Ring species
In biology, a ring species is a connected series of neighbouring populations, each of which interbreeds with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" population. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region (sympatry) thus closing a "ring". The German term , meaning a circle of races, is also used. Ring species represent speciation and have been cited as evidence of evolution. They illustrate what  happens over time as populations genetically diverge, specifically because they represent, in living populations, what normally happens over time between long-deceased ancestor populations and living populations, in which the intermediates have become extinct. The evolutionary biologist Richard Dawkins remarks that ring species "are only showing us in the spatial dimension something that must always happen in the time dimension". Formally, the issue is that "interfertility" (ability to interbreed) is not a transitive relation; if A breeds with B, and B breeds with C, it does not mean that A breeds with C, and therefore does not define an equivalence  relation. A ring species is a species with a counterexample to the transitivity of interbreeding. However, it is unclear whether any of the examples of ring species cited by scientists actually permit gene flow from end to end, with many being debated and contested. History. The classic ring species is the "Larus" gull. In 1925 Jonathan Dwight found the genus to form a chain of varieties around the Arctic Circle. However, doubts have arisen as to whether this represents an actual ring species. In 1938, Claud Buchanan Ticehurst argued that the greenish warbler had spread from Nepal around the Tibetan  Plateau, while adapting to each new environment, meeting again in Siberia where the ends no longer interbreed. These and other discoveries led Mayr to first formulate a theory on ring species in his 1942 study "Systematics and the Origin of Species". Also in the 1940s, Robert C. Stebbins described the "Ensatina" salamanders around the Californian Central Valley as a ring species; but again, some authors such as Jerry Coyne consider this classification incorrect. Finally in 2012, the first example of a ring species in plants was found in a spurge, forming a ring around the Caribbean Sea. Speciation. The biologist  Ernst Mayr championed the concept of ring species, claiming that it unequivocally demonstrated the process of speciation. A ring species is an alternative model to allopatric speciation, "illustrating how new species can arise through 'circular overlap', without interruption of gene flow through intervening populations…" However, Jerry Coyne and H. Allen Orr point out that rings species more closely model parapatric speciation. Ring species often attract the interests of evolutionary biologists, systematists, and researchers of speciation leading to both thought provoking ideas and confusion concerning their definition. Contemporary scholars recognize that examples in nature have proved rare due to various factors  such as limitations in taxonomic delineation or, "taxonomic zeal"—explained by the fact that taxonomists classify organisms into "species", while ring species often cannot fit this definition. Other reasons such as gene flow interruption from "vicariate divergence" and fragmented populations due to climate instability have also been cited. Ring species also present an interesting case of the species problem for those seeking to divide the living world into discrete species. All that distinguishes a ring species from two separate species is the existence of the connecting populations; if enough of the connecting populations within the ring perish to sever the  breeding connection then the ring species' distal populations will be recognized as two distinct species. The problem is whether to quantify the whole ring as a single species (despite the fact that not all individuals interbreed) or to classify each population as a distinct species (despite the fact that it interbreeds with its near neighbours). Ring species illustrate that species boundaries arise gradually and often exist on a continuum. Examples. Many examples have been documented in nature. Debate exists concerning much of the research, with some authors citing evidence against their existence entirely. The following examples provide evidence that—despite  the limited number of concrete, idealized examples in nature—continuums of species do exist and can be found in biological systems. This is often characterized by sub-species level classifications such as clines, ecotypes, complexes, and varieties. Many examples have been disputed by researchers, and equally "many of the [proposed] cases have received very little attention from researchers, making it difficult to assess whether they display the characteristics of ideal ring species." The following list gives examples of ring species found in nature. Some of the examples such as the "Larus" gull complex, the greenish warbler of Asia, and the  Ensatina salamanders of America, have been disputed.