Canid taxonomy and evolution have long-stand ining debates, but one I don’t think has been discussed much is the evolution of the arctic fox-kit fox-swift fox clade.
For most of my life, there was always a long standing debate as to whether kit foxes and swift foxes are distinct species. The current thinking is the they are distinct but closely related species with a very narrow hybrid zone in parts of Texas and New Mexico.
And for most of my life, arctic foxes weren’t even considered part of the group. Indeed, they were considered so different from other foxes that they were placed in their own genus (Alopex). We’ve since discovered that arctic foxes have mitochondrial DNA sequences that are very similar to swift and kit foxes, and in terms of their mtDNA, they are as distinct from kit and swift foxes as they are from each other.
Since then, arctic foxes have been classified within genus of “true foxes” (Vulpes), with the arctic fox being V. lagopus, the kit fox as V. velox, and the kit fox as V. macrotis.
Broader genomic analysis has revealed that the arctic fox is actually quite closely related to the kit fox, and analysis of mitochondrial DNA suggests that arctic and swift foxes diverged only 250,000 years ago. If similar results are confirmed in nuclear DNA comparisons, it means that these foxes diverged from each other after Old World and New World red foxes split. A recent genome-wide study revealed that red foxes from North America diverged from those of the Old World 400,000 years ago, and in recent decades, it’s generally been accepted that red foxes are the same species. (There is currently a move to split them into two species).
Now, it’s pretty obvious that a similar study needs to be performed on swift, kit, and arctic foxes, and there is actually an obvious question that one of these studies could answer:
Are arctic foxes polar-adapted swift or kit foxes or are swift and kit foxes arctic foxes that have adapted to the Great Plains, Rockies, and arid regions of the West?
Arctic foxes are found in the arctic of North America and the arctic of Eurasia. They come in two basic phases: white, which sheds out to brown and white in summer, and blue, which sheds out to blackish gray in summer and is gray-tinged in winter.
Arctic foxes could be descendants of swift or kit foxes that wound up adapting to a polar environment, which then allowed them to access the Old World. However, there is a paleontology study that says arctic foxes evolved in the Himalayas. I am a bit skeptical of this study, because it seems to contradict the genetic data that connects arctic foxes with swift and kit foxes of the Americas.
However, if swift and kit foxes are temperate-adapted arctic foxes, then it could be possible that arctic foxes did evolve in the Himalayas. They came across the Bering Land Bridge, and then some of them became isolated in environments that became more moderate in climate, and they lost their adaptations for changing their coat colors. After all, least weasels, long-tailed weasels, and stoats (ermines or short-tailed weasels) have some populations where the animals turn white in winter, and populations where they don’t.
It could be that this is the real difference between arctic and swift/kit foxes is they just represent divergent populations where some populations turn white and some don’t. We only think of them as separate species because they are quite geographically different from each other.
It also could be that these animals are actually more genetically distinct from each other than we’re currently thinking, but preliminary genomic analysis suggests a very close relationship between kit and arctic foxes.
And those two are much more geographically isolated from each other.
So we do know that these three fox species do form a clade within the Vulpes genus, but how they exactly fit together is a good question. Maybe they actually are more closely related to each other than Old and New World red foxes. Maybe they aren’t.
And we don’t know which type came first.
But we can answer these questions, and we can answer them the same way we figured out that red foxes in North America aren’t derived from English imports from the colonial period.
This is a study that I’m pretty sure will be done but just hasn’t yet.