Natural History

One of the most interesting pieces of evidence for speciation through evolution is the ring species concept.

Ring species are several interbreeding populations that can generally interbreed, but there are always two populations that make up the “end series “that are too genetically divergent to interbreed. The populations gradually range from one “end” to the other.

Such is the case with the Ensatina salamanders, which range through the coastal and forested regions of California. Those that came down the coast evolved to look like the poisonous newts of the Pacific Coast, while those that came down the forested regions of the Sierra Nevada evolved splotches for camouflage.

Their distribution forms a horseshoe around the Central Valley of California, which has acted as a geographic barrier. The two populations at the southern end of the horseshoe cannot interbreed. These two populations actually overlap in range, but they do not produce offspring. They have become too genetically distinct. And any hybrids that happen between the potentially interbreeding newt mimic populations and camouflage populations are poorly adapted to be either.

Source.

Something very similar is happening with dogs. The reproductive barrier isn’t a geographic one. It also isn’t the result of having closed registries, because even dogs within closed registry breeds still exchange genes with other dogs, whether the breeders like it or not. Closed registries could become a reproductive barrier that could lead to the development of a ring species, but another factor is probably much more likely to be the cause of the first domesticated ring species.

The factor is size.

It is likely that all the dog breeds are going to be able to interbreed with each other, closed registry or not. However, an exception exists at the two extremes in size. Very small dogs cannot breed with giant dogs unless someone uses AI. It is simply mechanically impossible for a male chihuahua or yorkie to mate  with a Great Dane or mastiff. It would be a near death sentence if a female toy breed tried to mate with a giant dog. These two extremes are genetically isolated from each other and probably will be forever.

Now, toy dogs can breed with large dogs. I know of several goldendoodle breeders who use toy poodle studs for their golden bitches without having to use AI. So large and very small dogs will not be reproductively isolated. Large dogs can cross with giant ones with very little trouble, so they will not be reproductively isolated. And there are always stories of relatively small dogs mating with big ones. For example, one of my childhood beagles knocked up a dobermann. He was a medium-small dog, and he was able to exchange genes with a very large dog with no human assistance. (The dobermann’s owners didn’t want that breeding to happen at all, so there was no assistance.)

But no one is going to find a Great Dane/Chihuahua mix that happened without human intervention.

Over time, those two extremes are going to become so genetically distinct from each other that they can’t interbreed at all.

And thus, we would have the first artificially created ring species.

It is not that far fetched.

It is what is likely to happen, but it is going to take a while.