Ring species are really quite startling evidences of biological change. A classic ring species is a single species that is distributed around some kind of geographic barrier to dispersion. Around the barrier, the species populations have differentiated into varieties or subspecies. Most of the adjacent subspecies are interfertile, except for one pair that is either reproductively isolated or experiences reduced fitness. For those of you who just tuned out from all the biology jargon, here's a simpler explanation based on what we think ring species are: Imagine a species spreading out (dispersing) from a location, say in northern California. This species (say it's a salamander) is a mountain species and doesn't like the lower elevations. As this salamander spreads southward over many generations, it will avoid California's central valley and spread down the hills and mountains that run around the central valley. Since salamanders don't really move very fast, spreading through California takes many generations, and during that time, the salamander populations become slightly different as they (presumably) adapt to local conditions. Finally, when the salamanders finish dispersing around the central valley, they come into contact again in the south, around Frazier Park. One set of salamanders went east around the central valley, and one went west. When they meet in the south, though, these salamanders are no longer able to interbreed normally, but if you test adjacent populations along the eastern dispersal route, you'll find that they interbreed just fine. Likewise with the western route. It's only the salamanders from the very southern ends of the dispersion routes that can't interbreed normally. If you're looking to reproductive capabilities to define what is a species (the biological species concept), these salamanders are kind of weird. As you might have guessed already, evolutionary biologists think that these salamanders (and other ring species like them) are examples of speciation in progress. I agree.
A new paper from Monahan et al. takes a different approach to studying ring species. Rather than looking for the species, they instead looked at geographic barriers that would be conducive to the formation of ring species. They found that these barriers are pretty rare, which could explain why there are so few examples of ring species. The paper is open access, and I definitely recommend you check it out, if you're into that sort of thing.
Monahan et al. 2012. Ring distributions leading to species formation: a global topographic analysis of geographic barriers associated with ring species. BMC Biology 10:20.
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