What is a species?
It first meant a distinctly-describable type.
Then, a distinct type that could not interbreed;
Then, a distinct types that could breed and produce fertile offspring.
Today, a species is defined as: A group that, in natural surroundings, breeds exclusively within the group.
Like any definition, it has exceptions, such as coyotes, dogs, and wolves, which can interbreed, yet are considered separate species. But this definition works fairly well.
– Adapted from “An Online Introduction to the Biology of Animals and Plants” by Michael McDarby, Fulton-Montgomery Community College.
Example of salamanders evolving today
Happening in the San Joaquin Valley, central California.
…a rare but fascinating phenomenon [is] 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.
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.”
Evolution in action