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Archive for the ‘Taxonomy’ Category

coyote wolf cross.PNG

An F1 cross between a gray wolf and a coyote that was produced through artificial insemination.

A few months ago, I wrote about a discovery that two species of howler monkey have evolved greater genetic divergence in a hybridization zone in southern Mexico. The hybrids were less fit to survive or reproduce, so natural selection has favored those individuals in both species that were genetically more divergent where their ranges overlap. This phenomenon, known as “reinforcement,” is a powerful tool that maintains both species as distinct.

I have been thinking about how this phenomenon may have played out in wolf evolution in North America.  We have found that gray wolves across North America have at least some amount of coyote introgression, which has been revealed in several full genome comparisons.

The wolves that have most evidence of coyote introgression are those that live in areas that were not in the historic range of coyotes, while those with the least coyote introgression tend to be in the areas where gray wolves and coyotes were sympatric.

It is possible that something like reinforcement went on with wolves and coyotes living in the West. Hybrids between gray wolves and coyotes were probably less likely to be able to bring down large prey or were too large to live on small game, which is the staple diet of most Western coyotes. Over time, reinforcement through natural selection could have caused greater genetic differences between Western wolves and coyotes, and Eastern wolves were without coyote and thus never developed these greater genetic differences.

When coyotes came into the East, they mated with relict wolves, so that we now have whole populations of wolf with significant coyote ancestry.

Now, this idea is not one that I find entirely convincing. One is that ancient mitochrondrial DNA analysis from wolves in the East suggests they had coyote-like MtDNA, which, of course, leads to the idea that the wolves of the East were a distinct species.

Further, the discovery of the recent origins of the coyote makes all of this much more murky.  Again, reinforcement is a process that is only just now being sussed out in the literature, and gray wolves and coyotes are unique in how much introgression exists between them.  Their hybridization has essentially been documented across a continent. The only wolves that have no evidence of coyote ancestry live on the Queen Elizabeth Islands of the Canadian arctic. No coyotes have ever lived on these islands, so they have never introgressed into the wolf population.

The howler monkeys in the reinforcement study hybridize only along a narrow zone in the Mexican state of Tabasco. They are also much more genetically distinct than wolves and coyotes are. The monkeys diverged 3 million years ago, but the current estimate of when gray wolves and coyotes shared a common ancestor is around 50,000 years ago.

So gray wolf and coyote “speciation” is a lot more complex than the issues surrounding these monkeys.

But reinforcement is something to think about, even if it doesn’t fit the paradigm exactly.

 

 

 

 

 

 

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arctic fox, swift fox kit fox

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.

 

 

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american-robin

American robin (Turdus migratorius)

The only thing American robins have in common with European robins is that they both have reddish orange breasts.

european-robin.

European robin (Erithacus rubecula)

American robins are classified as “true thrushes” in the genus Turdus.

The European robins were once thought of as thrushes, but now they are classified with the Old World flycatchers.

Behaviorally, the two species aren’t exactly alike, and the American robin is much larger than the European one.

However, if European explorers had really been thinking (or thinking the way I do). The American robin is almost exactly like another species of true thrush that Europeans know very well.

I’m talking about the European blackbird.

Now I’m a North American. I was raised where the American robin was so common that you often saw them hopping around on lawns in search of worms.

A few  years ago, I was in London’s hide park, and I saw a very similar bird to the American robin. Its behavior was almost identical when searching for worms. It is roughly the same size and shape. In fact, I think of a blackbird as an American robin covered in soot.

If you don’t believe me, have a look at a blackbird.

The common blackbird (Turdus merula)

The common blackbird (Turdus merula)

Both of these species are true thrushes and are classified in the same genus. However, in the public mind, I don’t think many people think of them as similar. After all, American robins live in North America.  The blackbirds are native to Eurasia and North Africa, although a few of them have been blown to the North American coast.

Further, I have a hard time telling these species apart if I am shown a picture of an albino or partially albino specimen of either species. Strangely, albinism isn’t that uncommon in either species.

This bird is a partially albino robin.

This bird is either a leucistic blackbird or a partial albino one.

I have no idea of why early explorers to North America didn’t think of the American robin as an American equivalent of the European blackbird. But then Europeans though the red deer relative of North America was an “elk.” They called the actual elk the moose. They also thought the large bison of North America was very similar to the Asian and African buffalo but ignored its similarity to its close relative the  wisent. They called the pronghorn an antelope and the mountain goat a goat. The coyote was placed with the jackals when the jackals had their own genus, and the gray fox and island fox aren’t closely related to the true foxes.

These early explorers really confused us with their bizarre classifications, but it makes sense. I seriously doubt that the average English explorer would have ever seen a wisent or a Scandinavian elk. So it is easy for someone like me to sit back here and call them fools for not having all the information that we can easily access today.

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