Ring species

[5] 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.

Also in the 1940s, Robert C. Stebbins described the Ensatina salamanders around the Californian Central Valley as a ring species;[7][8] but again, some authors such as Jerry Coyne consider this classification incorrect.

[9] The biologist Ernst Mayr championed the concept of ring species, stating that it unequivocally demonstrated the process of speciation.

[4] 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.

[1] Other reasons such as gene flow interruption from "vicariate divergence" and fragmented populations due to climate instability have also been cited.

Some of the examples such as the Larus gull complex, the greenish warbler of Asia, and the Ensatina salamanders of America, have been disputed.

In a ring species, gene flow occurs between neighbouring populations of a species, but at the ends of the "ring", the populations don't 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.
Ensatina salamanders example of ring species
Speculated evolution and spread of the greenish warbler, Phylloscopus trochiloides .
P. t. trochiloides
P. t. obscuratus
P. t. plumbeitarsus
P. t. ludlowi
P. t. viridanus
Note: The P. t. nitidus in Caucasus Mountains not shown