Ring Species: Salamanders
The various Ensatina salamanders of the Pacific coast all descended from a common ancestral population. As the species spread southward from Oregon and Washington, subpopulations adapted to their local environments on either side of the San Joaquin Valley. From one population to the next, in a circular pattern, these salamanders are still able to interbreed successfully. However, where the circle closes -- in the black zone on the map in Southern California -- the salamanders no longer interbreed successfully. The variation within a single species has produced differences as large as those between two separate species. 
Some critics of the theory of evolution argue that it doesn't convincingly explain the origin of new species. They say that members of one species couldn't become so different from other individuals through natural variation that they would become two separate non-interbreeding species.
One of the most powerful counters to that argument is the rare but fascinating phenomenon known as "ring species." This occurs when a single species becomes geographically distributed in a circular pattern over a large area. Immediately adjacent or neighboring populations of the species vary slightly but can interbreed. But at the extremes of the distribution -- the opposite ends of the pattern that link to form a circle -- natural variation has produced so much difference between the populations that they function as though they were two separate, non-interbreeding species.
In concept, this can be likened to a spiral-shaped parking garage. A driver notices only a gentle rise as he ascends the spiral, but after making one complete circle, he finds himself an entire floor above where he started.
A well-studied example of a ring species is the salamander Ensatina escholtzii of the Pacific Coast region of the United States. In Southern California, naturalists have found what look like two distinct species scrabbling across the ground. One is marked with strong, dark blotches in a cryptic pattern that camouflages it well. The other is more uniform and brighter, with bright yellow eyes, apparently in mimicry of the deadly poisonous western newt. These two populations coexist in some areas but do not interbreed -- and evidently cannot do so.
Moving up the state, the two populations are divided geographically, with the dark, cryptic form occupying the inland mountains and the conspicuous mimic living along the coast. Still farther to the north, in northern California and Oregon, the two populations merge, and only one form is found. In this area, it is clear that what looked like two separate species in the south are in fact a single species with several interbreeding subspecies, joined together in one continuous ring.
The evolutionary story that scientists have deciphered begins in the north, where the single form is found. This is probably the ancestral population. As it expanded south, the population became split by the San Joaquin Valley in central California, forming two different groups. In the Sierra Nevada the salamanders evolved their cryptic coloration. Along the coast they gradually became brighter and brighter.
The division was not absolute: some members of the sub-populations still find each other and interbreed to produce hybrids. The hybrids look healthy and vigorous, but they are neither well-camouflaged nor good mimics, so they are vulnerable to predators. They also seem to have difficulty finding mates, so the hybrids do not reproduce successfully. These two factors keep the two forms from merging, even though they can interbreed.
By the time the salamanders reached the southernmost part of California, the separation had caused the two groups to evolve enough differences that they had become reproductively isolated. In some areas the two populations coexist, closing the "ring," but do not interbreed. They are as distinct as though they were two separate species. Yet the entire complex of populations belongs to a single taxonomic species, Ensatina escholtzii.
Ring species, says biologist David Wake, who has studied Ensatina for more than 20 years, are a beautiful example of species formation in action. "All of the intermediate steps, normally missing, have been preserved, and that is what makes it so fascinating."