Holy crap, that’s some pretty amazing hybridization! Fluffly down pillows and milk and cheese, all from the same animal!
Imagine a flock of them perched above your newly-washed car.
Sailboat
And eggs.
Examples have been given here of hybrids that could reproduce and those that cannot (mule, jenny). What is the difference? Genetic distance between the parents? Can you predict before the mating that the offspring will be sterile?
This is a matter of species definitions. At the moment there is considerable debate between proponents of the Biological Species Concept, and those supporting the newer Phylogenetic Species Concept. The decision made under each concept could be different. And even under the BSC, how the two forms were classified would depend on the details of the interaction.
The BSC demands reproductive isolation of the two forms. Under the strictest interpretation, this would mean that they do not normally produce any fertile hybrids in the wild. (This is the standard by which Baltimore and Bullock’s Orioles were lumped.) Under a more lenient interpretation, it just means that there is no significant gene flow between the two populations. This would be the case if hybrid zones were narrow, or hybridization occurs but is infrequent - genes are not passing from the main body of one species into the other. (This is the standard under which the orioles were re-split.)
If two populations do not naturally come into contact, there is no way to rigorously apply the BSC. In this case, it’s a judgement call by taxonomists. They have to decide whether or not the differences between the two forms are sufficient to preclude regular reproduction if they did come into contact. This is usually done by comparing the degree of similarity of the two forms in question to related species that do co-occur, or to subspecies that do interbreed. It is possible to do experimental tests as well, for example, by playing courtship calls of one form to the other to see it they respond to them or ignore them. Nowadays genetic tests can be brought to bear as well, to see if they are as genetically different as known “good” species.
Under the BSC, if chimps and bonobos, presently considered to be good species, came into contact and freely interbred, they would be reclassified as belonging to the same species. However, if they interbred only in a narrow zone of contact, or if hybrids were rare, each would be held to be retaining its genetic identity, and under present standards they would continue to be classified as two species.
The PSC doesn’t care about reproductive isolation. As long as two forms represent separate evolutionary lineages, and can be reliably distiguished from one another, they are considered to be species. The PSC generally considers all isolated forms, as well as distinctive subspecies, to be good species. It would not only recognize bonobos as a separate species, but also all the subspecies of regular chimpanzees as separate species. The PSC would continue to recognize chimps and bonobos as separate species after they came into contact, unless interbreeding was so widespread that the two forms could no longer be reliably distinguished. (The important factor being the distinguishablity, not the interbreeding.)
I am glossing some things over, of course, but this is the gist of the two concepts.
It is very difficult to predict a priori. Usually the greater the time the two forms have been isolated and the greater the genetic distance, the less likely they are to be able to produce fertile hybrids. However, there are some very closely related forms that do not produce hybrids at all, and some quite distantly related forms that do produce fertile hybrids. There is a pretty good correlation, but it is not perfect.
One factor that helps prevent fertile hybrids being produced is differences in chromosome number, as in the case of horse vs donkey. But some forms with different chromosome numbers are completely interfertile.
(Nitpick: the other horse x donkey hybrid besides the mule is the hinny, not jenny. A jenny is a female donkey, the male being a jack (jackass). A mule is a cross between a male donkey and a female horse; a hinny is a cross between a male horse and a female donkey. Both are normally sterile.