Are mules an example of speciation?

I'd be more inclined to look at the genus Canis as a good example of speciation in action. Dogs, wolves, red wolves, coyotes, and jackals have reproductive isolating mechanisms in terms of behavior, vocalizations, pack structure, etc. But they are able to fully interbreed and produce fertile offspring. In the northeastern USA as wolf populations plummeted due to hunting, coyotes moved into their niches and hybridized with remaining wolves. The result is the Eastern coyote. At 60 - 70lb It's bigger than a pure coyote but very coyotelike in behavior and vocalization. It is able to take larger prey than a typical 25-30lb coyotes. In the southern USA the red wolf (Canis rufus) plunged to extinction levels and incoming coyotes crossbred with the few remaining. The result was a hybrid swarm that spread across several states. If left undisturbed they might have formed a unique species eventually, though certainly not in our lifetimes. The genus Canis is a good example of incomplete speciation.