... a dog, by any other name is still a dog ...
(Top Posts - Science - 050803)

Some members of the dog family, Canidae (domestic dogs, wild dogs, wolves, coyotes, jackals), can interbreed with constraints (location and social constraints, but not genetic constraints). Foxes are a member of the dog family, Canidae, which cannot interbreed with the aforementioned members (having become a distinct member some 11 to 12 million years ago, with a different number of chromosomes).

Domestic dogs and wild dogs, for example, only developed their modern-day affinity / association with humans up to 14,000 years or so ago, and are very closely related to wolves.

As pertains to speciation causality (such as reproductive isolation and lack of breeding over a substantial time period for interbreeding members in Canidae, for example), there has not been significant genetic dissimilarity to result in non-breedable populations among the mentioned interbreedable members.

The following contains an excerpt from a National Geographic magazine article describing how eating in proximity to humans was a likely key evolutionary impetus for the domestication of wolves into dogs. Also described is the close genetic similarity of the DNA of wolves and dogs, and the way in which dogs (even wild dogs) are dependent on proximity to humans for survival:

Wolf to Woof: The Evolution of Dogs
@ nationalgeographic.com

The following article gives a good overview of the complexities involved in the use of the word "species" (i.e., one definition does not fit all) ...

The Species Concept
(University of California Davis)