DEMES, CLINES & RING SPECIES

==Demes== A deme is a sub-population (a smaller group within the population) that can freely interbreed [image:http://i.imgur.com/zUrzO7W.jpg?2] A population is usually made of several demes (groups) that are partially isolated from each other. Often a geographical barrier limits gene flow between demes, but not sufficiently to result in speciation. Enough gene flow occurs between demes ensuring that all of it's members remain the same species. Highlighted in green is a deme, a gourp that is partially isolated form the rest of the population. Other groups in this image could also be considered demes. ==Clines== A cline is the gradual change in the phenotypes and genotypes of a species across a geographical gradient [image:http://i.imgur.com/42BII3o.png?1] When a species is distributed across a large geographical area some of its members will occupy areas with slightly different environmental conditions than others. Allele frequencies and traits might vary slightly across the geographical gradient because of different selection pressures. For instance, the colouration of these snails changes as you sample across their range. [image:http://i.imgur.com/bLuqAnU.png?1] Despite genetic and phenotypic differences, members of a cline are all still the same species because each member can interbreed with its immediate neighbours resulting in gene flow through the entire population. ==Ring Species== [image:http://i.imgur.com/yXmMCMc.png?1] Individuals at each end of a cline may be so different from each other that they can no longer interbreed. You might therefore conclude that they are different species, but it would be impossible to draw a dividing-line between them. Not all members could be classified as belonging to one group or the other, as there is such a gradual change in genotype and phenotype that each member of the population can interbreed with its immediate neighbours. Gene flow can still occur between each end of the cline via intermediary individuals. Therefore they are considered the same species. Remove enough individuals from the middle of the cline and you may in fact be left with two separate species. [image:http://i.imgur.com/PEayQJA.png?3] A ring species is simply a cline that is circular in shape, so that the two ends of the cline overlap with one another. This produces two neighbouring populations that rarely interbreed due to the cumulative effect of the changes in genotype and phenotype along the cline. For instance, the ''Larus'' gulls interbreed in a ring around the Arctic. There is a gradual change in genotype and phenotype resulting in seven different subspecies across the cline (1 → 7). Each of these different subspecies can interbreed with its direct neighbour, except for 1 and 7 (where the cline loops back onto itself).