A cline is a geographic gradient in the frequency of a gene, or in the average value of a character.

Clines can arise for different reasons:

• Natural selection favors a slightly different form along the gradient.

• It can also arise if two forms are adapted to different environments separated in space and migration (gene flow) takes place between them.

Geographic variation normally exists in the formof a continuous cline.

A sudden change in gene or character frequency is called a stepped cline. An important type of stepped cline is a hybrid zone, an area of contact between two different forms of a species at which hybridization takes place.