They allow for a reconstruction of the history of speciation, but they are relatively rare because in order for one to form, a single species must typically expand around a geographic barrier that is large enough to disperse the front before the species meet again.
One example of this concept is the circumpolar Larrus gulls, which live in America, England, and Siberia, and another is the Siberian green warbler, in which eastern and western forms do not interbreed. They allow for a reconstruction of the history of speciation, but they are relatively rare because in order for one to form, a single species must typically expand around a geographic barrier that is large enough to disperse the front before the species meet again. Most famously exemplified by the North American Ensatina salamanders, for 10 points, identify these types of species whose existence is made possible when populations encircle an area of unsuitable habitat and at one location in the circle, two distinct noninterbreeding forms exist.
One example of this concept is the circumpolar Larrus gulls, which live in America, England, and Siberia, and another is the Siberian green warbler, in which eastern and western forms do not interbreed.
{ring} species