[wai-ming]

Species...the usual, can interbreed with one another to create fertile offspring.

Different species...cannot interbreed and create fertile offspring.

Question:

Are all dogs of the same species?

If not, where do they draw the lines between different "species" of dogs? Most dogs can interbreed and produce fertile offspring, but I just can't imagine a Saint Bernard and a chihuahua... They are both dogs, but are they of the same species?

[anguish]

I just can't imagine a Saint Bernard and a chihuahua

I think it's a sign of good mental health not being able to imagine that :p ... However, they can still interbreed and indeed create fertile offspring, even if the 'act' itself has become somewhat cumbersome due to the enormous difference in size.

The various dog breeds we have are very recent, from a evolutionary perspective, so they have yet to speciate. Given enough time I don't think many biologists would disagree that the Saint Berhard and Chihuahua are likely subjects for speciation though :)