When they share habitat, orangethroat and rainbow darters tend to avoid one another, even though they are closely related and can produce “hybrid” offspring. The males compete with males of their own species and will almost always ignore females of the other species. A new study offers an analysis of the genomic changes that occur when these fish hybridize, offering insight into the gradual accumulation of incompatible traits that likely drives them to diverge.

Scientists once thought that female mate choice alone accounted for the eye-catching color patterns seen in some male fish. But for orangethroat darters, male-to-male competition is the real force behind the flash, a new study finds.

The research, reported in the Royal Society journal Proceedings B, suggests that separate populations of orangethroat darters are evolving differing color patterns as a result of the males’ ability to distinguish their own from other species.