Yes. Do a google search for “Ring Species” for real-world examples. Technically X,Y, and Z would all be considered to be the same species, with intra-species variation enough to prevent X and Y from breeding.
By definition, a species is a group of animals that can inter-breed, so the answer to your question is no.
In any event, what does this have to do with evolution. The claim of evolutionists is not that Animal X bred with Y to produce Z, but that X bred with X and, over the course of generations, the offspring mutated enough to become X+. Add up enough plusses and it’s an entirely new species.
I’m not sure whether it occurs as a “can not” or as a “do not” (and it requires a lot more than just three individuals), but there is a phenomenon known as a ring species in which two populations are known to not mate, but each mates with neighboring species so that you can create a “ring” in which every group except the final two interbreed. This is most common example given are the herring gulls.
There have been cases of small male dog/large female dog. After a report of one such incident, Tommy Newsome (asst. band leader on Johnny Carson’s Tonight Show) quipped “Somebody probably put him up to it.” (I suspect that wasn’t spontaneous.)
Yes, dogs are all one species. All dogs are interfertile with each other, at least theoretically. But I remember when I was a kid, a large male dog jumped in our yard and mated with our small Basenji when she was in heat. If she had carried those puppies to term she probably would have died.
She was not able to mate successfully with a dog of breed A, but she would have been able to mate with a dog of breed B that was smaller than A but larger than a Basenji. And a female of breed B would have been able to mate successfully with dog A. Therefore A and B could mate, B and C could mate, but A and C could not mate, satisfying the conditions of the OP.
In ring species like this, they are all considered one species, even though not all of the populations can interbreed, because genetic information can still flow from one extreme to the other through intermediate populations.
In your example, a mutation in X could get to Z through Y.
It would make them a different species if you defined species according to the Biological Species Concept. At first BSC seems objective. But is it? The thing is, there are many things that prevent organism A from mating with organism B that have nothing to do with their biology, and if you used artificial insemination you would create a fertile offspring.
But if this never actually happens in a real world population, are they still separate species? If Lions and Tigers could mate and produce fertile offspring, but never do so because they have different mating seasons, live in different parts of the world, and live in different habitats, are they really the same species?
Species is like pornography. Biologists might not be able to define it, but they know it when they see it. Except that not everyone sees it the same way. And when you try to write objective rules you find that the rules sometimes lead to strange results.
The case with the domestic dog is further complicated by the fact that it isn’t a natural species to begin with - it is the product of artificial, rather than natural (in the Darwinian sense) selection. Because the species wouldn’t even exist as we know it were we not directly involved, it becomes even more difficult to pin down its status as a species or group of species. The BSC requires successful breeding under natural conditions - where does this leave Fido?
Identifying species is, as Lemur pointed out, largely subjective because lineages exist as a continuum. We can identify certain instances wherein the organisms are certainly diffrent species (e.g., dog vs. moose), but as we examine populations wherein the organisms are more and more closely related (e.g., dogs vs. wolves, or even chihuahuas vs. mastiffs), the certainty begins to disappear.
No, it does not make them different species by the Biological Species Concept. The basic idea, according to the BSC, is that species represent groups of organisms among which there is regular gene flow. Just because two particular individuals within the population can’t interbreed doesn’t make those individuals members of different species.
As Darwin’s Finch states, the case of the domestic dog is anomalous because it is not a true species, but instead is a form produced by artificial selection by humans, and this goes for its many breeds as well. However, for the sake of argument, let’s look at it as if it were a wild species, because the Chihuahua-St. Bernard example does illustrate the principle behind “ring species”.
A Chihuahua can interbreed with, say, a terrier, a terrier with a poodle, a poodle with a retriever, a retriever with a mastiff, and a mastiff with a St. Bernard. Therefore it is possible for genes to be transferred from Chihuahuas to St. Bernards over several generations, and therefore they are not separate species.
However, if something caused the extinction of all intermediate breeds of dogs, then, under the BSC, it would be proper to regard Chihuahuas and St. Bernards as separate species, since they would no longer be able to transfer genes. And it is quite likely that this sort of process has produced some of the species we see in nature (although rarely if ever with the extreme differences we see between Chihuahuas and St. bernards.)
Well, I have only a duffer’s understanding of the science, but I think that the Chihuahua and the St. Bernard’s problem isn’t genetic incompatibility but rather womb space. If their pup were brought to term in a surrogate bitch, everything would be fine. Whereas the ring species (or rather, the ends thereof) have genetic differences, not merely physical (or if you will, mechanical) differences which preclude viable, fertile offspring even if a surrogate mother were available.
In the ring species mentioned, such as the gulls, the primary barrier to interbreeding is probably behavioral. The end subspecies in the ring have different courtship behavior, and thus don’t recognize each other as the same species and accept each other as mates. I suspect that some would be completely interfertile in captivity, or if artificially inseminated.