Cline (biology)

In biology, a cline (from the Greek “klinein”, meaning “to lean”) is a measurable gradient in a single character (or biological trait) of a species across its geographical range. First coined by Julian Huxley in 1938, the “character” of the cline referred to is usually genetic (e.g allele frequency, blood type), or phenotypic (e.g. body size, skin pigmentation). Clines can show smooth, continuous gradation in a character, or they may show more abrupt changes in the trait from one geographic region to the next. In biology, a cline (from the Greek “klinein”, meaning “to lean”) is a measurable gradient in a single character (or biological trait) of a species across its geographical range. First coined by Julian Huxley in 1938, the “character” of the cline referred to is usually genetic (e.g allele frequency, blood type), or phenotypic (e.g. body size, skin pigmentation). Clines can show smooth, continuous gradation in a character, or they may show more abrupt changes in the trait from one geographic region to the next. A cline refers to a spatial gradient in a specific, singular trait, rather than a gradient in a population as a whole. A single population can therefore theoretically have as many clines as it has traits. Additionally, Huxley recognised that 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. Because of this, clines were defined by Huxley as being an “auxiliary taxonomic principle”; that is, clinal variation in a species is not awarded taxonomic recognition in the way subspecies or species are. While the terms “ecotype” and “cline” 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. Clines are often cited to be the result of two opposing drivers: selection and gene flow (also known as migration). Selection causes adaptation to the local environment, resulting in different genotypes or phenotypes being favoured in different environments. This diversifying force is countered by gene flow, which has a homogenising effect on populations and prevents speciation through causing genetic admixture and blurring any distinct genetic boundaries. Clines are generally thought to arise under one of two conditions: “primary differentiation” (also known as 'primary contact' or 'primary intergradation' ), or “secondary contact” (also known as 'secondary introgression', or 'secondary intergradation'). Clines produced through this way are generated by spatial heterogeneity in environmental conditions. The mechanism of selection acting upon organisms is therefore external. Species ranges frequently span environmental gradients (e.g. humidity, rainfall, temperature, or day length) and, according to natural selection, different environments will favour different genotypes or phenotypes. In this way, when previously genetically or phenotypically uniform populations spread into novel environments, they will evolve to be uniquely adapted to the local environment, in the process potentially creating a gradient in a genotypic or phenotypic trait. Such clines in characters can not be maintained through selection alone if lots of gene flow occurred between populations, as this would tend to swamp out the effects of local adaptation. However, because species usually tend to have a limited dispersal range (e.g. in an isolation by distance model), restricted gene flow can serve as a type of barrier which encourages geographic differentiation. However, some degree of migration is often required to maintain a cline; without it, speciation is likely to eventually occur, as local adaptation can cause reproductive isolation between populations. A classic example of the role of environmental gradients in creating clines is that of the peppered moth, Biston betularia, in the UK. During the 19th century, when the industrial sector gained traction, coal emissions blackened vegetation across northwest England and parts of northern Wales. As a result of this, lighter morphs of the moth were more visible to predators against the blackened tree trunks and were therefore more heavily predated relative to the darker morphs. Consequently, the frequency of the more cryptic melanic morph of the peppered moth increased drastically in northern England. This cline in morph colour, from a dominance of lighter morphs in the west of England (which did not suffer as heavily from pollution), to the higher frequency of melanic forms in the north, has slowly been degrading since limitations to sooty emissions were introduced in the 1960s. Clines generated through this mechanism have arisen through the joining of two formerly isolated populations which differentiated in allopatry, creating an intermediate zone. This secondary contact scenario may occur, for example, when climatic conditions change, allowing the ranges of populations to expand and meet. Because over time the effect of gene flow will tend to eventually swamp out any regional differences and cause one large homogenous population, for a stable cline to be maintained when two populations join there must usually be a selective pressure maintaining a degree of differentiation between the two populations.