Rath on 23 November 2009
@stof. If you killed everything apart from the Chihuahua's and the Great Danes you would probably have to consider them seperate species, there would be a barrier between them breeding.
Also many different species can produce fertile offspring, there only has to be a barrier to them breeding, range for example. That is why wolves and dogs are different species despite being able to produce fertile offspring. Wolves occupy a range that dogs don't.
Same goes for a huge number of species.
