Yesterday I began “deconstructing” (as the cool kids say) the claims in the new issue of New Scientist, below, stating that evolutionary theory needs a reboot. I don’t intend to go through all 13 “novelties” that supposedly call for an “Extended Evolutionary Synthesis”, but I’ll tackle just a few this week, for “unpacking” (as the cool kids say) all the errors and distortions of the entire article would wear me out. And the
rag magazine probably enjoys these posts as all they care about are clicks, not scientific accuracy.
Yesterday I criticized the magazine’s claim that “genetic plasticity”—the observation that the expression of genes and the traits they produce depend on the internal and external environment—is something novel that was just discovered recently, and that it refutes the widespread idea of genetic determinism. Well, this kind of plasticity isn’t new (it’s been around for a century), it doesn’t refute “genetic determinism” construed in some ways, and almost no biologists accept the form of genetic determinism that New Scientist claims is widespread. Today we take up an area I know something more about: speciation.
Point 5 of their article is the assertion, in caps, “SPECIES DON’T REALLY EXIST.” That will be news to the many of us who already see Homo sapiens as a species that’s different from gorillas, orangs, and the two chimp species. It will also surprise those of us who can instantly recognize a local bird as a robin, a starling, a pigeon, a mallard, and so on. Field guides, after all, would be useless if species weren’t distinct.
For, as Ernst Mayr and Theodosius Dobzhansky recognized in the 1930s, nature is not a continuum in which one form blends imperceptibly into another. Rather, nature is “lumpy” if inspected in a single area, and the lumps correspond to species. (This and the other issues below are all discussed in the first chapter and Appendix of my book Speciation with H. Allen Orr.)
The issue is then not to define species a priori, forcing the lumps in nature into the Procrustean bed of that definition, but rather to conceptialize species: describe in words what they represent. In the first paragraph, then, author Colin Barras gets it wrong:
FOR most of history, we have had little trouble defining species. There was a general assumption that a finite number of distinct forms of life had existed unchanged since creation, each sitting in a clearly defined pigeonhole: human, housefly, hawthorn and so on. Within the past few centuries, and particularly after Darwin, evolutionary theory has emerged as a more satisfactory way to explain how species came into existence. Yet in doing so, it has made species far harder to define.
Well, the issue isn’t how to define species but to find out how to recognize them. And yes, evolutionary theory since the 1930s has provided not only a good criterion for recognition, but also a good explanation of how species come into existence: how the process of speciation works. The explanation is, contra New Scientist, intimately connected with how we conceptualize species, for if we don’t know what these discrete units of nature are, how can we possibly understand how they came into being? Yes, there are lots of new species “definitions” that have arisen in the last several decades, but only one has stood the test of time, and is recognized as an accurate conceptualization of nature’s lumpiness by evolutionists. It’s called the “biological species concept” (BSC), and is roughly this:
A species is a group of populations whose individuals have the ability to exchange genes with other members of the group where they coexist in nature. In contrast, individuals belonging to different species cannot exchange genes in nature: they are reproductively isolated from each other.
Thus the key to understanding why you have no trouble telling birds or insects apart in one plot of land is because they remain genetically distinct from one another, with the reproductive barriers (mate discrimination, hybrid inviability, and so on) preserving the differences that accumulate within each species as it adapts to its environment. In other words the species is the thing that evolves. Now of course some populations of a single species can evolve differently from others, and some species show a limited amount of gene exchange with other species: I deal with these complications in my book, which I urge you to consult for further information.
Things go really haywire in the next paragraph:
There are several aspects to the problem. One is that if we accept the idea of species evolving from other species, then we must allow that an ancestral species can gradually morph into one or more descendants. We would still like to place organisms in discrete categories, but doing so is difficult if species blur into one another through time. “As we have come to terms with evolution, it has highlighted a problem with the machinery in our heads we use for classifying,” says Frank Zachos at the Natural History Museum of Vienna in Austria.
Change in a single lineage over time is a non-problem. Of course lineages slowly transform over time, as ours did. If we evolved, for example, from Homo erectus, it becomes a purely arbitrary matter when to give the later segment of that lineage the name Homo sapiens. Everyone recognizes that this is a matter of naming, not of making a crucial and meaningful biological decision. As for the splitting of one species into several, which occurs via (usually) gradual differentiation of geographically isolated populations to the point where they can’t interbreed, it’s also arbitrary when you call the descend moieties “different species”. We know that when no gene exchange can occur, good biological species have come about, but at intermediate stages of the process, I prefer to say that populations are “becoming more and more species-like.” What New Scientist sees as problems here have been dealt with amply in the last 80 years.
Here’s another non-problem:
For Jody Hey at Temple University in Philadelphia, the more important problem is that biologists often have two objectives in mind when they define species: one is the traditional desire to divide nature into easily recognisable packages; the second is to explain, in evolutionary terms, how those species came into existence. “Humans have conflicting motivations towards species,” he says.
Some researchers argue that these two objectives can never be achieved simultaneously. Down the decades, biologists have come up with a few dozen clever ways to define species. Some make it easy to classify the organisms we encounter – by their physical appearance, for example – but tell us little about the evolutionary process itself (see “Sadistic cladistics”, page 49). Other definitions get to the heart of how species come to exist, but can be difficult to use in the real world.
But other researchers, including me and other evolutionists, do think these objectives can be achieved simultaneously. Are we mentioned, and our reasons given? Nope.
I’m a friend of Jody’s, and he’s a terrific scientist (and a reader here), but I disagree with him on this issue. If you read Speciation, you’ll see that the BSC in fact fuses these two objectives. You first conceptualize species as units of nature that have limited or no gene exchange between them where they co-occur. Then the second problem arises immediately, and comes with a built-in research program: “how do the reproductive barriers arise in the first place?” That is the problem of speciation, and the problem that Darwin, despite the title of his 1859 book, couldn’t solve, for he had no notion of species as reproductively isolated units. In fact, the two objectives have already been achieved simultaneously by evolutionists who accept the BSC. Somehow Colin Barras seems to have missed this. No species concept other than the BSC can explain the palpable lumpiness of nature, and also how it comes about.
The third issue, which comes up often, is that gene exchange between apparently distinct species occurs more often than we used to think. (We know this because we have DNA-based ways of detecting such exchange—”introgression”—that we didn’t have a few decades ago. So here’s the supposed problem of “hybrid bonanza”:
In principle, advances in genetic sequencing could have helped by indicating how genetically distinct different groups of organisms are and how long ago lineages diverged. But sequencing has arguably made the problem worse by revealing that interbreeding – more technically, introgression – between closely related “species” is common across the tree of life. “It does seem to be the rule, not the exception,” says Michael Arnold at the University of Georgia in Athens. Indeed, evidence of introgression stretches right to our front door: our ancestors interbred with various ancient hominins that might, in the eyes of some, count as distinct species.
Well, interbreeding is not ubiquitous (humans and orangs, for instance, don’t exchange genes with any other species), and even when hybrids are formed they sometimes are sterile or don’t mate back to one parental species, necessary for introgression. Hybrid ducks, for example, can be fertile, but introgression is limited because the hybrids look weird and aren’t seen as acceptable mates. Yes, introgression is more common than we thought, but often it occurred in the distant past or, if it occurred more recently, is limited. Yes, we had gene exchange with Neandertals and Denisovans, and it appears to have been more than rare, so I tend to see these groups as subspecies of H. sapiens rather than separate species. When there’s that kind of gene exchange, the problem becomes a judgment call. But this problem hasn’t persisted: now all H. sapiens belong to the same species, and there’s no question of an other species of hominin existing now.
In fact, if gene exchange were pervasive and ubiquitous, we couldn’t make family trees of plants and animals very easily: the gene exchange would blur out the twigs. But it hasn’t.
The article goes on:
Another problem is that looking at genes rather than observable features makes it easier to find new species, leading to what some researchers have called taxonomic anarchy. For instance, a biologist can argue that a previously recognised species should really be split into two or more “new” species, as happened when genetic analysis of the African elephant led to its being separated into savannah and forest-dwelling species.
This is a non-problem as well. If you insist on calling geographically isolated populations, like giraffes, as “different species” if they have a certain amount of genetic or morphological differentiation, then that’s also a judgment call, for one can never be sure what degree of genetic differences (usually judged by DNA differences) would correspond to reproductive isolation. If you don’t care about reproductive isolation, then you have no threshhold degree of genetic difference that is biologically meaningful.
The one sure criterion for species delimitation is this: “do the forms interbreed fairly extensively where they co-occur in nature?” If yes, then they’re members of the same species. If not, they’re members of different species. (One other way to demarcate separate species is that if you cross the forms in captivity and the hybrids are completely sterile or inviable, they are separate species, for hybrids would also be sterile and inviable in nature. But if two forms hybridize in zoos and produce fertile offspring, as lions and tigers sometimes do, then it’s a judgment call. In fact, lions and tigers co-occurred in the Middle East in historical times and there are no records of hybrids. Hybridization is an artifact of captivity, as it breaks down the reproductive barriers that kept these cats isolated in nature. Lions and tigers are different species because they don’t exchange genes where they cooccurred in nature.)
The giraffes, living in different parts of Africa, can’t be tested this way because they don’t co-occur, so calling them four different species on the basis of DNA differentiation is a purely subjective exercise (see my post on the giraffes here).
There is one way that looking at genes can help us find new species that aren’t “subjectively described.” This is when you find what seems to be a single species in one area, but then genetic analysis shows that there are actually two forms that each are “fixed” for a different set of genes or chromosome patterns. This is prima facie evidence of non-interbreeding, and we have what biologists call sibling species. Two of the species I worked with, Drosophila pseudoobscura and D. persimilis, for example, were originally thought to be a single species (you can’t tell them apart by looking at them), but research showed that each group is “fixed” for a different set of chromosome arrangements, and you don’t find both arrangements in any individual, so there are no hybrids.
The last bit:
To help add more rigour to the business of defining new species, earlier this year Zachos and other biologists proposed establishing the first single authoritative list of the world’s species. “Species” itself will remain a slippery concept, but at least we could all agree on where to draw the lines.
No, we won’t all agree on where to draw the lines. The giraffe is merely one out of many, many cases in which biologists will quibble about which populations are different species.
To summarize, New Scientist is wrong: species do exist, regardless of some introgression, and we understand not just what they represent—that is, why nature is lumpy rather than continuous—but also how the lumps come to be.