Cline (biology)

[1] Clines usually have a genetic (e.g. allele frequency, blood type), or phenotypic (e.g. body size, skin pigmentation) character.

Such clines in characters can not be maintained through selection alone if much 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.

[9] 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.

[2] 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.

This secondary contact scenario may occur, for example, when climatic conditions change, allowing the ranges of populations to expand and meet.

For example, aposematic signals in Heliconius butterflies sometimes display steep clines between populations, which are maintained through positive frequency dependence.

These individuals are then predated more heavily relative to their counterparts with "normal" markings (i.e. selected against), creating populations dominated by a particular pattern of warning signal.

As with heterozygote disadvantage, when these populations join, a narrow cline of intermediate individuals could be produced, maintained by gene flow counteracting selection.

[7] While selection can therefore clearly play a key role in creating clines, it is theoretically feasible that they might be generated by genetic drift alone.

Both extrinsic and intrinsic selection can serve to generate varying degrees of reproductive isolation and thereby instigate the process of speciation.

If the local genetic differences are great enough, it may lead unfavourable combinations of genotypes and therefore to hybrids being at a decreased fitness relative to the parental lines.

[20][2] Clines can be portrayed graphically on maps using lines that show the transition in character state from one end of the geographic range to the other.

Although the term "cline" was first officially coined by Huxley in 1938, gradients and geographic variations in the character states of species have been observed for centuries.

One commonly cited example of a gradient in morphology is Gloger's Rule, named after Constantin Gloger, who observed in 1833 that environmental factors and the pigmentation of avian plumage tend to covary with each other, such that birds found in arid areas near the Equator tend to be much darker than those in less arid areas closer to the Poles.

[22] One of the proposed reasons for this cline is that larger animals have a relatively smaller surface area to volume ratio and therefore improved heat conservancy – an important advantage in cold climates.

[22] The role of the environment in imposing a selective pressure and producing this cline has been heavily implicated due to the fact that Bergmann's Rule has been observed across many independent lineages of species and continents.

For example, the house sparrow, which was introduced in the early 1850s to the eastern United States, evolved a north-south gradient in size soon after its introduction.

From this, it has been possible to infer that some Asian populations migrated towards Europe around 2,000 years ago, causing genetic admixture in an isolation by distance model.