What does the term cline refer to in biology?

In biology, a cline refers to a geographical gradient of variation within a species. This concept is important in understanding how populations of a species exhibit gradual changes in physical or genetic traits across different environments or locations. These variations may occur in response to environmental gradients such as temperature, altitude, or other ecological factors. For instance, as one moves from a lower altitude to a higher altitude, one might observe changes in the coloration or size of a particular species of plant or animal due to adaptation to varying environmental conditions.

The presence of a cline illustrates how populations are not static, and instead, they can show continuous variation that is influenced by geographical distribution. This understanding is crucial in the study of evolution and adaptation, as it highlights the relationship between an organism and its environment, demonstrating how species evolve traits that best suit their specific habitats.