Most closely related species have evolved differences in courtship behavior (calls, displays) that prevent mating from ever taking place. It is believed that this is because hybrids are in some way less capable of surviving and producing offspring, so that individuals that are willing to mate with members of other species are selected against. In fact, this is why courtship behavior exists in the first place: so that two members of a pair can determine that they are in fact members of the same species, and will produce offspring that won’t be selected against.
This can break down in captivity, or in places where one or both species are rare, and no other member of their own species is available as a mate. Basically, it’s “beggars can’t be choosers.” In such cases hybrids, even completely fertile hybrids, can be produced. But the requirement for recognition as distinct species is that such hybrids are rare or don’t occur under natural conditions, not that they are sterile.
The more time two lineages have been separated, the less likely they are to be able to produce fertile hybrids, as genetic differences accumulate between them. Eventually they will be unable to produce viable hybrids at all, even sterile ones. But this process takes time, and sometimes only distantly related species like camels and llamas may still be able to produce hybrids.
No biologist would regard that as “successful” breeding. In evolutionary terms, producing sterile offspring is even worse than producing no offspring at all, because the individuals put time, effort, and energy into producing an offspring that was a dead end.
With all due respect, this being GQ and all, your personal opinion doesn’t mean much. Biologists have a definition of what a species is, and the most commonly used definition for animals is the Biological Species Concept. If two populations regularly interbreed and produce fertile offspring in the wild, they are usually considered the same species. If they don’t interbreed (or rarely do so), they are considered separate species. If two population can interbreed and produce fertile offspring, but they don’t do so because they are separated by some barrier, they will usually be classified as subspecies.
There are also ring species, where several populations have overlapping territories. Imagine 4 populations A, B, C and D such that A can interbreed with B, B can interbreed with C, C can interbreed with D, but A can’t interbreed with D. You do get gene flow between A and D, but not directly.
ETA: I started this before Colibri posted and just realized I didn’t finish. Most of this I learned from him, so hopefully I got it right.
My apologies. I was responding to a question where someone asked me what my definition of successful breeding was. Since I am not a biologist, I probably should not have answered.
