conditions. Such variation is called irregular variation. Second, when the gradual variation of a character of a species can be correlated with a gradual geographic variation, the variation of the character is called a cline. Clines occur because of altitude gradients or latitude gradients, for example. Third, when an abrupt step is plotted in the graph of an otherwise gradually varying clinal character, the two populations on either side of the step are designated as being subspecies or races. Even with the sometimes great morphological differences between populations due to intraspecific variation, populations are still considered part of the same species if they are not reproductively isolated from each other.
