Defining species and subspecies is a complex task. Lot of ink gets used in peer-reviewed journals debating if a population is a true species, a subspecies, an ecomorph, or a mere local variant. There is no cut-and-dry definition for any of these classifications, which is actually one of the strongest pieces of evidence for evolution. If all things were specially created, it would be very easy to determine species statuses. You wouldn’t have all this debate at all. It would be very easy to settle.
Long-term readers know that I’m skeptical of considering dogs a separate species from the wolf, even if it is not politically correct in some dog training circles to say so. I’m also skeptical of species status of the red wolf and the “Eastern wolf,” which are almost certainly just hybrids with wolves and coyotes. I’m skeptical that the Island fox of the California’s Channel Islands is separate species from the mainland gray fox, but I don’t deny that it is a very unique subspecies.
Part of my reasoning for being skeptical of the Island fox’s species status is that it only recently derived from the mainland species. These two populations have split from a common ancestor only within the last 9,200 years and may have been introduced to the islands by humans only about 7,000 years ago. For those of you keeping score, dogs split from their most recent common ancestor with wolves at least 15,000 years ago, and when you realize that there are some populations of wolves, especially in the New World, that are even more distantly related to dogs than the common subspecies of Eurasian wolves are, you can see where this gets really tricky.
So if I’m not calling a dog a species, then I really shouldn’t be calling an Island fox one.
If you call an Island fox is a species, then it is perhaps the youngest mammal species on earth.
The only other species that comes close to it is the pygmy three-toed sloth. That species derived the Panamanian brown-throated three-toed sloth about 8,900 years ago, when the island of Isla Escudo de Veraguas became separate from the mainland. Unlike their more generalist ancestor on the mainland, the pygmy sloth lives solely on mangrove leaves, and it spends a lot of time swimming from mangrove to mangrove. It is 40 percent smaller than its mainland ancestor.
And there are only about 70 of them left.
This sloth and the Island fox are good examples of how rapidly simple barriers to gene flow can result in phenotypic divisions between related populations.
It is at this stage I realize that I have contradictions in my own concept of species. I think everyone who has looked at the issue does at some point.
I’m less willing to think of the Island fox as a species than the pygmy three-toed sloth. But the truth is they are both kind of in that nebulous area.
I’m pretty sure that if you caught a gray fox here in West Virginia and turned it out on the island of San Nicolas, it would mate with the island foxes. If it were a dog fox, I’m pretty sure it would dominate the island, not only because a mainland gray fox is quite a bit bigger than an island but also because these foxes are known for their very tight genetic bottleneck. These foxes have been able to keep their MHC haplotypes diverse and heterozygous because they have a way of picking up on these differences when they choose their mates. My guess is a big mainland gray would be in demand with just about any vixen on the island. Monogamy be damned.
But somehow, I think adding a brown-throated three-toed sloth to Isla Escudo de Veraguas would mess up the uniqueness of the pygmy three-toed sloth. Maybe the hybrids would be unable to live so well on mangrove leaves. Maybe.
Or maybe I’m just finding a way to rationalize it.
This isn’t an easy question. It’s not an easy answer.
We talk a lot about conserving endangered species, but the truth is we don’t have a very good concept of what a species is. We have a lot of different concepts for defining species– all of them good. But every single one of them leads to contradictions like these.
These contradictions are wonderful in science. They give us things to question, debate, and explore.
But when we come up with something called an Endangered Species Act, what exactly do we mean?
The law cannot handle such nebulous and contradictory definitions. Science explores natural phenomena. The law derives requires clear definitions, or lacking such clear definitions, case law that fleshes out the nebulousness. (At least in common law countries…)
I don’t have any good answers to these questions, and the truth is that no one really does.
That’s because at the core of it, evolution isn’t just something that happened. It is something that is happening, and our human brains, which try to make clear distinctions so that we can understand, intuitively cannot handle the gray area.
We know that the Urocyon genus of wild dogs is a very distant offshoot from all other canids. We know that three-toed sloths are not that closely related to two-toed sloths, which are the last survivors of the lineage that included those giant ground sloths that once roamed much of North America.
When we get to the finer distinctions between really closely related populations, we really don’t have good answers.
But it’s pretty clear that if we call the Island fox and the pygmy three-toed sloth species, they are very new ones.
Evolution can happen rapidly, but it usually doesn’t.
And islands have a way of making it happen fast.
Those concepts are fascinating in themselves– much more so on which Latin or Latinized binomial we use.
Similarly, I’m pretty skeptical towards considering the domestic cat a separate subspecies from the Near Eastern wildcat (Felis silvestris lybica).
So, the three mammals species that are younger than 15,000 years are the Pygmy Three-Toed Sloth (Bradypus pygmaeus), the Island Fox (Urocyon littoralis), and the mule deer (Odocoileus hemoinus). The Island Fox descended from the Grey Fox (Urocyon cinereoargenteus) and the Pygmy Three-Toed Sloth descended from the Brown Throated Three-Toed Sloth (Bradypus variegatus).
The mule deer (the youngest of the three deer species in the genus Odocoileus) came about from matings between white tailed deer (Odocoileus virginianus, the oldest of the three species) and the black tailed deer (Odocoileus columbianus, usually listed as a subspecies of mule deer). So, the mule deer came from the black tailed deer, not the other way around.
So, the mule deer black-tailed deer taxonomic dispute is inverse of that of the Near Eastern wildcat domestic cat taxonomic dispute. While the domestic cat is pretty much virtually genetically identical to the Near Eastern wildcat, the mule deer is genetically distinct from the black tailed deer.