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john hawks weblog

paleoanthropology, genetics and evolution

Photo Credit: Patas monkey (probably not from the population discussed in this article). Avi, on Flickr, CC-BY-SA 2.0

Are these moustached monkeys a species unto themselves?

I ran across a news article from the BBC by Paul Rincon, about a proposed taxonomic revision to patas monkeys in northeastern Africa: “Moustached monkey is separate species”.

Scientists took a fresh look at the distribution and physical appearance of patas monkeys in Ethiopia, confirming there were two species rather than one.
It was originally described as a separate species in 1862, but was later folded in - incorrectly - with other patas monkeys to form a single species.

This is not “a new species being discovered” as some might expect from a news story. It’s one researcher, Spartaco Gippoliti, who has written a paper presenting some history of the taxonomy of patas monkeys. He suggests that a species name (Erythrocebus poliophaeus), first given in 1862 but later discarded, now be revived to apply to a geographic population of patas monkeys in Ethiopia. The paper is in the journal Primate Conservation and is available online: “On the Taxonomy of Erythrocebus with a Re-evaluation of Erythrocebus poliophaeus (Reichenbach, 1862) from the Blue Nile Region of Sudan and Ethiopia”.

In the paper, Gippoliti refers directly to the possible importance of this species to broader issues of conservation in Ethiopia:

Monkeys of the genus Erythrocebus are potential flagships for important African ecosystems, and may well be at greater risk than is generally believed.

Recognizing more monkey species would cause greater awareness of the threat to regional populations. Patas monkeys are not listed as threatened, but the populations in Ethiopia are at much greater risk than patas monkeys in other parts of Africa.

But is it scientifically valid to name species for conservation reasons?

I don’t want to criticize this specific proposal about patas monkeys. I agree with Gippoliti that scientists need to know a lot more about them, and this is an urgent conservation concern. They should be conserved where they are at risk.

What surprised me when I followed up the original research paper, is just how little evidence it presents about the variation in patas monkeys and how this population compares to other patas monkey populations.

Truth is, I’ve become accustomed to seeing genetic evidence about population divergences. Most recent papers that claim evidence to support a novel distinction at the species level between primate populations include some evidence about genetic variation within and between populations. Genetic evidence is valuable because it can demonstrate that populations have been evolving along separate trajectories with little gene flow between them for a long time.

That’s not to say that genetic data is always sufficient to establish that a species is valid. Even large genetic differences may not be enough demonstrate that speciation has taken place between two populations. Two populations may exchange genes rarely, yet often enough to prevent the evolution of reduced hybrid fertility or viability.

The paper does discuss coat and skin coloration as evidence for a distinction. But it doesn’t present evidence about the variation of these traits in either population, nor does it present other morphological data about their variation. Mainly, the paper presents a case that scientists don’t know enough about the patas monkey variation.

Considering the geographic separation and distinctive external appearance, I have no hesitation in considering poliophaeus to be a distinct species. Its closest taxon in appearance seems to be baumstarki, for which species’ status is also warranted. The recognition of these patas monkeys as species, highlights the need for field surveys to assess their geographic range and conservation status in both Ethiopia and Sudan.

To me, this is a very sharp example of the old debate between splitters and lumpers as applied to conservation.

Gippoliti discusses the history of the naming of these monkeys, and suggests that prior workers who lumped the monkey populations may have been overly influenced toward lumping them together, by variation and developmental changes to the skin color and coat changes. For him, the geographic separation and differences in appearance are enough to justify a separate species designation.

My philosophical inclination is toward lumping groups together, and recognizing geographic variation below the level of species. Occasional gene flow between genetically differentiated groups has been important to the evolution of ancient humans and other primates. Since long-range gene flow can make such a difference to adaptation within species, and since it has happened so often, I’m inclined to be conservative in naming such groups. Ideally, I’d like have some evidence of reduced fertility before recognizing them as different species.

Yet biologists have come to accept more and more that hybridization and introgression between populations are common in mammal evolution, even when hybrids have reduced fertility. Speciation is much less of a barrier to adaptive gene flow than biologists once assumed.

For those of us who care about the mechanism of adaptation and the particular history of adaptation in hominins, that makes species names much less important than they once seemed. Morphological traits don’t predict evolutionary potential in the way many anthropologists used to think.

For scientists who are deeply engaged in conservation biology, species names make a lot more difference.

Still, with genetics, we’ve become accustomed to having actual data on diversity, rather than the often-subjective assessments of taxonomists. Hopefully some further genetic information from patas monkeys will help to clarify the variation in these populations.

UPDATE (2018-01-19): Reflecting on this, I realized what is bothering me about this instance. We should have many more short papers discussing the history of taxonomies and their problems matching current data on species’ natural distributions. That’s helpful to advancing new data collection and research. I learned a lot about patas monkeys from this paper I didn’t know.

But those kinds of papers, lacking any new empirical data, should not be trumpeted in the press as “discoveries”.

Scientific discussion about species is valuable, and recognizing underappreciated diversity can advance conservation goals. But when a data-free paper is reported in the press as a “discovery of a new primate species”, it damages the credibility of conservation efforts. Such publicity makes it appear that scientific judgments about species are merely arbitrary, motivated by politics and not data.