Did you know that one of the wolf species protected under the Endangered Species Act isn’t actually a unique species?
That is the conclusion of a recent paper in Science Advances (Open Access) showing that, among other interesting results, the Red wolf is a very recent hybrid of wolves and coyotes. The paper used whole genotypes of wolves from around the world to come to several conclusions, the biggest being that the data do not support the Eastern and Red wolf subspecies and that North American wolves and coyotes split very recently, only 6,000-117,000 years ago!
But hybrids are sterile right? That’s why mules can’t breed.
Yes, many hybrids are sterile, but not all. Hybridization is seen as a larger and larger part of the way new species are formed. Along with physical or behavioral separation hybridization can be a powerful way to recombine genes and create organisms adapted to new niches.
So what makes a species then? If you were to ask people overwhelmingly you would be answered with, “animals that look the same”, or “animals that can breed with each other.” These ideas each come from different concepts of what defines a species, and while there is plenty of agreement about how to define a species, biology is messy and often doesn’t fit the rules we lay down. The Wikipedia entry on “Species problem” gives some idea of how difficult it can be to define a species, with seven sections and eight subsections. Basically, scientists have a pretty good handle on how to define species, but it can get messy when you look under the hood.
The authors of the new wolf article explain it well in a quote from a companion article in Science:
“People think that species should be genetically pure, that there should be tidy categories for ‘wolf’ and ‘coyote.’ That’s not what we found,” says Bridgett vonHoldt, an evolutionary biologist at Princeton University and the study’s lead author. “The study shows that mixed ancestry is common, even in animals [in the western United States] we’ve traditionally identified as ‘pure,’” adds Linda Rutledge, a postdoc in VonHoldt’s lab”
But, at what point can we call a hybrid population a new species, or a population worth protecting? After all, hybridization is one way that a species might evolve to outpace habitat impacts. In fact the article indicates that the Red wolf hybrid happened during the time that wolf populations were being hunted to extinction, indicating that it may have been an evolutionary reaction to predation pressure. Hybrids, therefore, aren’t just a mistake but a way that species can keep their genomes relevant as their environment and niches change…or even open up brand new niches.
Sculpin in the Rhine and Scheldt rivers offer an interesting glimpse of hybridization in action. When the two rivers were joined by a canal scientists soon saw sculpin with intermediate features that were able to persist in habitats that no one expected sculpin to live in. It turns out these fish were a hybrid population, better adapted to river mouths than the parents of either species. They are not a unique species because on each end of their range they still breed with the parent species…but in the habitat for which they are best adapted, they are a distinct population.
A species of fruit flies have also been shown to be the result of very recent hybrid speciation (pay wall). The flies mate on certain, specific, types of fruit which creates breeding isolation between species. Scientists found that a unique fly species bred on honeysuckle plants, and that the fly was a hybrid of two other species which colonize other fruit. The honeysuckle has only been introduced in the last 250 years, indicating that speciation through hybridization can be a relatively quick process. Rhagoletis pomonella, a related fly has also speciated to parasitize apples in the Pacific Northwest, though not through hybridization.
So, what does this mean for the protection of Red wolves under the ESA? Their reintroduction has been hampered by the fact that they readily interbreed with coyotes, though the parental population seems to have more genetics in common with wolves than coyotes. The ESA does insist on attempting to preserve genetic diversity within populations where it exists, hence the many ESU (Evolutionarily Significant Unit) designations of threatened Pacific Salmon, but the legislation does not specifically recognize hybrids.
The authors themselves spend significant space discussing this very topic. Calling the use of taxonomy in the current Endangered Species Act legislation, “Overly strict” and “antiquaited” they point out,
“Species and taxonomic concepts are varied, complex, and difficult to apply in practice. Of greater importance are the preservation of evolutionary and ecological processes and the role of an endangered taxon in this dynamic. Admixture is one critical example of a process that may enhance adaptation and evolution in the rapidly changing environment of the modern world.”
They go on to say that smaller, more coyote-like canids, might be better adapted to the modern world than wolves who require large expanses of territory and herds of large animals on which to prey. The spread of the eastern coyote, which is also a wolf-coyote hybrid but not discussed in the paper, would indicate that this idea has merit.
Further, they argue that species are a moving target over time and that conservation cannot by hidebound to an idea that omits the effects of natural selection on a population, that instead these new traits must also be conserved,
“These suggestions follow the “ecological authenticity” concept, in which admixed individuals that have an ecological function similar to that of the native endangered taxon, and that maintain a portion of the endangered genetic ancestry, warrant protection.”
What all of this will mean for wolf conservation, or for how the Endangered Species Act recognizes hybrids, has yet to be decided. It will play out among scientists, and potentially in the halls of Congress. What it illustrates is that all the models we apply to classify one species or another are less than perfect, and that nature has a way of bending all the rules in order to fill niches as they change. If you thought being a taxonomist would be an easy job…think again!