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Obviously, a very large dog would have difficulties mating with a very small dog and vice versa.

big dog and small dog

But putting that problem aside (using, say, insemination), considering the large variation of dog breeds, are there any two breeds that are so different genetically that they can't produce viable offspring together?

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Isn't that what distinguishes species from breeds/races? Whether they can reproduce with each other? –  Armatus Jun 4 '12 at 9:26
    
Wolves can breed with dogs, still they are considered different species. –  Zano Jun 4 '12 at 9:49
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@Zano: wolf and dogs are the same species (canis lupus), different subspecies (canis lupus familiaris vs canis lupus lupus). Dogs are domesticated wolves. –  nico Jun 4 '12 at 12:41
    
@nico: Fair enough. –  Zano Jun 11 '12 at 16:04
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up vote 7 down vote accepted

I actually found some sort of reference for this. Apparently in the case of a beagle and irish setter pairing, they had a lot of difficulty in producing pups, but as breeds have a lot of genetic quirks, this might be due to a genetic accident; this is probably a case of mutual infertility rather than speciation. There are probably mutations in their genomes which are causing non viability in the pups the way that it does with some human parents who have difficulty having children - an immunological incompatibility or traits which cause most offspring non-viable.

In fact the discussion points out that the species designation does not always mean they cannot produce offspring, but simply do not. Coyotes and wolves will produce offspring too if they are encouraged to do so, but are competing and antagonistic in the wild and so never do mate in practice.

So the answer is probably no. (the beagle/setter pups did show up, but with a much smaller litter than usual with just 2 pups).

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I was looking for something like this. The definition of species was never an issue as far as I was concerned. –  Zano Jun 4 '12 at 14:11
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