I understand this question might fall into opinion based, but I'm trying to write it so that it meets the "Many good questions generate some degree of opinion based on expert experience" part and not get closed.
A closed creationist question about directly observing the evolution of new species made me think about just declaring dog breeds as new species just to provide an answer to that question. Turns out I'm not the first one to think of it, and to be honest I probably read that article and forgot about it.
That article quotes geneticist Jerry Coyne as saying:
“If somehow the recognized [dog] breeds existed only as fossils, paleontologists would consider them not one species but many—certainly more than the thirty-six species of wild dogs that live in nature today.”
There is also this question about breeds of dogs being different enough they can't breed. The answer posted to that question discusses attempts to breed beagles with irish setters, and having great difficulty.
I'm aware that the commonly used definition of different species is individuals who can't interbreed and make a fertile offspring. However the Scientific American article I referred to above mentions that animals are often classified as separate species because they are separated by a geographical barrier, and if they could mingle they would make fertile hybrid offspring. The same articles mentions fruit fly research that bred two different species of fruit fly and made fertile offspring, even though they would never interbreed in the wild.
This is speculation, but let's imagine taking an island and putting several breeds of dogs on that island. Many of those breeds would easily interbreed and create mutts. But I'm pretty sure there would some sorting, the large dogs like st. bernards and great danes aren't going to easily interbreed with tiny dogs like chihuahuas. If we came back to this island after 50 or 100 years, we'd probably see small dogs and large dogs (assuming the island had decent ecological niches for both sizes of dogs to survive in)
Creationists like to ask why we haven't observed the evolution of a new species and don't want to listen to arguments about the fossil record or DNA sequences. If we just define some dog breeds as different species, we'd have an easy answer. I think some dogs would at least come close to meeting that definition for these reasons:
- Dog breeds with large size differences would have great difficulty breeding.
- Some pairs of breeds seem to have trouble interbreeding (Beagles + Irish Setter).
- Some animals recognized as different species can create fertile hybrid offspring.
So why don't we recognize some dog breeds as different species?