Cline (population genetics)

In biology and ecology, an ecocline or simply cline (from Greek: κλίνω "to possess or exhibit gradient, to lean") is an ecotone in which a series of biocommunities display a continuous gradient. The term was coined by the English evolutionary biologist Julian Huxley in 1938.

More technically, clines consist of ecotypes or forms of species that exhibit gradual phenotypic and/or genetic differences over a geographical area, typically as a result of environmental heterogeneity. Genetically, clines result from the change of allele frequencies within the gene pool of the group of taxa in question. Clines may manifest in time and/or space.

In ecology, spatial clines have led to gradient analysis where the abundance and distribution of organisms is rendered by sinusoidal curves on the plane. From these curves can be extracted that populations occupy zones of maximum and minimum presence, according to their special needs and tolerances imposed by their environment.

Typically, a well-marked cline does not allow for a delineation of subspecies, as it is then impossible, by definition, to draw any further clear dividing lines between populations. In population genetics, a cline could include a spectrum of subspecies, as allele and haplotype frequencies tend to vary over a larger space; moreover, in evolution, genetic lineage sorting usually lags behind the establishment of locally differentiated phenotypes. Regardless, in neither case will such a variation yield different species, as long as the populations, though geographically spread, can interbreed one with another.

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