How are chihuahuas and St. Bernards the same species?
The same way Verne Troyer (81cm tall) and Andre the Giant (224 cm tall) are the same species, or Shaquille O’Neil (216 cm), or Gwendolyn Christie (190 cm).
Physical differences in appearance do not immediately relate to a difference in speciation or genetic compatibility.
Dogs are all the same SPECIES (canis familiaris) but different BREEDS as humans have looked for different traits in different dogs and promoted them through selective breeding to give the different breeds we have today.
In the same way cabbage, broccoli, cauliflower, kale, Brussels sprouts, collard greens, Savoy cabbage, kohlrabi, and gai lan are all the same species, Brassica oleracea, but bred for different properties.
Seeing as dogs are descendants of wolves, the thing that perplexes me is how did wolves eventually filter their appearance from breeding? Wolves have a very homogenous look about them so if they are the original “source”, where did the bear like features of a Chow Chow come from? And where did the rodent like features of a Chihuahua come from?
The technical scientific definition of species is pretty different from how it is used colloquially. Technically any two animals that are genetically similar enough to theoretically breed and produce viable children are the same species. So for examle, llamas and alpacas are technically the same species since they can crossbreed, while humans and chimpanzees are different species as they cannot.
This doesn’t mean members of the same species can breed reliably or without complications, it just means it’s genetically possible.
So, because of this, basically all dogs are the same species, including chihuahuas and St Bernards.
They are close genetic relatives and can – with some difficulty – produce viable offspring.
Arguably even *wolves* and chihuahuas are the same species (some classifications list dogs as their own species, and some list them as a subspecies of wolf). Like most biological classifications, these lines can be a bit fuzzy around their boundaries, but all dogs are quite closely related to one another (their last common ancestor is only a matter of thousands of years ago, which is much more recent than it is for most species).