Ensatina

Ensatina is a genus of lungless salamanders. They are a complex of subspecies of "Ensatina eschscholtzii", found in forests in British Columbia, Washington, Oregon, California and Baja California, Mexico.
Ring species.
The "Ensatina" salamander is a ring species in the mountains around the Californian Central Valley. The complex forms a horseshoe shape around the mountains. Though interbreeding can happen between each of the 19 populations around the horseshoe, the "Ensatina eschscholtzii" subspecies on the western end of the horseshoe cannot interbreed with the "Ensatina klauberi" on the eastern end. It is an illustration of "nearly all stages in a speciation process" (Dobzhansky). Richard Highton argued that "Ensatina" is a case of multiple species and not a continuum of one species.