How might secondary contact result in a cline?

Question

Quizplus · Accepted Answer

Secondary contact occurs when two previously isolated populations come into contact again after evolving separately. If there is partial reproductive isolation, meaning that some hybrid mating occurs but has low fitness, a spatial gradient of hybrids may form, resulting in a cline. This cline represents a gradual change in the frequency of alleles between the two populations as well as a transition zone where hybrids are more common. This can occur as a result of natural selection favoring individuals with intermediate phenotypes that are better adapted to the local environment.

How Might Secondary Contact Result in a Cline