In biology, an ecocline or simply cline (Greek: κλίνω = to possess or exhibit gradient, to lean)^[1][2] is a term (coined by Julian Huxley in 1938)^[3] used to describe an ecotone in which a series of biocommunities display continuous gradient.

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.^[4][5][6] Clines may manifest in time and/or space.^[7]

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.^[8]

Map of indigenous skin color distribution in the world based on Von Luschan's chromatic scale.

In the context of anthropology and particularly human genetic variation, clines are related to races and can be genetically defined in a rigorous fashion, for they are readily submissible to straightforward, quantitative metrics.^{[citation needed]} Perhaps the most notable example of human continuous cline is the skin color gradation across latitudinal paths on the globe. Indigenous populations of the tropics have darker skin than indigenous populations outside of these regions due to variation in received sunlight.^[9]

The Larus gulls hybridize in a circular cline around the Arctic.

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.^{[citation needed]}

A remarkable example of cline is that of a ring species ^[6],^[10] where all neighboring (yet genetically distinct) populations across a geographical path can interbreed, but the two populations at the furthest positions of the range are so genetically divergent that they do not normally interbreed. Thus the two "end" groups are eligible to be classified as different biospecies. This phenomenon is possibly due to the cumulative effect of numerous genetic changes that have occurred or are still occurring along the cline.^{[citation needed]} Chain species are closely related to speciation.

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.^[11] However a recent study (Liebers et al., 2004) has provided genetic evidence that this example is far more complicated than presented here, and likely does not constitute a typical ring species.^[11][12]

[edit] Types of clines

• Thermocline - A cline based on difference in water temperature,
• Chemocline - A cline based on difference in water chemistry,
• Halocline - A cline based on difference in water salinity,
• Pycnocline - A cline based on difference in water density.

[edit] See also

• Isolation by distance
• Ring species
• Race (classification of human beings)
• Human skin color
• Larus
• Lesser Black-backed Gull
• Speciation
• Polymorphism (biology)
• Ecotype
• Allele frequency

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[edit] References

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (July 2009)

1. ^ http://www.greek-language.gr/greekLang/modern_greek/tools/lexica/search.html?lq=%CE%BA%CE%BB%CE%B9%CE%BD%CF%89&dq=
2. ^ http://www.merriam-webster.com/dictionary/cline
3. ^ Julian Huxley
4. ^ Microsoft Encarta Premium 2009: "cline"
5. ^ King, Stansfield, Mulligan: A dictionary of genetics, 7th ed. (2006), Oxford University Press:"cline"
6. ^ ^{a b} Begon, Townsend, Harper - Ecology: From individuals to ecosystems, Blackwell Publishing, 4th ed. (2006), p. 10
7. ^ Eric R. Pianka: Evolutionary Ecology, 6th ed. (2000), Pearson Education, chapter 4
8. ^ Eric R. Pianka: Evolutionary Ecology, 6th ed. (2000), Pearson Education, chapter 4
9. ^ race. (2009). Encyclopædia Britannica. Ultimate Reference Suite. Chicago: Encyclopædia Britannica.
10. ^ Fundamentals of biogeography - Richard J. Huggett, 2nd ed. (2004), p. 20
11. ^ ^{a b} Larus
12. ^ Liebers, Dorit; de Knijff, Peter & Helbig, Andreas J. (2004): The herring gull complex is not a ring species. Proc. Roy. Soc. B 271(1542): 893-901. doi:10.1098/rspb.2004.2679 PDF fulltext Electronic Appendix