of a domestic sow mated with the wild boar. (They're technically still the same
species, Sus scrofa.) A hybrid was said to be an offense against the gods, possibly because the Romans sacrificed pigs at funerals and temples. Hybrids were dangerous to keep, and would not do.
Domestic pigs have curly or kinky tails, are bred to various colors, but often with white and pink, and run to fat. They also have smaller tusks than wild pigs do, particularly boars.
Finally, domestic pigs are not wild. They tolerate people and will accept mildly rough handling without attacking out of fear. However, before the advent of guns, wild boar hunts were held to be dangerous, as hunters had to get up close to spear the boar, and the boar could gore them. Boarhounds had to be quick, large, and strong. They usually hunted boar while wearing spiked collars or armor. How different from the pig that can be handled, petted, and raised for food!
The wild boar in the US is a hybrid-- the European wild board crossbred with domestic pigs gone feral, but the constraints of wild life has kept them to a wild phenotype, but perhaps more dangerous as domestic pigs gone feral tend not to be afraid of man, and that boldness can be passed on, along with any favorable genes, such as for increased fecundity.
The pig species (Sus scrofa) has many subspecies and also a few chromosomal races: variants in chromosome numbers or arrangements that may or not may prohibit interbreeding, but make a population distinct. In pigs, those races are interfertile.
In some wide-ranging species like Mus musculus domesticus (the house mouse), there are chromosomal races which have reduced fertility in hybrids(but they still can reproduce somewhat.) They are considered the
same species, nevertheless, since the chromosomal translocations are recent, and they are still interbreeding with other mice. Some combinations are more fertile than others, so there is a partial fertility barrier. But eventually these races may split apart and become cryptic species-- species that look almost identical, or so close that one simply looks like a subspecies of another.
We often think the lack of fertile hybrids defines two species as separate, such as mules, zebrasses, etc., but this is an accident of chromosomal incompatibility, and occasionally female hybrids (molly mules) are fertile and have borne foals to either horses or donkey stallions, which in turn had offspring. (The fertile hybrids are female, in accordance to Haldane's rule.)
Strangely, species can be very different, even be classified in different genuses, and still be interfertile. Captive big cats of different species have interbred with each other-- this site is a good guide-- despite that in the wild, there are real reproductive barriers: lions routinely kill the cubs of smaller species, and will attack smaller big cats.
Behavioral reproductive isolation is, in a way, more effective than genetic isolation, because it ensures no energy is wasted on matings or pregnancies that will fail to yield good cubs. But those barriers will fail, temporarily, if there are no good mate choices of the right species. Also, species separated by geography have no reason to have strong reproductive or genetic isolation. This is why pumapards have been created.
Leopards are Leo panthera. They have the same rough shape as other big cats, can roar but not purr, due to the shape of their voice box (pumas can't roar, although they can purr), and can crossbreed with all cats of the Leo genus, but they're the smallest of their genus. Pumas are Felis concolor (now Puma concolor)-- considered the biggest of the small cats. They grew large after the large cats went extinct in North America, but their heads did not grow quite as big as their bodies. They may be the closest living relative of the cheetahs (a large cheetah went extinct in North America, too.) This may be why pumpards are so small: half the size of their parents; the leopard has evolved smaller size relative to its ancestors, while the puma is a small cat with genes boosting its size. The latter genes are cancelled out by the leopard genes.
Servals, wild longlegged African cats, have been hybridized with domestic cats to produce the Savannah Cat, a partly fertile hybrid, despite being of different genera, not just species. However the first generation of males are always sterile, and it may be that the breed, if it endures, will wind up being mostly domestic cat in origin. (The Savannah website I link to has a pretty good explanation of the various levels of reproductive isolation.)
Bengal cats are hybrid animals produced by the crossing of the domestic cat with the asian leopard cat (another inter-genera cross), and only Bengal cats with a single leopard-cat ancestor at least four generations back (F4 hybrids) are considered temperamentally suitable for pets. No lynx/bobcat-domestic cat hybrids have ever been proven.
Those hybrids are relatively small animals, and witnessing predatory behavior is part of the pleasure of owning cats; although what is cute in a small cat grows far less humorous if the animals are big enough to kill a dog, a child-- or even maul adults.
Grey wolves have many subspecies-- populations isolated from each other and adapted to local circumstances--like the tiger and other large predators. The existence of distinctive subpopulations of a species (subspecies) means we can speak of intraspecies hybridization, or hybrids within a species.
If you've heard of ring species, that's a species that varies according to environment, but maintains a continuous population made up of mutally interfertile subpopulations,without major reproductive isolation. The endangered Arabian wolf is a desert variant of the grey wolf-- it has short hair, large ears, pale coloring, lanky legs, short hair, and may be as light as 40-50 pounds-- all the better to shed heat with--while the Timberwolf of Canada may go over 120 lbs, has dense fur, smaller ears, huge paws for snow, etc. Those are examples of local adaptions that don't make the wolves very different beyond what genetic drift caused by geographic isolation might do: they both hunt small and bigger prey, mate-bond and raise pups in similar fashion.
Humanity is considered a ring species: we have the physical diversity to fit different environments from the Tropics to the Arctic, but we have no actual "races"-- just intergradations as populations adapt to their given environment and marry around them to slightly different populations. Our cultures are even more diverse than our bodies, thanks to our brains, though, allowing us to adapt to places our bodies aren't ideally suited for.
Until recent DNA analysis, the Indian and the Himalayan wolf were thought to be two subspecies of the grey wolf. But they have been separate and distinct from all other grey wolf populations for over
500,000 years, and from each other. These wolves are two separate species, Canis indica and Canis himalayenesis. Moreso, despite long co-existence with dogs, there is no genetic evidence that these wolves have interbred to or contributed to the domestic dog gene pool in India.
Dogs (Canis familiaris) are now known to have been domesticated a subspecies of the gray wolf (Canis lupus.) The most recent research indicates all dogs are descended from a single subpopulation of wolves in East Asia around 15,000 years ago or more, but dogs are reproductively, cognitively, behaviorally, and morphologically distinct from all other gray wolves. In fact, it seems the particular subspecies leading to dogs may have been distinct from other grey wolf populations for as long as 150,000 years!
Dogs also have radiated into many subtypes, partly due to selective breeding, but also for the environment they live in: the Aleut dog has thick fur like the Timberwolf, but a curly tail and doggy nature. DNA analysis has found that all New World dog breeds are descended from Old World wolves-- specifically, that small originating population somewhere in what is now China. So the oldest and purebred sled dog breed, the Canadian Inuit Dog, does not have Timberwolf blood, despite a similar appearance. Rather, the cold climate has forced the evolution of a similar coat and appearance to the local wolves, while retaining primitive dog behavior and nature-- convergence. Behaviorally, there is no convergence: the Canadian timberwolf lives in packs and roams vast territority; Inuit dogs stay near villages, in packs/ teams, often staked out in small territorities (and are pack-focused, like hounds.) They eat scraps and leftovers, rather than hunting their food.
Domestic dogs also have developed the ability to digest grain, making them distinct from the wolf. However, like their humans, they vary according to geographic origin in how well they may digest grain. Northern breeds do poorly on a grain diet, while herding breeds from Europe may become ill on a meat-heavy, starch-free diet. They also have adapted to a diurnal schedule favored by humans, unlike the crepulscent hunters other grey wolves are.
We won't go into the numerous cognitive, physical, and behavioral differences caused by dogs' neoteny, but those differences are what make dogs not only handable by people, but able to read people and stay bonded to people. Any introgression with other grey wolves threatens these key mental adaptions.
Practically, if you have a subspecies that has spread around the world in the same geography as the old species, but which occupies a distinct ecological niche (and domestication is a very distinct ecological niche); which other subspecies are not adapted for, and accordingly rarely interbreeds with the resident subspecies, this subspecies is already in effect a separate species.
1) Dogs and most grey wolf subspecies are physically separated and rarely meet due to their different ecological niches.
2) Different breeding cycles (domestic dogs cycle twice a year, wolves one.)
3) Behavioral isolation, in that a wolf is often far more likely to kill a dog instead of court it.
4) Morphological or mechanical isolation may exist: most dogs are too small to mate with a wolf; only a subset of domestic dogs are big enough to physically mate with wolves.
The taxonomic revision of Canis familiaris to Canis lupus familiaris, while confirming the ancestral relationship of the domestic dog, wrongly implies dogs are the same species as wolves.
Dogs are bred not to be wolves, but rather to live with us and do many jobs. While our ancestors didn't have fancy genetic tools, they knew how to tell a wolf from a dog and excluded or killed dogs that were too wolf-like or dangerous, consistently domesticating dogs for over 15,000 years. The word "hybrid" could have just been easily applied to a wolf-dog hybrid-- had they been common at all, which all evidence suggests they were not. Even our the proto-dog wolves that we domesticated were distinct from all other subspecies of wolves.
Hybridization may no longer be an offense to the gods, but wolf-dog hybridization is illegal in many states. Wolves are endangered and shouldn't be used in vanity wolf-dog breeding schemes. Today, we have genetic tests to find out if a dog is really a wolfdog or not. Eventually, we may be able to spot the specific subspecies of grey wolf used, too.