Cline (biology)

In biology, a cline is a measurable gradient in a single characteristic (or biological trait) of a species across its geographical range.^[1] Clines usually have a genetic (e.g. allele frequency, blood type), or phenotypic (e.g. body size, skin pigmentation) character. They can show either smooth, continuous gradation in a character, or more abrupt changes in the trait from one geographic region to the next.^[2]

A cline is a spatial gradient in a single specific trait, rather than in a collection of traits;^[3] a single population can therefore have as many clines as it has traits, at least in principle.^[4] Additionally, as Julian Huxley recognised, these multiple independent clines may not act in concordance with each other. For example, it has been observed that in Australia, birds generally become smaller the further towards the north of the country they are found. In contrast, the intensity of their plumage colouration follows a different geographical trajectory, being most vibrant where humidity is highest and becoming less vibrant further into the arid centre of the country.^[5] Because of this, Huxley described the notion of clines as an "auxiliary taxonomic principle,” meaning that clinal variation in a species is not awarded taxonomic recognition in the way subspecies or species are.^[1]

The term cline was coined by Huxley in 1938 from the Greek κλίνειν klinein, meaning "to lean.” While it and the term ecotype are sometimes used interchangeably, they do in fact differ in that ecotype refers to a population which differs from other populations in a number of characters, rather than the single character that varies amongst populations in a cline.^[4]