The conventional wisdom is that domestic dogs are all one species, and
in fact are the same species as the grey wolf, despite the wide and
bizarre array of forms the domestic dog takes.
However, this is generally based on a genetic analysis, showing that
the grey wolf and all dogs share common bits. However, the most test
of a species species is wether or not the "varieties" are willing or
able to interbreed. When they no longer can succesfully interbreed
(decent fertility rate, healthy offspring), we usually say that they
are seperate species.
To what extent is our familarity with dogs blinding us to
speciesation? Yes, different "breeds" of dogs produce pups all the
time, and willingness doesn't seem to much of an issue with dogs. But
what about at the extremes? Mechanics aside, is there any evidence
that the fertility rate between, say, the Bull Mastif and the Toy
Poodle are very low? Or would in-vitrio of these two breeds work just
In short, has anyone done a serious study of the cross-breed viability
(at a purely genetic-combination level) for a wide variety of dog
Request for Question Clarification by
01 Apr 2006 14:12 PST
Fascinating question, and one I addressed during my thesis defense
years ago, except I used a Great Dane and Chihuahua as my examples,
and asked my audience what an alien biologist visiting earth would
conclude, regarding whether these two animals were the same or
For all the obvious reasons, a Bull Mastif and a Toy Poodle (to stick
with your example) could not successfully mate -- at least not very
often. Even if an impregnation were successful, the difficulties
involved in birth (especially of a small dog birthing a large pup)
pose additional and extreme problems.
I don't think studies of the reproductive success (or lack thereof)
between extremes of breeds would really tell you anything more than
your common sense would -- they simply aren't going to be terribly
successful at mating...if at all.
But, their lack of success would not make them distinct species!
Why do I say that? It's not a matter of experimental evidence of
"cross-breed viability" (in fact, I doubt there is much evidence of
that...it seems a cruel experiment to try).
Instead, it's much more a matter of the concepts underlying the term
"species", that argue for the conclusion that breeds of dogs -- no
matter how different -- still all belong to the same species.
Would you like your question answered by reference to discussion of
the term "species" rather than by reference to experimental evidence?
If so, I'd be glad to take a crack at it.
Let me know your thoughts on this.