Asymmetric introgression between fishes in the Red River basin of Texas is associated with variation in water quality.

When ecologically divergent taxa encounter one another, hybrid zones can form when reproductive isolation is incomplete. The location of such hybrid zones can be influenced by environmental variables, and an ecological context can provide unique insights into the mechanisms by which species diverge and are maintained. Two ecologically differentiated species of small benthic fishes, the endemic and imperiled prairie chub, , and the shoal chub, , are locally sympatric within the upper Red River Basin of Texas.