This is not the only example of a “ring species” by any means. A more familiar one to many people is the encitina, a California salamander. It probably originated in Oregon around ten million years ago, and migrated south through the redwood forests of northern California to the northern end of California’s great central valley. There they split into two groups, one of which migrated south along each side of the valley.
Those on the east side developed a strategy of camouflage, blending in with their surroundings to hide from predators. By the time they reached the southern tip of the valley, their descendents had become black with big tan splotches.
Those on the west side, toward the Pacific ocean, followed a strategy of mimicry, looking more and more like a particular, poisonous newt living in the same area. When they reached the southern tip of the valley, their descendents there had become a solid, glossy, brownish-orange color. Very different from their cousins 0n the other side.
All down the eastern side of the valley, the encitinas change gradually from place to place, but they keep on mating with their neighbors and producing viable offspring. That is, their offspring live normal salamander lives and produce offspring of their own, keeping the species going.
Down the western side of the valley, the same process occurs. The encitinas change gradually from place to place, but they keep on mating with their neighbors and producing viable offspring to keep the species going. This pattern changes only when the cousins from the two sides of the valley meet again at the southern end.
The solid, glossy, brownish-orange salamanders from the west side of the valley often mate with the gaudy black ones with the big tan splotches from the east side; but their offspring are usually mis-shapen and off-color, so they neither blend into the environment nor fool predators into thinking they are dangerous. The hybrids are not suited (or “adapted”) to the environment, and seldom live long enough to reproduce. Therefore, they are not “viable.”
The changes from generation to generation, and from one place to the next, were so small that they continued to interbreed on both sides of the valley as they migrated south; yet they split from one species in the north into two species in the south over the course of a few million years and a few hundred miles.
These ring species are wonderful, current examples of micro-evolution becoming macro-evolution just by adding one tiny change to another tiny change until they add up to big changes.
1. Lest I be accused of “picking and choosing my facts,” I need to comment here that encitinas crossed the valley and interbred with those on the other side at least a time or two. Biologists can tell that by studying their offspring. But the valley was so inhospitable to them that it seldom happened. For all practical purposes, they evolved as I have described above. The point is that their descendents at the southern tip of the valley meet the definition of separate species. Many species are formed in exactly this way.
2. Phrases like “developed a strategy of camouflage” are not meant to imply that they did it intentionally. Of course they did not. Random mutations set the stage for this “development” and the non-random process of natural selection primarily brought it about.