Isolation–not just geographical!
Researchers working in Africa have reported strong evidence for speciation among cichlid fish in Lake Victoria, based not on geographical isolation, but on how they perceive color. Interesting!
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.
You can read more about it in Nature or in the New York Times.