Its actually not about whether they can interbreed, its about whether
there are barriers to reproduction.  Thus you can have hybrids, so long
as the populations are freely interbreeding theyre distinct species.  For
a strong advocacy of the Biological Species concept, the book Speciation
by Jerry Coyne is a decent place to get started.

Nicholas Johnson

The University of Chicago
Committee on Evolutionary Biology

On Fri, 8 Jul 2005, Monfils, Paul wrote:

> Fred,
>
> If interbreeding is the sole (or principle) criterion for speciation, where
> do hybrids come from?  Hybrids should be impossible if species, by
> definition, cannot interbreed.  Are the domestic dog, the wolf and the
> coyote conspecific because they sometimes interbreed?
>
> Paul M.
>
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