The “coywolf”: a new species of canid?

At least three readers have pointed me to articles, one in The Economist and the other in Raw Story, arguing that a new species of canid, the “coywolf” (also called a “wolfote”) is emerging before our eyes as wolves (Canis lupus), domestic dogs, and coyotes (Canis latrans) all hybridize to form a distinct entity.

Such “hybrid speciation” is indeed possible, and has been seen several times in animals and many times in plants. (Wikipedia gives a good summary of the facts, and a longer treatment appears in Chapter 9 of Speciation, the book I wrote with Allen Orr. There’s also a short but useful paper by Richard Abbott and Loren Rieseberg, free online, here.)

One way is “diploid (or “homoploid”) hybrid speciation,” which is what’s supposed to be happening in the coyowolf. In that case, two plants or animals hybridize, and the hybrid undergoes genetic change via natural selection, a change that involves sorting out the different genes from the two parental species into a novel mixed genome. If this new genome has features that prevent its carriers from reproducing with the parental species (“reproductive isolating barriers”), it could form an interbreeding unit that would be considered a new species because its members mate only with each other, and not with the two parental species.

This appears to have happened in some fish, butterflies, and birds, as well as plants, but it’s not common. That’s because hybrids between existing species are usually at a fitness disadvantage, and also would likely be “mated to death” because a rare hybrid would be more likely to mate with one of the parental species in the area than to find other rare hybrids to mate with. Also, it’s unlikely that a mixed genome from two parents would undergo genetic change that could produce a genome producing reproductive barriers from those parents.

But that could occur if the new hybrid species has different ecological requirements from those of both parents. Such a difference might segregate the hybrid into a new area where the parents don’t occur, allowing it to survive and reproduce. After all, a single hybrid individual is not a species, which most evolutionists recognize as a population of interbreeding individuals that has barriers to gene exchange with other populations. A hybrid species, then, has to comprise a lot of individuals that breed with each other, but not very much with the parental species.

Indeed, new diploid hybrid species of plants often show unique ecological requirements that sequester them in a new habitat, and give them evolutionary “breathing room” to reproduce with each other, and evolve further changes that restrict gene flow from the parental species. The work of Loren Rieseberg and his colleagues on diploid hybrid sunflowers, some of which are restricted to extreme habitats like sand dunes, is perhaps the best example of this. Below is a photo of a new species of sunflower, H. anomalus (middle) which, as genetic analysis shows, formed after hybridization of the parental species (left and right). As you see, the new species also occupies a novel new habitat: sand dunes:

Another form of hybrid speciation is more common: polyploidy. In such cases, new species form by hybridization of two distinct species, but the hybrid is largely sterile because the chromosomes of the hybrid fail to pair (this is a requirement for formation of gametes). But in some cases the semisterile can produce offspring in which its entire genome is doubled, so that each individual now has a full genome from each parental species. The chromosomes in such a “tetraploid” can pair properly, and it can be fertile.

This form of speciation, called “allopolyploidy,” still faces the twin problems of hybrid speciation: the need to form an interbreeding population, and the need for some kind of ecological segregation to prevent the new tetrapoloid from being mated to death with nearby parental plants, producing sterile “triploid” offspring that doom the hybrid to extinction. But this form of hybrid speciation is fairly common in plants. Using data from Sally Otto and Jeannette Whitton at the University of British Columbia, Orr and I estimated that roughly 7% of new speciation events in ferns and 2-4% of speciation events in flowering plants involve allopolyploid speciation. Why this kind of speciation is much rarer in animals than in plants is still unresolved, but there are various hypotheses you can find in our book.

That’s just background on how new species can form by hybridization between existing species. Now what about the coywolf?

The following information is taken from the Economist article and some genetic from Wikipedia (the Raw Story appears to be a condensed version of the Economist’s article). I’m going by that information since a scientific paper on the coywolf doesn’t seem to have yet been published.

• Habitat destruction and the killing of wolves has been forcing coyotes and gray wolves (which are closely related: about 3.3 million years diverged) into closer proximity with each other and with human-owned d*gs, leading to hybridization between all of three canids and the production of animals with mixed genomes. Wolf-coyote hybridization is also promoted by the increasing rarity of wolves in the eastern U.S., so that they see coyotes as potential mates. (I call this the “prison effect”.) The fact that hybrids can include coyote, dog, and wolf genomes has been verified by genetic analysis.

• About 10% of the coywolf genome comes from domestic dogs, 25% is gray wolf, and the other 65% is from coyote. I have no idea how variable this mixture is among coywolf individuals.

• The “species” (we’ll get to whether it really is a species shortly) shows a mixture of morphological traits of coyotes and wolves. As The Economist notes:

“At 25kg or more, many coywolves have twice the heft of purebred coyotes. With larger jaws, more muscle and faster legs, individual coywolves can take down small deer. A pack of them can even kill a moose.

Coyotes dislike hunting in forests. Wolves prefer it. Interbreeding has produced an animal skilled at catching prey in both open terrain and densely wooded areas.”

Here’s a picture of one; you can see more in the video below:

• “Coywolves” are common, and appear to inhabit areas that aren’t much frequented by their two parental species:

“Purebred coyotes never managed to establish themselves east of the prairies. Wolves were killed off in eastern forests long ago. But by combining their DNA, the two have given rise to an animal that is able to spread into a vast and otherwise uninhabitable territory. Indeed, coywolves are now living even in large cities, like Boston, Washington and New York. According to Chris Nagy of the Gotham Coyote Project, which studies them in New York, the Big Apple already has about 20, and numbers are rising.”

Now for the million-dollar question, at least for me: Is the coywolf a new species, as many articles have implied? Indented quotes are from The Economist; my own comments are flush left. To answer this question, we have to discuss what evolutionists mean by “species”, and of course there are dissenters.

Whether the coywolf actually has evolved into a distinct species is debated. Jonathan Way, who works in Massachusetts for the National Park Service, claims in a forthcoming paper that it has. He thinks its morphological and genetic divergence from its ancestors is sufficient to qualify.

But morphological and genetic divergence from ancestors is not sufficient, for there are many sterile hybrids that occur in nature that are morphologically distinct from their ancestors (they’re usually intermediate), and also genetically divergent (they have genes from two or more ancestors), but they’re not species because they don’t form an interbreeding population that is reproductively isolated from the parents. It’s the bit in italics from the last sentence that must be satisfied before we can affirm that coywolves represent a new species. This, indeed, is pointed out by the Economist:

But many disagree. One common definition of a species is a population that will not interbreed with outsiders. Since coywolves continue to mate with dogs and wolves, the argument goes, they are therefore not a species. But, given the way coywolves came into existence, that definition would mean wolves and coyotes should not be considered different species either—and that does not even begin to address whether domestic dogs are a species, or just an aberrant form of wolf.

We needn’t concern ourselves whether domestic dogs are a species, as they are an artificially selected variant of the wolf and their reproductive isolation isn’t tested in a purely natural setting.  But the fact that wolves and coyotes produce occasional fertile hybrids should NOT be taken to mean that they’re the same species, for that hybridization was very rare in the natural environment before humans began degrading it. Many “good” species, like the polar and grizzly bears, can hybridize when their habitat changes, which shows that they were good species whose reproductive barriers at one time involved ecological differences that kept them geographically segregated. When the environment changes, “good species” can become a hybrid swarm. As Orr and I argue in Speciation, the production of a few hybrids doesn’t completely negate the concept of a species, and many distinct species do hybridize occasionally, sometimes producing fertile offspring (this happens in ducks). But often hybrids are sterile, so species remain distinct, and even fertile hybrids might be inviable or have difficulty finding mates. (That’s the case for ducks, I think: hybrid ducks are unattractive to females of the pure species who have a search image for a proper mate.) The Economist goes on:

In reality, “species” is a concept invented by human beings. And, as this argument shows, that concept is not clear-cut. What the example of the coywolf does demonstrate, though, is that evolution is not the simple process of one species branching into many that the textbooks might have you believe. Indeed, recent genetic research has discovered that even Homo sapiens is partly a product of hybridisation.

“Species” is a concept invented by human beings, but that doesn’t mean they’re not real and meaningful entities. The concept of a “star” was also invented by humans! Remember that stars can sometimes fuse together, or destroy each other.

Homo sapiens, for example, is a meaningful entity, and, at least now, doesn’t blur into other primate species. We do have genes from Neandertals and Denisovans, but those were probably members of our own species, as the hybrids were fertile; they were equivalent to the subspecies of animals and places recognized by biologists. Indeed, both Denisovans and Neandertals are usually placed in the same species as modern humans: H. sapiens.

When you look at the birds in your neighborhood, you won’t find any difficulty placing any of them in its group: pigeons, cardinals, starlings, sparrows, house finches, and so on. If species weren’t in some sense real entities, nature would be continuous and different people would place species boundaries at different places. That doesn’t occur (see Chapter 1 of Speciation for a long discussion on the reality of species.) Further, studies have shown that in most groups, including plants, it’s not hard to identify discrete entities. Nature is simply not a continuum with the boundaries between “species” being completely arbitrary.

To be sure, there is some blurring. There has to be when new species are evolving from different populations of a single species, or when there’s occasional hybridization, or when reproductive barriers are breaking down, as they seem to be in coyotes and wolves in some locales. All we can say is that speciation is a process, which can culminate in entities that are completely unable to exchange genes (“good species”), but that during that process, some entities can be more or less “species-like,” depending on gene flow. We will sometimes be faced with a judgment call, but very often, as with our own species or with lions, we aren’t.

There’s more information in this nine-minute clip, calld “Meet the coywolf,” from a PBS Nature documentary that appeared earlier this year:

What’s the upshot?  Given the information that coywolves seem to be semi-social and breed largely with other coywolves, that they seem to inhabit an ecological niche different from their parental species, and that they’re genetically distinct from either parent in possessing a hybrid genome, I’d say that the coywolf is going through early stages of hybrid speciation similar to that which occurred in the hybrid sunflowers. Before I’d call them a new species, though, I’d want to know how often they breed with either pure coyotes or domestic dogs, and how homogeneous the coywolf genome is . If they comprise a variety of diverse admixtures of coyote, wolf, and dog genes, so that their genomes haven’t become relatively homogenous among individuals, I’d be less inclined to call them species.

But, in the end, at this stage the question is a semantic one, for the coywolves do exchange genes with dogs and wolves, so they’re not what I call “good” species.

What’s more important is that we’re seeing, in the human habitat, a new form of animal emerging, one that, by combining genes from different species, has developed traits that allow it to exploit a new ecological niche. This is precisely what happened in the sunflowers that are now considered different species. So we may be seeing a case of speciation in statu nascendi—in the process of being formed. Only time will tell if coywolves will become so distinct, and so homogeneous among themselves, that they’ll deserve their own Linnaean binomial. Because of that, it’s premature for The Economist and The Raw Story to say that “a new species is emerging right before our eyes.”