Dog Skulls, Evolution, and Speciation

Though I teach genetics at the university level, I’ve never been overly comfortable with the concept of species, and speciation in particular. The definitions, criteria, and explanations of what is and what isn’t a separate species have always seemed somewhat arbitrary and capricious to me. Fortunately, my background in molecular and cellular biology is predominantly related to protein structure/function relationships, so this particular difficulty has never affected my work in any way. I can be uncomfortable with it, and still function in the scientific community.

An article published in the Proceedings of the National Academy of Sciences (PNAS) on the genetic origins of canine brachycephaly, the particular skull shape associated with English bulldogs, pugs, and to a lesser extent boxers, has brought this issue to the forefront of my mind again.

I’ve always thought that dogs in particular were an interesting case. To my knowledge there isn’t a species that exhibits such a wide degree of internal variation. From tea cup yorkies, to bulldogs, to great danes and everything in between, you can get a dog in just about any size or shape, with a variety of temperaments, skill sets, and breed-specific idiosyncrasies.

Yet, each of these highly distinct breeds is a member of the same genus and species, physical limitations aside (Great Dane & Chihuahua), they’re all sexually compatible and produce viable offspring; they are for all intents and purposes the same species. For that matter, the notion of a separate species for dogs is further complicated by the fact that wolves, dogs, and coyotes can all interbreed with one another and produce viable offspring. Thus, the designation of three separate species for dogs, wolves, and coyotes, has always seemed dubious at best. They appear to be variations on a specific canine theme, that are adapted to and with specific environmental and behavioral patterns.

Perhaps that in and of itself is enough to designate them each a separate species; but if so, couldn’t the same be said of dog breeds? They’re certainly adapted to exhibit specific behaviors and have specific strengths within a breed. How is this dramatically different than coyotes or wolves when compared to dogs.

Consider for a moment, figure one below, depicting a variety of canine skulls and note the lack of lateral uniformity that exists among these skulls. You’ll note a wide degree of variation in just about every feature of these skulls, from the size of the eye sockets, the length of the snout, the size and shape of the brain case, etc. I further submit that you could likely arrange these skulls in some sort of ‘evolutionary’ order that made sense morphologically.

Figure 1. Skull morphologies of a variety of skulls from domestic dogs.

It’s a pretty phenomenal demonstration of the effect that selection, in this case artificial selection, can have on the morphology of a species.

Imagine for a moment, an archaeologist in the future, say 10,000 years from now. Somehow, records of our civilization have been lost or destroyed, and in the future archaeology alone is used to infer information about the past. Archaeologists would unearth a human civilization that was in general, fond of pets. When they come across these different dog skulls, will they infer that the bulldog, tea cup yorkie, and great dane are members of the same species?

I submit that they would not.

Based on the current methods of using morphology to indicate species, there would likely be dozens, if not hundreds of canine species.

Consider the variation in skull morphology for something such as Darwin’s Finches. Though absolute quantification of the range of differences between canine skulls and the shapes of the heads from different species of finch is out of the scope of this blog post, I’m willing to speculate that there is at least as much variation between dog skulls, a single species as there are between finch skulls, multiple species (Figure 2)

Figure 2. Comparison of head morphology among different species of finch.

None of what I’m saying disproves evolution, nor was it even offered in that spirit. More than anything, I chose to post this to highlight some of the very real difficulties that are associated with attempting to identify different species via morphology alone.

Finally, consider for a moment some of the members of hominid family, and the morphology that exists between their skulls (Figure 3). Though a wide degree of variation exists across the spectrum of hominid skulls, and again, I’m not offering this post as a refutation of evolution in any way, given the lack of lateral uniformity in canine skulls within a species, how sure can we be about some of the variations that exist between hominid skulls when we call them a different species?

Figure 3. Comparison of skull morphology of hominid skulls

In my opinion, based on what I’ve written above, the use of morphology to distinguish one species from another is dubious at best, and only serves to highlight the lack of clarity, confusion, and arbitrary nature of distinguishing one species from another.

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