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."