A while back I discussed a paper in Current Biology by Julian Fennessy et al. . That paper used genetic analysis (the total genetic divergence among groups) to claim that there are actually four species of giraffe instead of a single species with nine subspecies. Using the Biological Species Concept (BSC), however, I argued that there was no objective basis for recognizing four distinct species on the basis of genetic distance and monophyly alone, for such recognition is purely subjective. How much genetic divergence between geographically isolated groups is necessary before we call them “separate species”? Any decision must necessarily be subjective, since no cut-off point of genetic distance is biologically meaningful.
I concluded that although the press gave the Fennessy et al. paper a ton of publicity, there’s no good reason to recognize four instead of one species of giraffe so long as all the “species” are geographically isolated from one another. (Greg Mayer and Matthew Cobb, my biology co-writers here, agreed.)
Now, mirabile dictu, the Public Broadcasting Service (PBS) in the U.S. has taken up the issue, and I had several conversations about speciation with writer Becca Cudmore, who proved to be one of the more inquisitive and savvy science journalists I’ve encountered. And, miracle of miracles again, she gives substantial publicity to the idea that giraffes may not really comprise four species.
In her PBS NatureNow article “How many giraffe species are there, really?” Cudmore gives a good airing of the BSC and my take on the giraffes. The gist:
Unlike Coyne’s approach, the study used genetic differences to separate the giraffes. This is a method of defining species by their “phylogenetics,”or by their shared traits. In this case, Goethe University researcher Axel Janke found genetic markers, such as mutations, that were common among certain giraffes and not shared by the others. This suggested to him that there has been very little gene sharing between the groups.
But by Coyne’s definition, this doesn’t prove that giraffes are reproductively isolated. “The only way to show whether or not they are separate would be to move the wild giraffes into the same area and see if they produce a fertile offspring,” he says. While the different subspecies are known to hybridize in captivity, there is very little evidence of this in the wild.
“The Biological Species Concept is more meaningful because it helps to explain one of evolutionary biology’s most profound questions”: Why is nature discontinuous? he says—why is it not all “one big smear” that can exchange genes?
And so on. There is of course some pushback:
Still, with what we know about hybrids between species in the wild, Janke calls Coyne’s approach too “pure” and says that it’s going out-of-date.
Janke is just wrong here. (I have no idea what he means by “too pure”!) The fact that some species exchange genes is not a serious problem for a concept based on reproductive isolation between entire genomes, and in fact most closely related species do not exchange genes. The cases of gene exchange between biological species, while widely publicized, are not the rule but the exception. (See Coyne & Orr, Speciation, for the evidence.)
Most tellingly, virtually every paper I’ve seen on the process of speciation—that is, on the ways that new species come into being—deals not with the accumulation of genetic distance per se, but on the development of reproductive barriers that eventually prevent populations from exchanging genes. That’s a tacit admission of the importance of the BSC.
I think the impetus behind naming more giraffe species is largely connected with conservation, for with more named species we can put more species on the endangered list and save more of the phenotypic and genetic diversity in what was formerly one species. But while that may be an admirable goal, it should not be a motivation for recognizing species in nature.
It may not be a coincidence that Fennessy works for the Giraffe Conservation Foundation. Cudmore notes this:
Whether one, four, or six species, giraffes have experienced a 40 percent plummet in population over the past 15 years. They’re currently listed as a species of “least concern” by the IUCN [International Union for Conservation of Nature] and unlike alarm bells ringing for Africa’s elephants, gorillas, and rhinos amid the poaching crisis, they receive relatively little attention.
and the press release for the paper gives a quote from Fennessy:
“With now four distinct species, the conservation status of each of these can be better defined and in turn added to the IUCN Red List,” said study co-author Julian Fennessy of the Giraffe Conservation Foundation in a release. For example, said Fennessy, there are now less than 4,750 Northern giraffes and fewer than 8,700 reticulated giraffes in the wild. “As distinct species, [this] makes them some of the most endangered large mammals in the world.”
This makes me suspect that behind the “splitting” of giraffes is a conservationist motivation, not an attempt to partition out nature in biologically and evolutionarily meaningful ways.