In the Calnali River in Mexico, two pure species of swordtail fish live side by side with a third population that’s a mix of the original two. Classical models of evolution predict that pure females would not mate with these hybrids, as the hybrids are considered less fit from an evolutionary perspective. But a new study shows that’s not the case. Researchers examined the two pure swordtail species, Xiphophorus birchmanni and Xiphophorus malinche, as well as the hybrids, tagging the animals and extracting DNA to probe their mating patterns. They discovered that X. birchmanni and X. malinche did not mate with each other, but that females of both species mated with the hybrids. Hybrids can therefore act as a channel for genes to flow between the pure species populations, the team will report next month in The American Naturalist. But if the two pure species can’t mate, how did the hybrids form in the first place? The researchers suspect that contaminants in runoff from rural villages confused the fishes’ sense of smell, which typically prevents them from mating with other species. Those contaminants may have weakened over time, re-erecting the olfactory barriers to mating.