The Biological Species Concept and Hybridisation in Primates

The most popular species concept in use today, the Biological Species Concept (BSC) defines a species through reference to the limits of reproductive compatibility: essentially, through the idea that any pair (male and female) within a single species will be capable of producing viable and fertile offspring, while a couple which belong to different species will not. The boundaries of successful reproduction, then, can be used to delineate species, at least in sexually reproducing animals.

Of course, it's not actually that simple. Baboons, for example, of the genus Papio, have been the subject of extensive debate, with some authors recognising as many as five separate species on the basis of morphology while adherents of the BSC, while not denying that these putative species are constant and stable, note that hybridisation between the various populations Papio means that only on species can be present. So biologists who favour the BSC lump all members of the genus Papio into one species, despite their differences, while many other researchers do not (Jolly 2001).

Interestingly, though, Papio beboons do not only hybridise with one another. Dunbar and Dunbar, for instance, noted as early as 1974 that apparently fertile and reproductively successful hybrids can be produced between at least one Papio species and the gelada baboon, in the genus Theropithecus. These two genera are closely related, to be sure, next to one another on most phylogenetic trees of the old world monkeys, but have been distinct lineages for several million years. In addition to Dunbar and Dunbar (1974)'s wild hybrids between the gelada and anubis baboons moreover, Jolly et al. (1997) report hybrids between hamadryas baboons and geladas in the wild, and Markarjan et al. (1974) between Papio baboons and both geladas and rhesus macaques, the baboons' even more distant relatives in the genus Macaca. These so-called "rheboons", however, may not be fertile or capable of attracting mates (Jolly 2001).

In light of these papers, I have been reading about hybridisation in monkeys, and it seems to be a lot more prevalent than I previously realised (there are papers galore out there, but to go into detail on all of them would take far more space than I have here!) At the same time, though, I started thinking about this after reading Jolly's paper from 2001, as cited in the last post, which is about the use of papionin monkeys as analogues for our ancestors. One interesting suggestion of Jolly's is that if baboons and macaques, or baboons and geladas, can hybridise after several million years as distinct lineages, why do we believe that all the species of australopithecine-type hominins (genus Australopithecus and genus Paranthropus) necessarily behaved as biological species? Or, for that matter, why do we think that early species of our own genus, Homo, couldn't have hybridised with one another? To me, although there is no clear evidence for hybridisation in our own lineage, there is no reason to rule it out; the evidence simply isn't clear enough. However, it is difficult to envision what evidence we might find that would inform us about hybridisation in past hominins. I suppose the real question is whether the morphological differences between the hominin species we have already identified are real discontinuities or simply artifacts of the incomplete fossil record.

Whether we can tell from the fossil record or not, this is interesting stuff, and has substantial implications for hominin taxonomy and our understanding of the evolutionary process.



References

DUNBAR, R., & DUNBAR, P. (1974). On hybridization between Theropithecus gelada and Papio anubis in the wild☆ Journal of Human Evolution, 3 (3), 187-192 DOI: 10.1016/0047-2484(74)90176-6

JOLLY, C.J., WOOLLEY-BARKER, T., BEYENE, S., DISOTELL, T.R., & PHILLIPS-CONROY, J.E. (1997). Intergeneric hybrid baboons. International Journal of Primatology, 18 (4), 597-627

JOLLY, C. (2001). A proper study for mankind: Analogies from the Papionin monkeys and their implications for human evolution American Journal of Physical Anthropology, 116 (S33), 177-204 DOI: 10.1002/ajpa.10021

MARKARJAN, D., ISAKOV, E., & KONDAKOV, G. (1974). Intergeneric hybrids of the lower (42-chromosome) monkey species of the Sukhumi monkey colony Journal of Human Evolution, 3 (3), 247-255 DOI: 10.1016/0047-2484(74)90183-3