1938, in biological use, back-formation from incline or from Greek klinein "to slope, to lean" (see lean (v.)). Middle English had clinen (v.) "to bend, bow," from Old French cliner, from Latin clinare.
A gradual change in an inherited characteristic across the geographic range of a species, usually correlated with an environmental transition such as altitude, temperature, or moisture. For example, the body size in a species of warm-blooded animals tends to be larger in cooler climates (a latitudinal cline), while the flowering time of a plant may tend to be later at higher altitudes (an altitudinal cline). In species in which the gene flow between adjacent populations is high, the cline is typically smooth, whereas in populations with restricted gene flow the cline usually occurs as a series of relatively abrupt changes from one group to the next.