We had some good discussion on the thread about elephants yesterday, and by “good” I don’t mean “everyone agreed,” because they didn’t. I was pleased to see people exchanging and defending their divergent views.
The quarter starts today, teaching looms, and I don’t have a lot of time to write this morning, but I’d like to mount a brief defense of the biological species concept (BSC). Today I’ll just explain what the BSC is and state what I consider to be the “problem of speciation,” recognizing that not everyone will agree. Later today, or tomorrow, I’ll explain why I think the BSC is the best concept to address that problem.
The BSC defines (or rather conceptualizes) species as groups of interbreeding individuals that are separated from other species by reproductive isolating barriers. By “reproductive isolating barriers,” I mean genetically based traits of organisms that prevent them from exchanging genes with members of other species. (Gene exchange can occur only when members of two different groups can form fertile and viable hybrids).
These barriers can include genetically based ecological differences that limit species to different habitats, so that they never meet and thus can’t mate; “sexual isolation”, the very common tendency for individuals to court and mate preferentially with members of their own species; the inability of pollen from one species of plant to grow down the stigma of members of another; the preference, when a female is multiply inseminated by a member of her species as well as a member of another, for using the same-species sperm to fertilize eggs; and the various forms of “postzygotic isolation” that prevent gene flow after members of different species have already mated and formed fertilized eggs. These last forms include the inviability of hybrids or their sterility (the mule, a hybrid between a donkey and a horse, is a classic example of hybrid sterility).
All of these reproductive barriers keep members of different species from exchanging genes, that is, the barriers maintain the integrity of species. That doesn’t mean that they evolved to maintain the integrity of species. In most cases they didn’t. As Allen Orr and I show in our book Speciation (from which all of these ideas are taken), in nearly all cases reproductive isolating barriers (RIBs, a good Chicago acronym), are the accidental byproduct of divergent evolution between populations.
For example, two geographically isolated populations (don’t mix up geographic isolation with reproductive isolation!), may evolve by natural selection to adapt to different habitats. When that divergent evolution has proceeded sufficiently far, the genomes of the different “populations” may have diverged so much that they can’t work well when combined in a hybrid individual. The hybrid could then be inviable or sterile, a form of reproductive isolation that prevents gene exchange. As this example shows, such barriers can simply be “accidents”: the byproduct of what happens when populations evolve along different paths.
I think the existence of species, in most cases, simply reflects these accidents of evolution. These discrete groups of organisms (see below) are simply what transpires when geographically isolated populations evolve away from each other. And their discreteness then becomes evident when those newly-evolved species come back to coexist in the same area. (For truly, one can see the discreteness of species only when they’re living in the same place. That gives a clue to the connection between the species problem and the BSC.)
In a few cases, however, natural selection can directly favor the production of discrete groups. One of them is “reinforcement”, which we’ve discussed before, in connection with my student Daniel Matute’s recent paper. The other, of interest mostly to evolutionists, is sympatric speciation.
If you read Speciation—and I realize that most readers haven’t—you’ll see that we have extensive discussion about alternative species concepts, and of the problems of both those concepts and the BSC. (By the way, if you’re at all interested in speciation you should read the book. I don’t like sounding like Chris Mooney with incessant repetitions of “read my book,” but I’m quite proud of it. It took Allen and me six years to write, and involved reading hundreds and hundreds of papers.)
I know that the BSC can’t extend to asexual organisms, and in many cases is somewhat subjective. What do you do, for example, if two groups have just a limited amount of gene exchange? What about if two populations occur in different areas, like the elk and the red deer, so there’s no possibility of them encountering each other—something that is almost required to determine whether they can exchange genes? (You can force-mate them in zoos, of course, but that’s a one-way test. If they do produce fertile hybrids in a zoo, that doesn’t tell you that they’d do so in nature, for the enforced confinement of a zoo may break down reproductive barriers that would operate in the wild. But if they do produce inviable or sterile hybrids in a zoo, like the Indian and African elephants, that tells you that they’re almost certainly biological species.)
But let’s put this aside for the nonce. I’d recommend reading Chapter 1 and the Appendix of Speciation if you’re interested in species concepts. (You don’t have to buy the book: many libraries have it.) Many of you might not agree with our take, but at least read it before you belabor me for not considering the problems of the BSC and the “advantages” of other species concepts.
Why I like the BSC is because it has a natural connection to what I—and many early evolutionists at the beginning of the “Modern Synthesis“—consider the “big species problem.”
The problem is to explain why nature is discontinuous rather than continuous. Why, in one patch of forest, do all the birds, insects, and mammals (and yes, most of the plants, too!) fall into a limited number of discrete, objectively recognized categories? Nature is not a continuum, with blackbirds blurring into starlings blurring into sparrows and so on. If that were true, what good would Peterson’s bird guide be? Nature—at least that moiety of nature that reproduces sexually—is discrete.
This problem was stated at the very beginning of the book that is widely regarded as having launched the modern synthetic theory of evolution, Genetics and the Origin of Species (1937), written by my academic grandfather, Theodosius Dobzhansky:
Right at the beginning of this book (and I recommend the first edition to budding evolutionists) Dobzhansky sets out the “species problem”. On the first page he describes how many species there are on earth (he estimated 822,765 at that time!), and then, on the second page, is a section called “Discontinuity.” I quote at length, because this is perhaps the best existing statement of the problem of speciation. (Note, by the way, what a splendid writer Dobzhansky was. And his native language was not English, but Russian!)
Organic diversity is an observational fact more or less familiar to everyone. It is perceived by us as something apart from ourselves, a phenomenon given in experience but independent of the working of our minds. A more intimate acquaintance with the living world discloses another fact almost as striking as the diversity itself. This is the discontinuity of organic variation.
If we assemble as many individuals living at a given time as we can, we notice at once that the observed variation does not form a single probability distribution or any other kind of continuous distribution. Instead, a multitude of separate, discrete, distributions are found. In other words, the living world is not a single array of individuals in which any two variants are connected by unbroken series of intergrades, but an array of more or less distinctly separate arrays, intermediates between which are absent or at least rare. Each array is a cluster of individuals, usually possessing some common characteristics and gravitating to a definite modal point in their variations. . .
Dobzhansky then goes on to talk about the discreteness of cat species (bless his heart), and shows that there’s no continuum between house cats, lions, and other felid species. Cat species are discrete, and any cat from nature is easily recognizable (that’s why field guides work!). He continues:
What has been said above with respect to the species Felis domestica and Felis leo holds for innumerable pairs of species, genera, and other groups. [JAC: I'd maintain that Doby was wrong about groups above the species level: genera, families, etc. are not as discrete, or as objectively recognized, as species.] Discrete groups are encountered among animals as well as plants, in those that are structurally simple as well as in those that are very complex. Formation of discrete groups is so nearly universal that it must be regarded as a fundamental characteristic of organic diversity. An adequate solution of the problem of organic diversity must consequently include, first, a description of the extent, nature, and origin of the differences between living beings, and second, an analysis of the nature and the origin of the discrete groups into which the living world is differentiated.
(My emphasis.) Although students no longer seem to be interested, or even aware of, this fundamental problem, it was something that preoccupied the founders of the modern synthesis, including Dobzhansky and Ernst Mayr.
As we point out in Speciation, there is no “best” species concept. Different species concepts are useful for different purposes. But I contend that the biological species concept is the only one that enables you to tackle, and ultimately to solve, the “species problem” described by Dobzhansky. It’s the only one you can use to address the problem of why animals and plants in one area fall into a finite number of discrete groups.
In the next (and final) post on this, I’ll explain why.