Here's the thing with tiny dogs and huge dogs: Even if you put them on an island and wait 10,000 years, not only will they likely be the same species at the end, they'll be the same breed. Before reproductive isolation due to incompatible size splits them off from each other, the big dogs will very likely have gotten smaller and/or the small dogs will very likely have gotten bigger, and those descendants will very likely interbreed. Without human involvement they can be expected to 'return to the mean' over generations, sans environmental pressures keeping them from varying in size (maybe a prey animal only tiny dogs can get at coupled with a food source only big dogs can reach with a dearth of food sources for medium dogs, for example). On the other hand, they will probably be a subspecies or even a different species from all the other dogs in the world, because of their reproductive isolation from dogs outside the island.
I'm not anti-Christian. I'm anti-stupid.
