What is a species? Biologists are often frustrated by the diffuculty of defining what a species is, in an exact, scientific, unmistakable way. The official definition of a species is usually said to be: "If two groups of animals cannot produce fertile offspring together, then they are two species, otherwise, they are one species". This definition is usually modified by adding that if the species can produce fertile offspring via artificial insemination, but cannot mate because their sexual organs are incompatible, or because species A and species B have reproductive cycles timed so that they could never or very rarely mate, then they are two species. Sometimes other provisions are added. But there are some very big diffuculties with this. Take lions and tigers for an example. Lions and tigers never mate in the wild, even when an oppurtunity arises. They look different, fill different niches in the wild, have different inherited systems of social organization, hunting, and temperament. Yet when kept in captivity, it can be seen that Lions and Tigers readily mate, and produce comepletely fertile offspring. And there are other species of wild cat that we all consider separate species, yet they do produce fertile offspring in captivity. There is also the example of black bears, grizzly bears, and polar bears. Once again they do not mate in the wild, and no hybrids are found, yet, biologically, they are one species. They all produce perfectly fertile, healthy offspring together. In fact, biologists were sure that they were separate species before captive breeding experiments were done. And then there is the dog family. Dogs, Coyotes, Jackals, Wolves, Dingos, Dholes, Pariahs, and some species of fox (but very few species of fox) all produce fertile offspring together, and would thus be one species. Yet they all have different appearances, different in-born behavior patterns, fill different niches in the wild, and although they may mix a little in the wild, they hardly mix at all. And then there are chimps and humans. Officially, as far as we know, no-one has ever tried to produce a human/chimp hybrid. Yet many experts on chimp biology think there is a very good chance that such hybrids could be produced and be fertile. No-one doubts chimps and humans should be classed as separate species. And then there are tricky cases, such as buffalo and cattle. For years, no hybrids at all were produced despite numerous attempts. Then some hybrids were produced who were fertile with each other, buffalo, and cattle, and now we have a number of herds of various mixes of "beefalo". And there are also "theretically impossible" hybrids, even fertile hybrids. Such as cow/whitetail deer hybrids. Yes, cows and deer are really far from being related. Yet I have seen one such hybrid, and heard neighbors tell of many more, with photos. They are often odd-looking critters, but they exist. They are very rarely produced. Many groups of similar species can (and do in captivity) produce fertile offspring. Perhaps we don't have hybrid swarm obliberate species in the wild because:

1.) These species consider themselves different species, and either don't mate with each other, or persecute hybrids (such as excluding them from their group). One species might see hybrids, and the other species, as horribly ugly or even deformed by their standards of beauty (mate selection criteria). We've all seen how people, even the most enlightened people, instinctivly try to avoid, or persecute, people who look odd.

2.) Hybrids are not suited for either the niche of their father's species or the niche of their mothers species, by being in the middle, they are not competitive (survival of the fittest) in either niche. This would be especially true of wolf-dog hybrids, dogs are so physically inferior to wolves in the ways that count when living in the wild and hunting for a living (even though they look fairly alike to humans on the outside) that a wolf-dog hybrid would almost certainly not be anywhere near the standards needed for wolf survival, especially considering that many pure wolves are not up to the standards of surviving for long. But whatever the reasons, wild animals do often seem to have a strong sense of species, and hybrids are not common in the wild.