In population genetics, a cline is a gradual change of a character or feature (phenotype) in a species over a geographical area. The change in phenotype does not result in different species as long as the geographically spread populations can interbreed with one another. This meaning of cline was introduced by Sir Julian Huxley.

A ring species is a special kind of cline where the geographical distribution in question is circular in shape, so that the two ends of the cline overlap with one another, giving two adjacent populations that rarely interbreed due to the cumulative effect of the many changes in phenotype along the cline. The populations elsewhere along the cline interbreed with their geographically adjacent populations as in a standard cline.