Hybrid swarms. What an interesting concept. I first encountered this phrase hiking to Gregory Bald in the Great Smoky Mountains NP. Four native azalea species are believed to be present on the bald (Rhododendron calendulaceum, cumberlandense, arborescens, and viscosum) and over the years have hybridized amongst themselves producing a wide range of bold and beautiful colors. Then, again, I heard it when a speaker marveled at two species of trillium, located fifty feet apart, with the full range of color hybrids existing between the two. Numerous plant families are known to exhibit prolific generic profundity. Violets, hawthorns, and even hickories are several noteworthy genera. Yet, it applies to animals as well as plants. Bobcats and our little kitty cats interbreed, as do western and Baltimore orioles (or, is it northern orioles?).
The historic definition of species not being capable of producing fertile offspring among other species is no longer an viable statement. Indeed, nature doesn’t fall into neat catagories so desired by taxonomists. Quite often, when discussing species, the phrase ‘primary isolating mechanisms’ is used to explain why two closely related species don’t interbreed in the wild, while they can (and do!) in captivity. As Man has removed barriers, we are seeing numerous examples of separate species producing ‘hybrid swarms’.
One of the best examples of a hybrid swarm involves the different members of the Canid genus. Domestic dogs, wolves, and coyotes can (and do) interbreed. Historically, coyotes were confined to western US states, with eastern wolves inhabiting our region, perhaps alongside the red wolf (more on that later…). With the removal of the wolf from the east and the cutting of the forests, the western coyote has now traveled eastward, settling throughout our mid-Atlantic region. In fact, over the past thirty or so years, coyotes have become established in every county in PA, MD, WV, VA and NC. Interestingly, eastern coyotes are much larger than their western counterparts (40 lbs versus 25 lbs), but still significantly smaller than gray wolves (80 lbs).
A 2009 study provides some fascinating insight into our eastern US coyote genesis (Rapid adaptive evolution of northeastern coyotes via hybridization with wolves, Curtis R. Kayes and J.J. Kirchman). Mitochondrial DNA sequence data was collected from 686 eastern coyotes obtained from Ohio east to New Jersey, north into Maine and southern Quebec. The results found 20% of the coyotes to have DNA typical of wolves from eastern Canada and the Great Lakes region. Only one sample had dog-like DNA.
Skulls from 196 coyotes were also collected and measured. Eastern coyote skulls were larger than western coyotes, with distinct sexual dimorphism (males larger than females). This sexual dimorphism is not found in western coyotes, but is found in gray wolves. It is believed that the larger skulls are not just an introduction of wolf genes, but reflect the evolutionary impact of the larger role of large prey (deer) in the eastern coyotes diet (larger prey requires larger jaws).
This research also supports the contention that our eastern coyotes came through two migration paths. One route was a slow-moving front coming from Ohio, while the other was a fast-moving front coming from Minnesota, Ontario, and south through New York. This northern front moved five times faster than the western front, hybridizing with wolves along the way, producing a larger, more rapidly expanding population. In addition to the wolf DNA signature, this northern front also exhibits very low diversity (only three genetic types), indicative of a very limited passage of coyotes; possibly just a handful. The western coyotes display many dozens of genetic types.
In contrast, the Ohio front exhibits high genetic variability and no wolf-like DNA, much like the western coyote population. Not surprisingly, this western front consists of smaller sized coyotes; more representative of their western counterparts.
Mitochondrial DNA is providing significant revelations among animal relationships.
For example, the gray wolf (Canis lupus) is now generally accepted to be divided into the western species and an eastern timber wolf (Canis lycaon). More interesting, perhaps, is the situation with the red wolf. Over the last ten years, there has been much debate over whether the red wolf (Canis rufus) is a separate species or a hybrid between coyotes and wolves. It’s status as a federally-listed endangered species hinges on this issue.
Much refinement and constantly improving techniques of DNA analysis now has found a unique sequence (haplotype) among red wolves that has not been observed in coyotes, wolves or dogs (Adams, 2003). But is that enough to justify a separate species? A lot depends on whether you are a ‘lumper’ or a ‘splitter’. Some experts place the red wolf with the new eastern timber wolf on the basis they most resemble each other. The interpretation of the USFWS in the five-year recovery plan is that the larger gray wolf stayed in the northern coniferous forests where large prey (moose, carribou, elk) was found, while the smaller red wolf stayed in the more southern mixed conifer-deciduous forests with smaller prey (white-tailed deer). Where the two overlapped, they hybridized over evolutionary time to produce the eastern timber wolf.
The only fact that is clear at this time is that full scientific consensus has not yet been reached regarding the eastern wolf concept.