http://en.wikipedia.org/wiki/Ring_species
In biology, a ring species is a connected series of neighboring 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" species. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing a "ring".
Ring species provide important evidence of evolution in that they illustrate what happens over time as populations genetically diverge, and are special 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. Richard Dawkins observes that ring species "are only showing us in the spatial dimension something that must always happen in the time dimension."[1]
Formally, the issue is that interfertile "able 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 that exhibits a counterexample to transitivity.[2]
http://www.talkorigins.org/indexcc/CB/CB910.html
Ring species show the process of speciation in action. In ring species, the species is distributed more or less in a line, such as around the base of a mountain range. Each population is able to breed with its neighboring population, but the populations at the two ends are not able to interbreed. (In a true ring species, those two end populations are adjacent to each other, completing the ring.) Examples of ring species are
• the salamander Ensatina, with seven different subspecies on the west coast of the United States. They form a ring around California's central valley. At the south end, adjacent subspecies klauberi and eschscholtzi do not interbreed (Brown n.d.; Wake 1997).
• greenish warblers (Phylloscopus trochiloides), around the Himalayas. Their behavioral and genetic characteristics change gradually, starting from central Siberia, extending around the Himalayas, and back again, so two forms of the songbird coexist but do not interbreed in that part of their range (Irwin et al. 2001; Whitehouse 2001; Irwin et al. 2005).
• the deer mouse (Peromyces maniculatus), with over fifty subspecies in North America.
• many species of birds, including Parus major and P. minor, Halcyon chloris, Zosterops, Lalage, Pernis, the Larus argentatus group, and Phylloscopus trochiloides (Mayr 1942, 182-183).
• the American bee Hoplitis (Alcidamea) producta (Mayr 1963, 510).
• the subterranean mole rat, Spalax ehrenbergi (Nevo 1999).
http://evolution.berkeley.edu/evolibrary/article/devitt_01
If you've skimmed a high school biology textbook, you've probably seen the picture: multicolored salamanders meander around California, displaying subtle shifts in appearance as they circle its Central Valley. This is Ensatina eschscholtzii, and it's so well known because it is a living example of speciation in action. Adjacent populations of the salamander look similar and mate with one another — but where the two ends of the loop overlap in Southern California, the two populations look quite different and behave as distinct species. The idea is that this continuum of salamanders — called a ring species — represents the evolutionary history of the lineage as it split into two.
Ensatina has been recognized as a ring species since the 1940s, when biologist Robert C. Stebbins trooped up and down California to investigate its range. Since then, several generations of scientists in Stebbins' institution, the Museum of Vertebrate Zoology at UC Berkeley, have continued these studies, digging deeper into Ensatina's history and biology. At this point, one might think we'd know it all. What more could there be to learn after 60 years of research on a common salamander? "Lots!" says Tom Devitt, a graduate student at the museum. Tom studies Ensatina to flesh out its evolutionary history — but not just for Ensatina's sake. This classic example sheds light on the basic evolutionary processes that shape all life.
