Differentiation in stature (sizes in cm) among yarrowplant populations (Achillea lanulosa) in the Sierra Nevada, California.

Cline in human skin color

An example of a stepcline. Changes between forms occur abruptly, without any obvious environmental change.

Genetic variation within and among human races. Notice that although phenotypic differences among races seem to be pronounced, the underlying genetic differences are very minor. Perhaps, our sensory systems are fine tuned to detect even very slight differences in our own species.