The giraffe, Giraffa cameleopardalis, was first described by Linnaeus, and gets its species name from its fancied resemblance to a hybrid beast (as Wikipedia notes, the name comes from the Greek καμηλοπάρδαλις” meaning “giraffe”, from “κάμηλος” (kamēlos), “camel” + “πάρδαλις” (pardalis), “leopard”, due to its having a long neck like a camel and spots like a leopard). It’s always been considered a single species, but divided into about a half dozen subspecies that live in different areas and are distinguishable by different patterns of reticulation in their coats. Here’s an old subspecies designation and map; note that the populations included in each of the six subspecies live in different areas:
Here’s a classification of nine subspecies based on pattern (the number of named subspecies has been between four and about nine (I haven’t searched extensively).
Note that this classification is more or less arbitrary because the populations are geographically isolated and so one can’t use the classical “biological species definition” (BSC), in which members of the same species are able to interbreed in nature and produce fertile hybrids, while members of different species, when present in the same area, either do not mate with each other, or, if they do mate, do not produce hybrids that are fertile. Note that to use the BSC, putatively different (or identical) species have to be “tested” when living in the same area (“sympatric”). If they do not encounter each other in nature, there’s little you can do to apply the BSC.
One way around this is to hybridize them in zoos. If different “subspecies” do not mate with each other, or can’t produce fertile hybrids when they do mate in captivity, they’re almost certainly unlikely to do so in nature, and can be considered members of different species. However, if two different types do hybridize and produce fertile offspring in captivity, that doesn’t mean they’re members of the same species, for in nature other “isolating barriers”, like different breeding times or a genetically-based aversion for mating with other types, could keep them genetically separated even though barriers could break down in the artificial environment of zoos.
“Ligers“, for instance, are hybrids between male lions and female tigers, made famous in the movie Napoleon Bonaparte. Lions and tigers will mate in captivity, and some liger hybrids are fertile. But we don’t consider lions and tigers to be members of the same biological species because, when they were living in the same place in nature—in India, though they no longer are “sympatric” there—no hybrids were produced.
Allen Orr and I discuss the rationale for using the BSC for investigating how species originate in Chapter 1 of our book Speciation (2004), and conclude that if you want to understand “the species problem”—why animals and plants in a given area divide into clumps and are not simply a schmear)—the BSC is the concept to use. But if you merely want to name species, rather than understand how discrete cluster originate, then you’ll have to engage in a subjective classification if those groups don’t live in the same area.
This is the problem with a new paper in Current Biology by Julian Fennessy and coauthors (reference and free link below). This paper tries to determine the number of giraffe species that really do exist, but since the subpopulations live in different places and don’t encounter each other (see below), they’re forced to use another criterion to determine the number of giraffe “species”. They use genetic divergence, as measured by the amount of sequence difference in the DNA.
Fennessy et al. sequenced eight genes: 7 nuclear genes and the mitochondrial DNA (effectively a single gene) in nine previously-described subspecies of the giraffe. Phylogenetic analysis of the sequences, shown below, showed that populations fell into four distinct clusters, which they decided to call “four species”. Here are the new species they distinguish and name:
- southern giraffe (Giraffa giraffa),
- Masai giraffe (G. tippelskirchi),
- reticulated giraffe (G. reticulata)
- northern giraffe (G. camelopardalis), which includes the Nubian giraffe (G. c. camelopardalis) as a distinct but related subspecies.
Here’s the phylogenetic analysis (turn sideways); the colors of the populations correspond to colors on the map below (note again that the subspecies do not inhabit the same areas, though they may have in the past; we just don’t know!) Note too that each species is “monophyletic”, containing all the individuals that descend from a single common ancestral population.
Here are some of the populations they sampled; the colors correspond to the dots in the phylogeny above.Finally, the authors do a genetic “STRUCTURE” analysis, which assumes the presence of different groups (varying how many groups they preconceive) and tries to see, using genetic differences, how distinct those groups. The assumption of both three and four groups (the latter corresponding to the four species they name) were the most useful for giraffes. At the top of the figure below you can see how well the clusters are demarcated with a K (group number) of four, and how the demarcation becomes marginally worse when you assume five clusters.
Also, below the cluster diagram is a DNA-based estimate of giraffe “species” divergence times calibrated from the fossil record of other mammals. You can see that the four groups diverged from each other between 1 and 2 million years ago.
So everybody seems to be happy with the conclusion that we have four species instead of one.
Everybody, that is, but Matthew Cobb and I. But more on that in a second.
The breathless conclusion that we have four species was accepted not only by the journal Current Biology, which published the paper (note the title), but by major news outlets, including the New York Times, the BBC, Science magazine, and so on. I haven’t found a word of criticism of this conclusion. So here’s some.
Dividing up geographically isolated groups into species is an exercise in naming and classification rather than understanding the problem of why nature is discontinuous—and by “discontinuous”, I mean the answer to the fascinating question “Why, in one area, are animals and plants divided into discrete groups rather than forming a continuum?” Using a genetic-distance measure, as the authors did in this paper, simply tells you how long the groups have been separated rather than whether they’d remain discrete were they to meet in nature. It tells you about evolutionary history but not about speciation.
So yes, we know the four groups they call species have been separated for 1-2 million years, and have developed coat color differences. But human “races” have also developed coat-color differences (and other genetic differences) over a period of about 60,000-100,000 years, also largely among geographically isolated groups. Now human populations cannot be distinguished from their DNA sequences nearly as unambiguously as can these giraffe subspecies, but if you gave them another million years of geographic isolation (now impossible because of travel), they might well have separated genetically to the same extent as these giraffes. Would you then be happy to call human populations different “species”? What if a human population became marooned on an island for a million years, and changed its hair color and became genetically distinguishable using sequencing of largely neutral sites. Would you be happy to say, unambiguously, that here was have a new species: Homo islandensis?
Also note that genetic STRUCTURE analysis of human populations shows a pretty neat division into six groups (Rosenberg et al. 2002), corresponding well with 5 geographic regions. (Using other assumptions of group numbers, with K = 2, 3, and 4, doesn’t give as clean a resolution, as shown by areas with mixed colors). The groups are from Africa, East Africa, Europe + Middle East, Central/South Asia, Oceania, and the Americas.
Do we have five human species here, based on genetic clustering? How much genetic difference would be sufficient to call them different human species? (These problems are all discussed in the Appendix of Speciation).
The correlation of genetic differences with geographic isolation, of course, reflects the fact that populations that don’t get a chance to exchange genes become more and more different through time via natural selection and—probably the major force in this case—genetic drift. But how much difference do we need to diagnose species? It’s arbitrary, since populations can become “reciprocally” monophyletic for even a single gene. Okay, so how many genes do you need?
So the designation that there are four genetic clusters of giraffes is sound, but do they correspond to what evolutionary biologists call “species”? Who knows? We don’t know whether these groups are reproductively isolated. I can find no evidence that these groups ever lived in the same area of Africa at breeding time—or any time. Further, they can apparently hybridize in zoos (see reference 20 for THE GIRAFFE STUD BOOK; the fertility of hybrids isn’t mentioned). So while these are “genetic clusters”, we have no idea whether they’re biological species.
Matthew in fact was quoted in the BBC piece from an email he sent them, but they left out his caveat about reproductive isolation. Here’s what the article said:
Matthew Cobb, professor of zoology at the University of Manchester explained that the “four groups of giraffes had “been separated for 1-2 million years, with no evidence of genes being exchanged between them”.
Here’s what the BBC chose not to quote from Matthew’s email (reproduced with permission):
Defining species is difficult, and neither number of genetic differences between two groups, nor the time they have been separated, are necessarily relevant. The key point is what is called the biological species concept – does mating between these groups produce fertile offspring? If not, they are species. If they can produce fertile offspring, then no matter how long they have not done so in the wild, they are still not true species. However, this technical aspect is probably the least interesting point about this study. Instead, we should be focusing on developing appropriate conservation strategies for the four groups identified in this study. Whatever we call them, these four groups have distinct genes and have had distinct histories for over a million years.
I agree almost completely with Matthew, though I have one quibble. Yes, Fennessy et al. have identified groups that we can use for conservation purposes—if we want to conserve the phenotypic and genetic diversity in what was once considered a single species. But do we want to do that? This question is never really discussed. ‘
If we’re trying to save the coat color genes, well, they’re probably segregating at low frequency in other “species” of giraffes, and we could reconstruct any extinct pattern through artificial selection. If we’re trying to save the DNA diversity, most of which probably involves nucleotide bases whose differences among the “species” is of no biological consequence, why are we trying to do that? My point here is that—and I may be missing some literature—is I have never seen a detailed and critical discussion of what exactly conservationists are trying to accomplish when they decide to preserve populations X, Y, and Z. As Dick Lewontin pointed out long ago, a single fertilized female within a species contains half of all the “additive genetic variation” (selectable genetic variation for various traits) of the entire species, and that kind of latent variation is probably present in several or all of the four “distinct” giraffe species. When we say “we have to save all four giraffe species”, we don’t consider what we’re trying to save? The populations themselves? The phenotypic differences that distinguish them? Or the genes that distinguish them? (My own view is that we should save everything just out of respect for animals and plants that were here before us.)
I’ve been hard on this study because I study species using the BSC, but I want to finish by saying out that the study of Fennessy et al. is indeed very good, except for its certainty that we have four “species”. They don’t discuss the difficult problem of what, exactly, a “species” is—something that Matthew summarized in one paragraph. That aside, Fennessy et al. have done great work. They have identified four distinct genetic clusters, which tells us about the evolutionary history of these groups and may ultimately give a key to their degree of reproductive isolation if the giraffes move around (perhaps due to climate change). It also gives us an idea of which groups were geographically isolated for long enough to allow such genetic differences to accumulate, and thus raises questions about biogeography of these populations.
But four species for sure? I can’t say. And I’m surprised that the major science journals and newspaper sections have accepted the authors’ conclusions uncritically.
Fennessy, J. et al. 2016. Multi-locus analyses reveal four giraffe species instead of one. Current Biology 26:1-7, online doi.org/10.1016/j.cub2016.07.036