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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cline (biology) | 2/4 | https://en.wikipedia.org/wiki/Cline_(biology) | reference | science, encyclopedia | 2026-05-05T09:07:03.435002+00:00 | kb-cron |
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. The mechanism of selection maintaining the clines in this scenario is often intrinsic. This means that the fitness of individuals is independent of the external environment, and selection is instead dependent on the genome of the individual. Intrinsic, or endogenous, selection can give rise to clines in characters through a variety of mechanisms. One way it may act is through heterozygote disadvantage, in which intermediate genotypes have a lower relative fitness than either homozygote genotypes. Because of this disadvantage, one allele will tend to become fixed in a given population, such that populations will consist largely of either AA (homozygous dominant) or aa (homozygous recessive) individuals. 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. Such hybrid zones where hybrids are a disadvantage relative to their parental lines (but which are nonetheless maintained through selection being counteracted by gene flow) are known as "tension zones". Another way in which selection can generate clines is through frequency-dependent selection. Characters that could be maintained by such frequency-dependent selective pressures include warning signals (aposematism). For example, aposematic signals in Heliconius butterflies sometimes display steep clines between populations, which are maintained through positive frequency dependence. This is because heterozygosity, mutations and recombination can all produce patterns that deviate from those well-established signals which mark prey as being unpalatable. 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. 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. Because both primary differentiation and secondary contact can therefore give rise to similar or identical clinal patterns (e.g. gently sloping clines), distinguishing which of these two processes is responsible for generating a cline is difficult and often impossible. However, in some circumstances a cline and a geographic variable (such as humidity) may be very tightly linked, with a change in one corresponding closely to a change in the other. In such cases it may be tentatively concluded that the cline is generated by primary differentiation and therefore moulded by environmental selective pressures.
==== No selection (drift/migration balance) ==== 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. It is unlikely that large-scale clines in genotype or phenotype frequency will be produced solely by drift. However, across smaller geographical scales and in smaller populations, drift could produce temporary clines. 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. Such clines are therefore unstable and sometimes called "transient clines".
== 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 ==