clinal

Clinal is an adjective used in biology to describe a gradual change in a trait or in allele frequencies across geographical or environmental gradients. A cline refers to the actual gradient, representing a continuous transition rather than abrupt shifts between populations. Clinal variation is common in natural populations, reflecting how gene flow and selection interact with geography and environment to shape differences across space.

The shape of a cline can be gradual or stepped, with smooth transitions or more abrupt changes at certain boundaries. Causes of clinal patterns include the balance between gene flow and selection along an environmental gradient, as well as the influence of genetic drift and historical demographic processes. Geographic features such as mountains or rivers, and ecological transitions, can create or reinforce clines by limiting dispersal or changing selective pressures.

Examples of clinal variation are found across taxa. Human skin pigmentation shows a broad latitudinal cline, generally increasing with proximity to the equator. The frequency of lactase persistence alleles varies progressively across Europe and adjacent regions in relation to historical dairy farming practices. In plants and animals, traits such as flowering time, leaf morphology, or body size often change gradually with altitude or latitude, reflecting different environmental conditions along those gradients.

Studying clines involves sampling populations along geographic transects and modeling trait frequencies or phenotypes as functions of position or environment. Cline analysis can estimate parameters such as cline width and centre, helping researchers infer the relative roles of selection, gene flow, and historical demography in shaping spatial variation.