If there’s a mountain, a body of water or other physical barrier between two populations of a species, then chances are good they’ll diverge over time.

But members of a species can become isolated from each other in other, nonphysical ways  through the way they sense the world, for instance. Evidence for this kind of speciation has been incomplete, however.

Image Color perception in cichlid fish like the Pundamilia pundamilia is evidence of speciation, a report finds. Credit... Inke van der Sluijs

Now, Ole Seehausen of the University of Bern and the Swiss Federal Institute for Aquatic Science and Technology and colleagues report strong ecological, genetic and molecular evidence for speciation among cichlid fish in Lake Victoria in Africa, based on how they perceive color. In two related species the females choose mates based on their coloration. In one, found in deeper parts of the lake, the males have red features. In the other, found in shallower waters, the males are blue. What’s more, in some parts of the lake the two aren’t really separate species, but rather are intermixed.