[GluteusMax]

Well, there are the ring species: each subspecies can readily interbreed with its neighbors, but the two end subspecies are completely incompatible, and thus constitute separate species.

Nah. Too parallel to the mule analogy. Breeding hybrids do not constitute a new species.

[Junior]

They aren't hybrids. These all fall along a spectrum of subspecies that are fully capable of breeding (and producing non-sterile progeny) with their direct neighbors, but less capable of producing said progeny with members further away. The two end subspecies are so far apart that they are incapable of interbreeding. What you are seeing with a ring species is speciation in the act.

[dangus]

Speciation is a process of numerous non-essential mutations occurring, renderring the two species' chromosomes incompatible for reproduction. Since it requires so very many chromosomes, it must occur over tens of thousands of years after the event which caused the populations to seperate.

We do, however, observe:
1. Emergence of a seperate population due to a mutation.
2. Chromosomal drift, so regular you can almost use it to measure time.
3. The emerging dominance of one subspecies over another.
4. The extinction of a subspecies.

In other words, we've observed every step in the process of speciation. We just haven't been watching long enough to witness the entire process at once.