Other forms: clines
In biology, a cline is a gradual change in a trait or characteristic across a population of a species. This gradient occurs over a geographic area where environmental factors, such as altitude or temperature, vary.
One example of a cline is the variation in zebra stripes across Africa. In the North, zebras have bold, black-and-white stripes over their entire bodies. Moving south, the stripes become fainter and more brownish-gray. In the most southern regions, their stripes are even more faded, and the lower legs are solid white. In a cline, there is no abrupt, sudden change; rather, traits change gradually across a spectrum based on the animal's geographic range.