I must say that while Science’s reporting about evolution and ecology is pretty dire—they’re mostly puff pieces with little critical thought—the work of Ann Gibbons is an exception. When presenting a new theory or discovery, she always seeks out any significant dissent in the scientific community, and weighs in critically herself. Gibbons was, for example, one of the first print journalists to note the problems with Darwinius (“Ida”), the putative missing link that turned out to be not so link-y.
The latest issue of Science contains a short but comprehensive piece by Gibbons (free, I believe) on the latest findings in human evolution, including the second discovery of hybridization between archaic humans and “modern” Homo sapiens. (This is the Denisova fossil that I wrote about recently.)
Do read Gibbons’s piece, as it’s a good way to get up to speed in 20 minutes or so. She discusses the controversy about whether modern H. sapiens evolved “multiregionally,” transforming itself in many different places, or via replacement from a band that left Africa less than 100,000 years ago. The hybridization data—Gibbons notes that up to 10% of some modern human genomes came from mating with “archaic” ancestors—makes it impossible to clearly demarcate the two theories. Gibbons ends her article by describing a wary rapprochement between the two main advocates of those competing theories:
As for Stringer and Wolpoff, both now in their 60s, their battle has mellowed. Their views, while still distinct, have converged somewhat, and they shared a beer at a Neandertal meeting last year. “The reason we get on well now,” says Stringer, “is we both think we’ve been proved right.”
But I want to discuss briefly a shorter second piece by Gibbons in the same issue of Science, “The species problem.” Here she brings up the controversy about whether modern humans, Neandertals, and Denisovans were members of different species or the same species. This question is far more important in dealing with humans than, say, with fruit flies, simply because there were far fewer types of hominins, and anthropologists’ careers depend on whether or not they name a new species. That’s why there are so many species names in the hominin family tree—names that turn on characters as tiny as a few millimeters in the measurement of a tooth. It’s likely that, several million years ago, three or four species of hominin did exist at the same time—and maybe at the same place—giving rise to fanciful scenarios about war and (especially) inter-group mating. There’s nothing more salacious, in an evolutionary sense, than imagining a burly, hairy, and robust hominin male copulating with a female from a more modern-looking species.
But that was more than a million years ago. What about the more modern groups of Homo, like Neandertals? According to Ernst Mayr’s biological species concept, which Gibbons describes, individuals are members of the same species if they can mate with each other when they encounter each other in nature, and, critically, produce fertile, viable hybrids. If they can’t, then there must exist genetic barriers to mixing of genes, the so-called “reproductive isolating barriers” that maintain the integrity of species.
But there is some slack in how biologists use the definition, for hybridization of this type can range from being very rare (e.g., the medium and small Darwin’s ground finches, which hybridize at a rate of about 2%) to more pervasive (e.g., the black duck and mallard form hybrid swarms when they meet). If there’s only a small amount of hybridization, and the species’ gene pools stay pretty separate, most biologists consider the case to involve different species (this is the situation in the Darwin’s finches I just mentioned).
A couple of other points:
- Mating between different groups is not enough to deem them conspecific: those matings have to produce viable and fertile hybrids. And “viable and fertile” means not only that the hybrids can have offspring, but that they do have offspring in the wild. Some interspecific hybrids in birds, for example, are viable and fertile, but are not recognized as proper mates by members of either parental species because those hybrids look weird or have strange mating behaviors. That is a form of reproductive isolation, too: it’s analogous to sterility, but sterility on the grounds of not being attractive as a mate.
- Therefore, if one just sees hybrids in the wild, that’s not enough to claim that the parental groups must be members of the same species. Those hybrids have to constitute a genetic bridge between the parental groups; that is, they have to be fertile and mate with the parental species. Often we simply don’t know this. I believe Peter Grant once had a paper in Science noting that about 10% of bird species are known to form hybrids with closely-related species. That’s been used to claim that hybridization is pervasive in birds, and that bird species might not be so “real” after all. But that’s a wrong conclusion. We simply don’t know anything about the fertility and viability (and sexual attractiveness!) of most of those hybrids. And the hybrids have to occur in nature, not in zoos or other artificial enclosures that might break down reproductive barriers that exist in nature.
Gibbons reports an opinion by Jean-Jacques Hublin, who works on human evolution:
In the real world, [Hublin] says, Mayr’s concept doesn’t hold up: “There are about 330 closely related species of mammals that interbreed, and at least a third of them can produce fertile hybrids.”
Not so fast! That 330 number means nothing, since most of these produce sterile or inviable hybrids. Too, how much of that “interbreeding” was observed in the wild versus zoos? (I don’t know the answer). And only the cases seen in nature count. Lions and tigers hybridize in zoos, producing fertile “ligers” and “tiglons”, but they didn’t hybridize when they once co-occurred in India. Zoos break down reproductive barriers: animals hybridize because they’re bored, horny, and there’s simply nothing else to mate with.
Finally, if we assume that 110 mammal species produce fertile and viable hybrids that interbreed with the parents in nature (that’s a generous estimate), this constitutes only 2.4% of all mammal species (there are about 4500). It’s misleading to claim that the biological species concept “doesn’t hold up” because it’s ambiguous at best 2.4% of the time. Think of all the other 97.6% of species where it’s not ambiguous. That’s the problem with using rare exceptions to invalidate a concept that works nearly all the time. Beware of these “anecdotal horror stories,” as one biologist called them.
So I don’t agree with Svante Pääbo, who is quoted by Gibbons as saying “I think discussion of what is a species and what is a subspecies is a sterile academic endeavor.” This may be true when arguing about human fossils in the past, or in a single lineage, but is not generally true in sexually-reproducing plants and animals. As I’ve noted before, species are real, objectively delineated entities in nature, and discussing why they are distinct, and how to diagnose them, is hardly a sterile exercise. Species are not arbitrary divisions of an organic continuum. In sexually reproducing taxa they form distinct groups, well separated in “morphospace” and “genospace.” We need to understand why that is.
- Finally, it’s dubious to define species based on the degree of morphological difference between them. As Gibbons notes, “There’s also no agreed-upon yardstick for how much morphologic or genetic difference separates species.” She’s absolutely right—to settle on such a yardstick is to make an arbitrary decision. The black duck and mallard, for example, look pretty different, but are probably members of the same species. If two species of geographically isolated birds were as different in appearance from each other as, say, Asians and Sudanese humans, they’d be called different species. Yet we don’t think of human “races” as different species because, despite their different appearances, they crossbreed easily and form mixed populations when they encounter each other. Morphological (and genetic) differences are often a poor key to species status, particularly when we’re so attuned to small differences, as is the case in human fossils.
So what about “modern” H. sapiens, Neandertals, and Denisovans? Clearly they hybridized, and some of the hybrids were fertile, for traces of Denisovan and Neandertal genes remain in our genomes. On this basis, anthropologist John Hawks deems Neandertals, modern humans, and Denisovans members of the same species; Gibbons quotes him as saying “They mated with each other. We’ll call them the same species.” (I hope by “mating” he means “mated and produced fertile offspring”.)
But a little bit of gene flow isn’t enough to convince most of us that these groups were conspecific. On that basis, the Darwin’s finches would be deemed conspecific, but nobody does that. The question is whether that gene flow reflected lack of opportunity for mating (in which case they might be the same species), or pervasive hybridization (between, say, modern humans and Neandertals) but only weak viability or fertility of the “hybrids” (in which case they’d be different species). We will probably never know the answer to this.
Does this make the species status of these three groups purely arbitrary? I don’t think so. What we can do is get a “yardstick” by seeing whether other species of primates that were separated for as long as Neandertals, Denisovans, and modern humans—roughly half a million years—have evolved into reproductively isolated groups. I’m not sure what the answer is (it’s probably sitting there somewhere in the literature), but I’d guess that they wouldn’t be separate species, especially because humans have much longer generation times than other primates and so would speciate even more slowly. If it were my call, I’d agree with Hawks (but for somewhat different reasons), calling Neandertals, Denisovans, and modern humans all members of Homo sapiens.
But as for the hobbits, H. floresiensis, I’d stick with calling them a different species. They diverged from modern H. sapiens much further in the past, although they may have been contemporaneous with us.