In a ring species, gene flow occurs between neighbouring populations of a species, but at the ends of the "ring" , the populations cannot interbreed.
The coloured bars show natural populations (colours), varying along a cline. Such variation may occur in a line (e.g. up a mountain slope) as in A, or may wrap around as in B. Where the cline bends around, populations next to each other on the cline can interbreed, but at the point that the beginning meets the end again, as at C, the differences along the cline prevent interbreeding (gap between pink and green). The interbreeding populations are then called a ring species.
In biology, a ring species is a connected series of neighbouring populations, each of which can interbreed 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 Rassenkreis, meaning a ring of populations, 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 can breed with B, and B can breed with C, it does not follow that A can breed with C–and thus 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.
Herring gull (Larus argentatus) (front) and lesser black-backed gull (Larus fuscus) (behind) in Norway: two phenotypes with clear differences
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.