Cline

In biology, a cline is a gradual change in the frequency of a trait or allele across a geographical area. This change is typically continuous, forming a gradient rather than discrete categories. Clines can reflect variation in morphology, physiology, or DNA sequence, and may occur across populations within a species or across closely related species along contact zones.

The shape and width of a cline are influenced by the balance between gene flow through migration and local selection from environmental differences, as well as genetic drift. A strong environmental gradient or strong selection relative to dispersal tends to produce steep clines; higher dispersal or weaker selection yields broad, shallow clines. Sometimes abrupt transitions occur in narrow regions called hybrid zones, which can themselves be studied as stepped or sigmoidal clines.

Clines are often analyzed by sampling genetic markers or traits along geographical transects and fitting mathematical models. Width, slope, and position of the cline provide information about the relative strength of selection and dispersal, and about the historical movement of populations.

Common examples include clines in human phenotypes or allele frequencies across latitude or altitude, and clines in adaptive traits such as beak size in island birds or resistance alleles in pest species along environmental gradients.

In evolutionary biology, clines are central to studies of gradualism, adaptation, population structure, and speciation. They illustrate how selection and gene flow shape variation across geographic space.