In a number of species, gradients exist in the area of occurrence in the degree of expression of certain phenotype traits. Simultaneously, these gradients need not be identical for the individual traits; a certain trait, e.g. body size, can change in the north – south direction while another traits, for example, hair color, can change in the east – west direction. In some cases, there exists a similar clinal variability in a certain trait in a greater number of even unrelated species. For example, body dimensions in warm-blooded animals often increase in the direction towards colder areas in the geographic area of occurrence (Bergmann’s rule), while the length of body appendages (for example, ears) becomes shorter in the same direction (Allen’s rule).
The mechanisms of formation and maintenance of this clinal variability are very diverse. Very frequently, geographic gradients of a certain factor in the environment are responsible for their existence, for example the average annual temperature or average annual precipitation. A biotic factor can also be responsible for clinal variability, for example a gradient in the population density of a competitive species limiting the width of the ecological niche of the species in question in a certain direction. If the ecological niches of two sympatrically occurring species thus overlap at least slightly, the clinal variability of one species can induce the emergence of clinal variability in another species in a similar manner. Finally, gene flow penetrating at a certain place into the gene pool of the population from the gene pool of another species, with whose members the representatives of the studied species can reproduce to at a limited degree, can be responsible for clinal variability.