In my introduction to the origin of horses I suggested that a horse is a horse of course, unless of course it isn’t a horse. But how do we know when we have stretched a horse beyond being a horse? Although I talked about the definition of horses in the context of creationist’ theory, I did not mean to suggest that defining the limits of species is an easy thing to do. In fact biologists and philosophers have debated the definition and meaning of species for the past century. Most of you learned in a general biology class in high school or college that species are “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.” This is called the biological species concept (the BSC) and was proposed by Ernst Mayr more than 50 years ago. This definition is not without its detractors in part because it only best applies to higher animals and doesn’t work so well for most other life on earth. I teach an upper-level biology course in which we spend upwards of three-week discussing species concepts and their philosophical roots with the point being that the exact definition of a species for all organisms is far from clear. This is because identifying where one species starts and another ends can be very difficult. In fact more the 20 species concepts have been proposed with some being species definitions for limited groups of organisms such as bacteria but most are attempts at a universal definition of species.
Regarding horses, the biological species concept (BSC) can be applied quite reasonably to define the modern species of equines. Horses and donkeys can interbreed to produce mules but mules are sterile providing evidence that donkeys and horses are “reproductively isolated” from one another. Horses and zebras can interbreed and produce offspring despite having different chromosome numbers. However there is typically an added caveat of the BSC that is usually not stated in the simple definition and that is that interbreeding “occurs in natural conditions.” Yes, a horse and zebra can interbreed, but in nature even if they are in close proximity to one another interbreeding is an exceedingly rare circumstance and therefore the horse and zebra genetic pools remain genetically distinct. Another example would be coyotes and wolves, they can interbreed and there have been cases of natural hybrids however, these two “species” have lived in the same geographical areas for thousands of years and they will generally won’t associate with each other in nature because of behavioral differences. As a result they are effectively isolated from one another and thus maintain consistent differences between each other.
So a horse is a horse because it recognizes itself as a horse and won’t, except under very unusual and unnatural conditions, interbreed with other animals that are not of similar morphological and genetic stock. All of the 20 or more species concepts that have been developed to define the limits of species would recognize the wild and domestic horse as a fairly good biological species. But clearly, horses are related to zebras and donkey’s even if they each form a well-defined morphological and genetic pool themselves. That horses, zebras and donkeys at some point in the past were not a horse, zebra or donkey but rather an ancestral species to all three seems quite evident. Even creationists appear to agree these species are all equines and have a common ancestor.
These equines illustrate why the biological species have been so difficult to define. If there are five or more species of equines today but only one in the past during the time that this one species was “evolving” into seven recognized living species they would have to go through a phase in which the distinction between one and then two and then three species would have been difficult to define because they would go through periods where they may have become isolated but didn’t yet have any significant genetic or morphological differences.
Today there is named subspecies of horse (Equus ferus) called Przewalski’s horse and they are the only truly wild horses as opposed to feral horses like the mustangs of the western US which are just domesticated horses that have escaped back into the wild. Przewalski’s horses are untamable and so truly wild. They are clearly (see below) the genetically most closely related horses to domesticated horses and can produce fertile offspring despite having one more chromosome than domesticated horses. And yet, interbreeding in nature is very rare and thus genetic exchange, especially given the different chromosome numbers, doesn’t really happen. Over time this genetic difference between domesticated horses and Przewalski’s horses will increase but on what day will we call these two different species? Some might call them species today but most call them each subspecies of one genetic stock. What is clear though is that segregated gene pools are on the route to becoming more and more genetically isolated and at some point will have
the types of mutation that will either prevent fertile offspring or will alter them behaviorally (think coyotes and wolves) so that the opportunities for future genetic exchange are reduced even further. Each becomes a step toward the formation or “creation” of a new species. If we look back in time, at some point zebras were probably just a populational variant of something like Przewalski’s horse but at a time slice in the past we may have called them all one species with subspecies. Going back further in time there may have been a time when there was only one clear species will only scattered variation that had not yet coalesce into clumps of genetically and morphologically distinct subspecies. So even if we define a species today, looking back through time that species will not always be as distinct as it is today.
How different are equine species today?
I can show you how genetically distinct horses are from other “horses” in the figure below. I generated this genetic similarity tree by comparing complete mitochondrial genomes (just over 16,000 total base pairs for each sample) between hundreds of breeds of horses and other species of living equines. What you see in the figure below is simply a comparison with total genetic similarity of this genome with the branches/lines representing the total genetic differences between a particular sample and all other samples. It is hard to read the names but I have labeled the general groups. What you see is that domestic horse breeds are remarkably similar genetically and thus very closely related to one another. The other “species” of horses are genetically distinct from horses and each species is nearly as different from one another as domestic horses are from those other species. This is similar to what one would observe with domestic dog breeds vs. wolves and coyotes and other dogs.
You can see that “wild horses” which are Przewalski’s horses are not much different from domesticated horses but as I said before, they are truly wild and so behaviorally differentiated from other horses. There are three species of zebra each of which is genetically distinct from one another. While domestic horses have what appears to be a lot of variability that variability is only skin deep. The zebras are far more different from one another than any of the domesticated horses.
Preview of Part III
In the next part of this series on the horse we will look at what happens when we extend the idea of ancestry to the fossil record of horses. I have mentioned that creationists don’t seem to have a problem of all living species of horses having a common ancestry liking them all to be in the same created kind. But when the fossil record enters the story things start to get a little more confused. How far can we push species concepts (or in the case of creationists, kind/barmin concepts) to accommodate greater and greater divergence of characters as being associated with a common ancestor? As we will see, creationists disagree about how far these boundaries can be pushed resulting in a diversity of opinions about the rate and mechanisms of evolution (they call it change or mico-evolution) that are needed to explain present day species.