I won’t say that philosophers in general have nothing to contribute to debates about the nature of biological species, but this philosopher certainly does: Henry Taylor, a fellow in philosophy at the University of Birmingham. His paper in The Conversation (click on screenshot below) not only says that the most used species concept in evolutionary biology—Ernst Mayr’s “biological species concept” (henceforth “BSC”)—is not only wrong, but that we should in fact have no species concept. Ignoring nature completely (has he been outdoors?), he concludes that nature is not divided into the discrete groups that gave rise to the notion of species. Rather, he thinks, nature—like some ideologue’s notion of biological sex—is a continuum. In fact, he concludes that “there is no such thing as ‘the human species’ at all.”
Well cut off my legs and call me Shorty! My whole life I’ve been interacting with (and mating with) what I thought were specimens of Homo sapiens. Now I find that I’m mistaken: we form a continuum with other species. Could I have mated with a chimpanzee or a badger by mistake?
Taylor’s list of publications gives exactly one (forthcoming in Synthese) related to the notion of species, and manages to make a big to-do about a geographically isolated population of brown bears that can hybridize with brown bears and polar bears. That is one of his beefs about the BSC, which I discuss below the screenshot:
The BSC is not really a definition, but, as I emphasize in my book Speciation—written with Allen Orr—an attempt to encapsulate in words the palpable lumpiness in nature that we see before us. And nature, at least in sexually-reproducing species, really is lumpy: it’s not the continuum, or “great interconnected web”, that Taylor sees. In Chapter 1 of Speciation, I give three lines of evidence for the reality of species: they aren’t just artificial constructs, or subjective human divisions of a continuum, but real entities in nature. Yes, there is some blurring in both sexual and asexual organisms, but by and large species exist as “lumps” in the pudding of Nature. If this were not so, biologists would be wasting their time studying species, and field guides would be of no use. There is no blurring, for instance, between our species, chimpanzees, and orangutans, nor between starlings, hawks, and robins on my campus. And so it goes for most of nature. Some hybrids may be formed between species, but they are often sterile or inviable, and so don’t blur the boundaries between groups.
What Mayr and others (e.g., Theodosius Dobzhansky) did was simply to describe what kept these lumps separate from one another where they coexist in the same location. And that factor was reproductive isolation: the existence of genetic barriers to hybridization that kept two species living in one place from forming fertile hybrids, and thus kept their gene pools separate. The BSC is this:
Members of different species are unable, when they live together in the same area, to hybridize and form fertile offspring: they are “reproductively isolated”. Members of the same species are able to mate and produce fertile offspring with other members of the same species.
Coexistence, or “sympatry”, is important in this determination because geographically isolated populations that show some differences can’t be fully tested under the BSC since they don’t encounter each other in a state of nature, and some species that hybridize in captivity don’t do so when they encounter each other in the wild (e.g., lions and tigers, which used to coexist in India).
There are of course intermediate cases—groups that are more or less “species-like”—depending on how much hybridization and gene flow they experience. But for sexually reproducing organisms, these cases are the exception (see Chapter 1 of Speciation). And of course, as we emphasize in the book, the BSC cannot be applied to species that lack sexual reproduction—like many species of bacteria. In those groups one may have to use other species concepts.
The advantage of the BSC is that it gives us an empirical program for studying how lumpiness arises in nature: it arises by the formation of genetic barriers, almost always between isolated populations that experience divergent evolution to the extent that, eventually, gene flow becomes impossible. (The barriers to gene flow aren’t directly selected for in most cases: they are simply byproducts of divergent evolution.) As I pointed out in my chapter, virtually everyone studying speciation in biology (as opposed to defining species), studies the origin of reproductive barriers. That’s a tacit admission that speciation does have something to do with reproductive isolation.
I won’t go on here: I recommend Chapter 1 of Speciation (it’s accessible to the layperson who knows a bit of biology), and, if you want to see the failures of other species concepts, read the Appendix.
Now, why does Taylor reject the BSC, and along with it all species concepts? He gives two reasons.
1.) Polar bears and grizzly bears, once living in different places (“allopatric”) are now meeting each other in nature due to the global-warming-induced disappearance of the cold habitat to which polar bears were once restricted. There is some hybridization between the two groups that now meet, and some of the hybrids are fertile.
Taylor says this shows that the two bears weren’t reproductively isolated, and thus weren’t species. But this is bogus: the two groups were biological species, isolated by what we call “ecological isolating barriers”: genetically based preferences for different habitats that kept two species from encountering each other. (The genetic basis of habitat segregation is important here: two groups isolated simply because they’re on different islands aren’t necessarily biological species because their spatial segregation is due to the contingencies of geography and not to evolution.) Thus the polar and grizzly bears were separate species, but their genetic barriers broke down due to climate change, making the differential habitat preference nonfunctional.
Species may not be permanently different: all of us recognize that groups that remain distinct in nature can, in the future, exchange genes because their genetic barriers have been circumvented by environmental change. Plants kept apart in nature because they are serviced by different pollinators (“pollinator isolation”) may, in the future, suddenly begin hybridizing if one of the pollinators goes extinct. Changes in habitat can efface genetically based ecological preferences, and so on. If you put lions and tigers together in zoos, this breaks down both the geographic and sexual preferences that kept them separate when they used to coexist in India. They can then hybridize and form fertile “ligers” or “tiglons”. Does this mean that lions and tigers are the same species? No, because the change in habitat (artificial confinement in this case) has broken down their genetically-based isolating barriers.
To say that the BSC is bogus because polar bears and grizzly bears now hybridize in some places is to throw out the baby with the ursine bathwater. And this isn’t even an intermediate case: it’s a case where a barrier has been effaced by climate change.
2.) Taylor then trots out the old canard (if ducks can trot) that organisms that don’t interbreed can’t be subject to the BSC. DUH! This is something I discuss at length in Speciation. Taylor:
The [BSC] definition makes use of the notion of interbreeding. This is all very well with horses and polar bears, but smaller organisms like bacteria do not interbreed at all. They reproduce entirely asexually, by simply splitting in two. So this definition of species can’t really apply to bacteria. Perhaps when we started thinking about species in terms of interbreeding, we were all just a bit too obsessed with sex.
Indeed, it’s hard (but not entirely impossible) to imply a reproductively-based species concept to bacteria. But different species do exchange genes, and there have been several attempts to discern bacterial species using reproductive criteria. The question hinges on whether there’s a problem to explain in bacteria: are they “lumpy,” like sexually-reproducing species, or do they form more of a continuum, and thus there’s not a biological observation that needs explaining? This question isn’t yet settled.
And that’s it: Taylor’s lame effort to topple the BSC—a concept that was not even meant to apply to asexual organisms. He then throws into the mix Darwin’s own confusion about what species really were (this is well known) and on that basis wants us to deep-six all species concepts and all ideas that species even exist as discrete entities independent of human judgment. (Tell that to a robin who is courting other robins but not pigeons! Animals are themselves good taxonomists!)
Here you go:
Scrapping the idea of a species is an extreme idea: it implies that pretty much all of biology, from Aristotle right up to the modern age, has been thinking about life in completely the wrong way. The upshots of this new approach would be enormous, both for our scientific and philosophical view of life. It suggests that we should give up thinking about life as neatly segmented into discrete groups. Rather, we should think of life as one immense interconnected web. This shift in thinking would fundamentally reorient our approach to a great many questions concerning our relation to the natural world, from the current biodiversity crisis to conservation. [JAC: Yeah, what would we now conserve if all of nature is one interconnected web? Would we need to conserve everything?]
And, in a way, this kind of picture may be a natural progression in biological thought. One of the great discoveries of evolutionary biology is that the human species is not special or privileged in the grand scheme of things, and that humans have the same origins as all the other animals. This approach just takes the next step. It says that there is no such thing as “the human species” at all.
That last sentence is risible: there is no species Homo sapiens?! Does Taylor know that we cannot form fertile offspring with any other species (yes, it’s been tried with our closest relative: inseminating female chimps with human sperm produces bupkes). And it’s not the “next step” in dethroning humans as the pinnacle of evolution to then say that they don’t exist as a group.
The danger here is that those who don’t know much about biology and evolution will read Taylor’s piece and think he’s onto something. He isn’t: these criticisms of the BSC have been made many times before, and dispelled equally many times—I do it in my book, which is 15 years old. The palliative for Taylor’s nonsense—and here I have to be a bit self-aggrandizing—is to read Chapter 1 and the Appendix of Speciation.