re: dogs, wolf spp, coyotes etc.
Diminished fertility is a sign that speciation is not complete. There are ring species (seriously neat...adjacent neighbors can interbreed but when they reach the extreme ends of their range, overlapping populations can't breed.
>=can breed with X=can't breed with
a > b > c > d > e > f X a.
somewhere along the line they (let's say a and d) can breed but have fewer or weaker offspring.
That's similar to what goes on with wolves, dogs and coyotes. No one is going to seriously argue that a chihuahua male could impregnate a wolf female, or that a chihuahua female could either be impregnated by, or bear the offspring of, a wolf.
The biologists' definition of species is critters that don't interbreed in nature, but on the way to species is diminished fertility. That would explain the occasional hybrid in various canid populations.