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cline,in biology, any gradual change in a particular characteristic of a population of organisms from one end of the geographical range of the population to the other. Gradients of characteristics usually accompany, and are responses to, environmental gradients; for example, a mountain range features gradients from top to bottom such as a temperature gradient (colder to warmer) and a humidity gradient (wetter to drier). They may also reflect patterns of individual migration or gene flow. In species of birds and mammals, there is usually a cline in body size, with smaller individuals in warm climates and larger individuals tending to be found in colder climates.
in biology, a gradual increase or decrease (quantitative gradient) of some character or property in populations in connection with marked change in physical geographic factors. A cline usually develops when a large area is more or less uniformly populated by a particular species and the populations and their groups are not separated by rigid isolating barriers. A cline provides an advantage in natural selection because it allows adjustment to any direction of change in the corresponding physical geographic factors.
A cline may also develop as a result of the rapid dispersal of a species.