The Hybrid History of North American Deer

There are five species of deer native to the US and Canada. Two of these, the moose and the caribou, are distinctive animals with no especially close living relatives. The elk, while perhaps not as distinctive in appearance as these two, is even more distant from the others in evolutionary terms. The other two, however, are the white-tailed deer (Odocoileus virginianus) and the mule deer (Odocoileus hemionus) which not only look similar to one another but are, indeed, each other's closest relatives.

When I discussed these two species in detail last year, I mentioned the existence of the "black-tailed deer" a subgroup of mule deer with tails of a more solidly black colour than others of their kind. The black-tailed deer are native to the Pacific Northwest and are generally considered to consist of two subspecies of mule deer that share a common ancestor that split off from other mule deer early on. Since are subspecies, not full species, it should come as no surprise to discover that they hybridise with other mule deer where the two come into contact, although, if anything, this happens more often than we might expect.

What might be less apparent is that mule deer can also hybridise with white-tailed deer. This may seem odd since the popular definition of a species is that two different species cannot crossbreed to produce fertile offspring. In reality, this definition is rarely used in practice by zoologists since it produces a number of unexpected complications and would also result in us ditching some species that are quite obviously unique (such as polar bears, which can crossbreed with brown bears). Instead, more reliance tends to be placed on whether species are distinctive, both physically and genetically, from each other. It's not so much an issue of whether they can crossbreed but whether they do so at all frequently in the wild even when the possibility presents itself.

In the case of white-tailed deer and mule deer, there does not appear to be any loss of fertility in the hybrids and, were they not obviously different in their appearance and behaviour - for example, preferring different types of woodland even where they live close together - it has been argued that we probably would consider them to be a single species. While it's probable that their habit of staying apart means that many hybrids identified visually in the wild are misidentification, we know from genetic evidence that crossbreeding between the two species does happen occasionally in the wild, over a broad region from at least Alberta to southern Texas. 

The existence of hybrids can have several implications beyond the academic interests of taxonomy. Hybrids, for instance, tend not to be protected in the way that 'natural' species or even subspecies are. That's not so much an issue here, where both species are common, but it can be elsewhere. For example, the status of the red wolf, a rare canid that might (or might not) be a hybrid between regular, grey wolves, and coyotes is potentially significant. Hybrid populations, where they exist, can also give rise to entirely new species, as I have previously discussed with dolphins, although we tend to consider this the case only when the original hybrids lived a long time ago.

And it turns out that this may also be relevant to the history of these deer. We can say this because of some puzzling findings about their genetics. When we perform genetic analysis on a wide range of nuclear DNA we find what our observations of the physical appearance, preferred habitats, and general behaviour of the deer would lead us to expect. That is, black-tailed deer and regular mule deer are subspecies of the same species, and white-tailed deer are something different. So far so good.

The problem is that, when we instead look at the mitochondrial DNA, we get a different answer. This time, black-tailed and white-tailed deer appear more closely related to each other than either of them are to non-black-tailed mule deer. Now, nuclear DNA is more reliable as a test for this sort of thing than mitochondrial, since there's far more of it, and it's inherited from both parents, while mitochondrial DNA is only ever inherited through the female line. So this doesn't do any damage to the idea that they are separate species, divided up in the way that we think they are... but it still needs explaining.

One of the limitations of these mitochondrial studies has been that they have tended to take their samples of the two species from places where they live close together. This is where they are most likely to have hybridised in the recent past and, if compared with black-tailed deer in a different part of the continent, may therefore have a greater chance of appearing similar than they otherwise would. A more recent study, however, sampled 130 deer from across North America, from the Yukon and Maine in the north down to Nicaragua in the south. Analysing these mitochondrial samples and comparing the rates of change with dates calibrated from fossil evidence allows us to piece together the history of the two species in more detail than ever before.

The oldest known fossils that we can confidently say were white-tailed deer come from northern Texas a little over 3 million years ago, while the oldest mule deer fossils are younger than this, and hail from southern California. Since these locations are not that far apart in the grand scheme of things, and the two species are so closely related, it seems likely that they actually originated at the same time, when some ancient species became split into two. Most likely, it lived somewhere around Utah or Colorado, and the populations became isolated when they headed south and ended up on opposite sides of the mountains. According to the study, this would have happened 3.13 million years ago, during the middle part of the Pliocene epoch, and probably not long after their unknown ancestor would have entered the Americas from Siberia.

From their original homes in California and Texas, both species expanded by heading east so that, by the start of the Ice Ages, white-tailed deer lived east of a rough line from Texas to Wisconsin, while the mule deer (not yet divided into their current subspecies) lived on the western side. Along the line, they could have met up, and the study shows signs of a major hybridisation event occurring 1.32 million years ago, with female white-tailed deer mating with male mule deer, and their hybrid descendants going on to live among the latter. Over the following generations, the white-tailed deer ancestry became so diluted that the hybrids were, to all intents and purposes, regular mule deer. But, since mitochondrial DNA is inherited solely in the female line, it had nothing to mix with, and the signal of those distant female ancestors remained in a population of what's now a different species.

Crucially, though, the descendants of the original hybrids did not spread throughout the entire mule deer population. That's probably at least partly because, by 1.32 million years ago, the Ice Ages were underway, limiting long-distance travel. Specifically, a population of mule deer had become isolated in the Pacific Northwest, prevented from joining up with their kin to the east by the Cascade Mountains. There is also some evidence that they had difficulty moving south, with San Francisco Bay being one of the geographical barriers in that direction.

This population became the first black-tailed deer and the "hybrids" from further east did not meet up with them until relatively recently - about 180,000 years ago - by which time the separate populations were too well established for the hybrid signal to make any inroads into the black-tailed group. This would imply that black-tailed deer, but not other mule deer, possess something close to the original mitochondrial DNA of their species since it is only they who can trace their direct female ancestry back to the founder population in Pliocene California.

Interestingly, the study also showed several divergent lineages among the white-tailed deer, probably the result of different populations being separated by the Pleistocene ice sheets and developing on their own for a time. For example, white-tailed deer from Maine are more closely related to those in Florida than they are to those in, say, Illinois, perhaps the result of unfavourably cold terrain in the Ice Age Appalachians keeping their ancestors separate, then remaining distinct as they headed north when the ice sheets retreated. (One might suspect that the Mississippi would have also been a barrier, but the study says otherwise... possibly because the deer are quite good swimmers). 

In a similar way, we can see that the white-tailed deer in Latin America probably descended from an Ice Age population in what is now the southeastern US, as had previously been suggested. These are also less closely related to the non-black-tailed mule deer than those further north are, which probably indicates a much more recent hybridisation event which is essentially still continuing today, albeit on a small scale. This would have begun when the ice sheets retreated and the two species started to come into contact again.

This may be a repeat of something that helped shaped the species we know today over a million years ago.

[Photo by Marshall Hedin, from Wikimedia Commons.]