I know it sounds like a definition dreamed up by Ambrose Bierce, but a species is "whatever the ranking expert on a given group says it is".

Members of different genera are usually unable to produce fertile offspring. See Liger and Tigeron.

It is more likely that different species within a genus can, but not always. Coyotes and Wolves seem to breed with no problems.

Subspecies should be able breed fully with each other since the criteria are usually just color variations and locality.

It has been said that "propinguity is the mother of romance" (Steven Stills?), but there can be no interbreeding when subspecies occupy completely distinct and isolated territories.

A few hundred years ago, the Coyote was not found east of the Mississippi River (as far as I know). It is able to walk accross bridges on our great rivers. The Coyote is a invasive species in this case.