Researchers and scientist have often defined a species as one that is isolated in terms of reproduction. This does not mean that two separate species cannot reproduce, but rather that they don’t. At least, that’s how I’ve always thought about species. A lion and a tiger can successfully reproduce to create a liger (and no – I didn’t know this until I watched Napoleon Dynamite), but these two species would never encounter one another in the wild, and therefore are reproductively isolated. And then I read about primate hybridization which forced me to approach this topic much differently.
Primate hybridization occurs when the geographic ranges or habitats of two separate species overlap a little, and individuals (typically males) “cross-migrate” to the boundaries of other species habitats. The outcome – a hybrid is an individual that Strier labels as “genetically unique,” and many of these hybrids can even reproduce themselves. This behavior has proven to be troublesome for taxonomists. Hybridization implicates that two distinct species are in fact not reproductively isolated – which, if you recall, is how I have always defined separate species.
In Cortés-Ortiz (2007), both positive and negative implications for hybridization are laid out at the beginning of the study. Hybridization could result in the breakdown of previous species and a potential loss of biodiversity or a “creative force” that would ultimately lead to the creation of new species.
I find the tension of hybridization to be quite complicated and without a clear answer. Strier’s textbook uses the example of two baboon species Papio hamadryas and Papio anubis to clearly illustrate the relatively common phenomenon of hybridization. The hamadryas and anubis babboons are two very different types of baboons: they have different social structures, foraging techniques, and they even look vastly different! But how can it be that they produce viable offspring? I believe that the hamadryas and anubis baboons are clearly two different species despite their ability to produce hybrids. I believe that these two types of primates are relatively close species with a limited amount of interbreeding.
If we look at the Neanderthals and early individuals of Homo sapiens there is a relatively similar debate. There is a small amount of Neanderthal DNA present in modern-day humans, and we know that they existed in similar locations in similar times. So the question is Homo sapiens and Homo neanderthalensis an accurate description of our relationship, or should we be classified as subspecies instead? This is one of the most heavily discussed issues in human evolution, and I find it fascinating that there is a similar and modern-day conundrum.
I think something interesting that might be a good point of discussion is the fact that evolution is a never-ending process. As humans, we love to think that we are the pinnacle of evolution. We can simply go no further because we have already achieved our optimal survival techniques. While modern health care and technology may help to make this myth a reality, species are always changing and evolving. The course of human and primate evolution is by no means a neat and clean one, and I imagine that hybridization could have occurred to assist this process somewhere along the way. Perhaps Papio anubis and Papio hamadryas are not separate species, but – as Cortés-Ortiz points out, perhaps they could be creating entirely new lineages.
When Strier discusses the baboon case study, she makes an interesting point that I find particularly interesting. I have already mentioned that hamadryas and anubis baboons behave very separately. If a “cross-migratory” male, let’s say hamadryas, reaches an anubis population, he may realize that anubis females respond differently to various mating behaviors. This makes relationships difficult to establish and stabilize between the two species. I also found it noteworthy that hybrids look and behave differently than either of their parental species. All of these social behaviors might make hybridization less likely to occur.
I particularly enjoyed looking at pictures of the two types of baboons. Here are my favorite pictures:
Strier, K.B. Primate Behavioral Ecology Fourth Edition. Prentice Hall: New Jersey; 2011.
Cortes-Ortiz, Duda, Canales-Espinosa, et al. Hybridization in Large-Bodied New World Primates. Genetics. 2007; 176: 2421-2425.