This post was triggered by an article that said that the Northwestern crow and the American crow are, well. Interbreeding so much that they are no longer separate species.
The fact is that “species” is a kind of arbitrary designation.
What is the Traditional Definition of Species?
A species is defined as the largest group that can interbreed and produce fertile offspring. Or the biggest gene pool under natural conditions.
The problem with this definition is that it just, plain, doesn’t work.
Birds, in particular, will quite cheerfully mate with related species, including in the wild, and produce healthy and fertile offspring. The same with canines — wolfdogs and coydogs are also healthy and fertile. In felines, male hybrids are infertile. but female ones are perfectly capable of breeding.
There have even been instances of female mules, the hybrid known for its sterility, producing offspring. In ancient Rome a “pregnant mule” was a euphemism for a possible, but highly unlikely event.
In other words, there are too many exceptions to “interbreed and produce fertile offspring” for it to be a meaningful definition. Instead, the working definition of species tends to be a discreet population, regardless of hybridization.
Species is a cultural invention. The animals, on the whole, don’t care.
How About Some Fun Exceptions
So, let’s have some fun here with examples of situations where species boundaries haven’t proved particularly…well…secure. In fact, in many cases the border between species are more like those in Europe than the border with Mexico.
Honestly, the idea of species not being able to interbreed probably came from the extremely low levels of fertility of the one species hybrid most people knew about: The humble mule.
The idea to breed mules probably came about from natural hybridization between wild asses (don’t snicker. A donkey is domesticated, if it’s not, it’s an ass, correct term!) and horses or between wild asses and zebras. (By the way, these hybrids are called zebrasses or zonkeys and a hybrid between a zebra and a horse is a zorse). In the American west, the BLM occasionally catches a mule in their roundups…Mustangs getting it on with feral burros. Mules are infertile because horses and donkeys have different numbers of chromosomes. Thus, mules end up with an extra, mismatched chromosome that causes them not to produce proper sex sells.
Male mules (johns) are routinely castrated (to make them easier to handle), but the general feeling is that they would not be fertile if left intact.
Female mules (mollies) are, like mares, left intact. And every so often one gets pregnant. Not all cases of mule pregnancy have been proven. Mollies have been known to steal foals from nearby donkeys or mares because although they can’t have babies, they still like babies. (In fact, some horse breeders have started using mollies as embryo transfer recipients because there is essentially no risk of the foal being genetically related to the surrogate).
Genetic testing has proven that pregnant mules are even more interesting than we thought. They get around the chromosome problem by simply not using the genes from one of their parents. One fertile mule was bred to a horse and produced a horse and then bred to a donkey…and produced a donkey.
Still, there have only been a handful of cases, and only three were confirmed through genetic testing.
Speciation through Hybridization in Dogs
I already mentioned that dogs, wolves, and coyotes can and will cheerfully interbreed and produce perfectly healthy puppies.
Which is how we now have two kinds of coyote in the U.S. The western coyote is still a coyote. The eastern coyote? Well, that’s what happened to the few remaining wolves in the east. They bred with coyotes. There’s also a few dog genes wandering around in there too, no doubt from strays.
The eastern coyote is, thus, larger than the western coyote.
Oh, and it gets even more interesting. The more deer and large prey, the more wolflike the coyotes. Meanwhile, suburban coyotes seem to have a bit more dog in them.
In other words, what we see here is animals obtaining valuable genes by mating with other species. Or, in some cases, mating with other species simply to survive.
Hybridization is thus a tool of evolution. It’s much quicker to get that size gene by making puppies with another kind of dog than it is by going through the lengthy process of evolving it yourself.
And then There’s us
Where our range overlapped with other types of human, we cheerfully bred with them. Even many Africans have now been shown to have Neanderthal markers, no doubt because some people left Africa, saw what was outside, and decided to just nope right on back to Africa.
Our own ancestors, in other words, were quite willing to mate outside their own species…and the instinctive motivation was probably the same as eastern coyotes. By sleeping with Neanderthals, we picked up some of their genes for cold adaptation, for example. And also some of their culture for cold adaptation.
One interesting thing, though, is that the human Y chromosome shows no Neanderthal genes whatsoever. We keep looking. We haven’t found them yet. This might indicate that, like cats, when humans mate cross species only the females are fertile.
Which might explain the stories of “fairy” women abducting men, giving them a fun night, and then abandoning them exhausted. Was that a way to have babies when the men in your own community tended to shoot blanks?
Point is: Species is an arbitrary line in the sand. Animals don’t care. Humans don’t care when it comes to picking a partner, or don’t when we aren’t having taboos laid on us to keep us from picking a partner who’s skin has a little bit too much melanin.
Instead of considering species, maybe we need to consider the interactive web of life as a whole, which includes animals living their lives, choosing their mates, and adapting to the environment.
And we definitely should consider how much hybridization may have speeded up evolution in some places.