Cline (biology)

Clines are often cited to be the result of two opposing drivers: selection and gene flow (also known as migration). 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 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. 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. Secondary contact 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. The cline of heterozygotes that is created when these respective populations come into contact is then shaped by the opposing forces of selection and gene flow; even if selection against heterozygotes is great, if there is some degree of gene flow between the two populations, then a steep cline may be able to be maintained. Because instrinsic selection is independent of the external environment, clines generated by selection against hybrids are not fixed to any given geographical area and can move around the geographic landscape. Secondary contact could lead to a cline with a steep gradient if heterozygote disadvantage or frequency-dependent selection exists, as intermediates are heavily selected against. Alternatively, steep clines could exist because the populations have only recently established secondary contact, and the character in the original allopatric populations had a large degree of differentiation. As genetic admixture between the population increases with time however, the steepness of the cline is likely to decrease as the difference in character is eroded. However, if the character in the original allopatric populations was not very differentiated to begin with, the cline between the populations need not display a very steep gradient. The fact that drift is a weak force upholding the cline however means that clines produced this way are often random (i.e. uncorrelated with environmental variables) and subject to breakdown or reversal over time. ==Clinal structure and terminology==

The steepness, or gradient, of a cline reflects the extent of the differentiation in the character across a geographic range. For example, a steep cline could indicate large variation in the colour of plumage between adjacent bird populations. It has been previously outlined that such steep clines may be the result of two previously allopatric populations with a large degree of difference in the trait having only recently established gene flow, or where there is strong selection against hybrids. However, it may also reflect a sudden environmental change or boundary. Examples of rapidly changing environmental boundaries like this include abrupt changes in the heavy metal content of soils, and the consequent narrow clines produced between populations of Agrostis that are either adapted to these soils with high metal content, or adapted to "normal" soil. Conversely, a shallow cline indicates little geographical variation in the character or trait across a given geographical distance. This may have arisen through weak differential environmental selective pressure, or where two populations established secondary contact a long time ago and gene flow has eroded the large character differentiation between the populations. The gradient of a cline is related to another commonly referred to property, clinal width. A cline with a steep slope is said to have a small, or narrow, width, while shallower clines have larger widths. ==Types of clines==

According to Huxley, clines can be classified into two categories; continuous clines and discontinuous stepped clines. Stepped clines can be further subdivided into horizontally stepped clines, and obliquely stepped clines. • Horizontally stepped clines show no intra-population variation or gradation in the character, therefore displaying a horizontal gradient. These uniform populations are connected by steeper sections of the cline, characterised by larger changes in the form of the character. However, because in continuous clines all populations exchange genetic material, the intergradation zone between the groups can never have a vertical slope. • In obliquely stepped clines, conversely, each population also demonstrates a cline in the character, albeit of a shallower slope than the clines connecting the populations together. Huxley compared obliquely stepped clines to looking like a "stepped ramp", rather than taking on the formation of a staircase as in the case of horizontally stepped clines. Discontinuous stepped clines Unlike in continuous clines, in discontinuous clines the populations of species are allopatric, meaning there is very little or no gene flow amongst populations. The genetic or phenotypic trait in question always shows a steeper gradient between groups than within groups, as in continuous clines. Discontinuous clines follow the same principles as continuous clines by displaying either • Horizontally stepped clines, where intra-group variation is very small or non-existent and the geographic space separating groups shows a sharp change in character • Obliquely stepped clines, where there is some intra-group gradation, but this is less than the gradation in the character between populations ==Clines and speciation==

It was originally assumed that geographic isolation was a necessary precursor to speciation (allopatric speciation). The possibility that clines may be a precursor to speciation was therefore ignored, as they were assumed to be evidence of the fact that in contiguous populations gene flow was too strong a force of homogenisation, and selection too weak a force of differentiation, for speciation to take place. This is known as reinforcement and plays an important role in parapatric and sympatric speciation. ==Clinal maps==

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. Character states can however additionally be represented using isophenes, defined by Ernst Mayr as "lines of equal expression of a clinally varying character". In other words, areas on maps that demonstrate the same biological phenomenon or character will be connected by something that resembles a contour line. When mapping clines therefore, which follow a character gradation from one extreme to the other, isophenes will transect clinal lines at a right angle. ==Examples of clines==

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. Indeed, some gradations have been considered so ubiquitous that they have been labelled ecological "rules". 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. Since then, this rule has been extended to include many other animals, including flies, butterflies, and wolves. Other ecogeographical rules include Bergmann's Rule, coined by Carl Bergmann in 1857, which states that homeotherms closer to the Equator tend to be smaller than their more northerly or southerly conspecifics. 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. This gradient reflects the gradient that already existed in the house sparrow's native range in Europe. Ring species are a distinct type of cline where the geographical distribution in question is circular in shape, so that the two ends of the cline overlap with one another, giving two adjacent populations that rarely interbreed due to the cumulative effect of the many changes in phenotype along the cline. The populations elsewhere along the cline interbreed with their geographically adjacent populations as in a standard cline. In the case of Larus gulls, the habitats of the end populations even overlap, which introduces questions as to what constitutes a species: nowhere along the cline can a line be drawn between the populations, but they are unable to interbreed. In humans, clines in the frequency of blood types has allowed scientists to infer past population migrations. For example, the Type B blood group reaches its highest frequency in Asia, but become less frequent further west. 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. In contrast to this cline, blood Type A shows the reverse pattern, reaching its highest frequency in Europe and declining in frequency towards Asia. ==References==