More on species. Part 2: Why the BSC solves the “species problem”

In yesterday’s post I outlined what I see as “the species problem” (the existence of discontinuities in nature in one area), outlined the “biological species concept” (BSC), and suggested that the BSC was the best species concept to use when studying that species problem.  Let me reiterate that I don’t think there’s a single “right” or “best” species concept—each has its weaknesses (some more than others)—nor do I think there’s a single species problem, either. On page 26 of Speciation, Allen Orr and I list five different species “problems,” though the one that I find most intriguing is the one I mention above.  I favor the BSC because I think it is the best one for solving that species problem.

I’ll try briefly today to show why the BSC, which conceptualizes species as those entities whose members are interbreeding among themselves but genetically separated from members of other species by reproductive isolating barriers (RIBs), is the species concept that not only explains the species problem (i.e. the discreteness of nature) but offers a solution to how that discreteness evolves.

It’s actually pretty simple.  Two sexually-reproducing and related taxa living in the same area cannot maintain their distinctness unless they have evolved substantial barriers to gene exchange. If all species exchanged genes with their relatives, and did so commonly, then nature would form a continuum: you would not be able to instantly discriminate between a sparrow and a starling, or a gingko and an oak.  (Some “species” do have limited gene exchange; more on that later.)  The mere observation that related and distinct groups in one location maintain their distinctness means perforce that there are reproductive barriers between them, even if those barriers involve the ecological or developmental inferiority of hybrids that do form between them.

Thus, the discreteness of nature must have some connection to impediments of gene flow between the discrete groups.  This may be why Dobzhansky, a geneticist, was the first person to draw this connection.  In a paper in Revista di Scienza in 1937, he made this explicit:

Any discussion of these problems [of discontinuities in the living world] should have as its logical starting point a consideration of the fact that no discrete groups of organisms differing in more than a single gene can maintain their identity unless they are prevented from interbreeding with other groups .  . Hence the existence of discrete groups of any size constitutes evidence that some mechanisms prevent their interbreeding, and thus isolate them.

For the professionals reading this, let me note that disruptive selection between two types of plants or animals in one area can also maintain them as discrete entities, but this is really a form of reproductive isolation as well, since by definition the intermediate forms are maladaptive under disruptive selection. Maladaptive hybrids constitute reproductive barriers known as postzygotic isolation.

This hybrid inferiority, for example, is the case in benthic and limnetic sticklebacks (Gasterosteus aculeatus) in the lakes of British Columbia, two groups with ambiguous species status. Some consider them different species, but most, like my friend Dolph Schluter, consider them “incipient species” or morphs. They forage in different places in the lake, and eat different food.

Here are females of the two types, showing how different they are (benthic on top, limnetic on bottom: photo by Todd Hatfield):

Regardless of whether we call these forms “species,” though, the point is that these two forms do interbreed in the wild.  Why, then, do they still remain distinct? Because, as Dolph and others have shown, the hybrids are at an ecological disadvantage.  They cannot forage as efficiently as either the pure benthic or limnetic forms and so leave few offspring.  This is a type of reproductive isolating barrier (we call it “extrinsic postzygotic isolation”) that helps keep the benthic and limnetic forms distinct.

This kind of barrier also operates in plants.  Several readers have vociferously asserted that related species of plants hybridize profusely in sympatry.  And, indeed, plants do seem to hybridize more often than animals (but not as often as everyone thinks; see pp. 40-45 of Speciation).  What people don’t consider when asserting this is that if plants really did hybridize profusely, or even moderately, with their relatives in the same area, the groups would no longer remain distinct.  Those plants would blur into a single variable taxon.  The fact that they do remain distinct despite hybridization indicates pretty clearly that there is some problem with the hybrids, which is, after all, demonstrates reproductive isolation.  And indeed, when you investigate these situations, you do find hybrid inferiority. We outlines some cases in Speciation.

One of the groups that’s often said to flagrantly violate the BSC is the oaks (genus Quercus; this is often said of cottonwoods, Populus, as well). On pp. 43-45 of Speciation I dissected this story and found it grossly exaggerated.  Boundaries between oaks are not nearly as porous as commonly thought.  Further, some of the evidence of “hybridization” between plant species is based on exchange of mitochondrial DNA (mtDNA) or chloroplast DNA (cpDNA) Biologists love to use these types of DNA because they’re easier to obtain than nuclear DNA, and also evolve more rapidly.  But we now know that both mtDNA and cpDNA move between species a lot more readily than the vastly greater amount of nuclear DNA, so claiming pervasive gene exchange based on observations of mtDNA or cpDNA alone is a perilous claim, and is known to be wrong in oaks.

The main point is that reproductive isolation is what keeps species distinct in sympatry (i.e., in the areas where they encounter each other and could potentially exchange genes). Without RIBs, nature would be a continuum.  Therefore, understanding the origin and evolution of RIBs is equivalent to understanding why nature is discrete in one area.  That is why the BSC is the best species concept to use for addressing this particular species problem. What is the origin of species? Under the BSC, that question becomes equivalent to “What is the origin of reproductive isolating barriers between closely related species?”.  And that is a much more tractable question.  Reducing the problem of speciation—or at least of discrete groups of sexually-reproducing organisms in sympatry—to the problem of the origin of RIBs is perhaps the greatest achievement of the modern synthesis, an achievement that, unlike many others, wasn’t foreshadowed by Darwin.

It’s telling that when evolutionists are studying the origin of species—how new species come to be—nearly all of them adhere to the BSC.  That is, they study the origin of reproductive barriers between incipient species.  This holds even if those workers adhere to other species concepts.  We give an example in Speciation: my friend Kerry Shaw at Cornell is a strong advocate of the phylogenetic species concept (PSC), and has written papers defending it and asserting its superiority over the BSC.  Yet in her own research on speciation in Hawaiian wingless crickets (Laupala), Shaw studies the origin of differences in mating song and cuticular hydrocarbons, factors that keep different cricket species from hybridizing in sympatry. This is an implicit admission of the value of the BSC in studying the origin of species in nature.

In fact, I don’t know of a single paper studying the process of speciation in real animals and plants in nature that doesn’t implicitly adhere to the BSC, concentrating on factors that impede gene flow.  There may be a few I have missed, but I try to keep up with the literature.  This, again, is a tacit admission of the usefulness of the BSC in understanding the origin of those entities we call “species.”

Finally, some species concepts that claim to differ from, and be superior to, the BSC really incorporate the BSC into their assumptions.  One of these is the “evolutionary species concept” (ESC), currently the most popular alternative to the BSC and one much beloved by systematists.  The ESC considers a species to be a lineage that is evolving “independently of other lineages.”

This concept has even more problems than the BSC.  First of all, its adherents never define what they mean by “independent evolution.”  If there is even a tiny bit of gene exchange, are lineages still “independent”  What about the very common situation within species (an example is different subspecies of birds) when some genes for morphology or behavior are evolving independently in different places, because the birds live in different habitats that select for different morphology or behavior, but the rest of the genome is not subject to divergent selection and so is not evolving independently in those two lineages.

A bigger problem is that of allopatry: when groups can’t exchange genes simply because they are geographically isolated (note: that is not necessarily the same thing as reproductively isolated). According to the ESC, if a few lizards travel to a distant oceanic island on a vegetation raft, and start breeding there, they instantly constitute a new species the minute they set foot (feet?) on the island, because from that moment they are “evolving independently.”  Would anyone really want to consider these a new species, though, simply because they land on an island that makes gene exchange with their ancestors impossible? Consider this: nothing biologically important has changed at the moment when those lizards land on the island.  Is that really the moment of “speciation”, then?

So one of the main weaknesses of the ESC is that it conflates geographic isolation with genetic isolation. And when the distinctness of taxa (and lineages) really is discernible—in areas where they coexist—lineages evolve independently precisely because they are reproductively isolated! After all, it’s the absence of gene exchange that constitutes “independent evolution.”  In that sense, the ESC really puts the phylogenetic cart before the genetic horse.  To explain why lineages evolve independently in sympatry, you must explain why there are barriers to gene exchange between those lineages—and that’s the species problem addressed by the BSC.

The common assertion of systematists that they alone—and not those pesky population geneticists—have successfully divorced pattern from process (see, for example, any paper by Quentin Wheeler), is simply wrong.  The ESC implicitly requires an evocation of process—the evolutionary process that creates reproductive barriers between lineages.

(A mini-rant about systematics here. I love systematics and think it’s ground zero for nearly all evolutionary studies. You can’t understand much about evolution, and certainly nothing about speciation, unless you understand the evolutionary relationships between species.  That’s why I’ve spent a lot of time making phylogenies, and why my most cited paper [Coyne and Orr 1989, Evolution] involves combining phylogenies of Drosophila species with data on reproductive isolation.

Nevertheless, many systematists, especially cladists, seem to have gone off the rails, making dumb and insupportable statements about evolution.  I don’t know why this is, since cladism has as its purview one of the most important insights in evolutionary biology: Hennig’s recognition that one could help reconstruct the history of life using shared derived traits (these traits are called “synapomorphies”).  Nevertheless, cladists like Wheeler constantly impugn evolutionary genetics and assert that only systematists have the right to construct species concepts. They also claim that their own systematically-based species concepts divorce evolutionary pattern from evolutionary process.  As I’ve just shown, they are wrong.  The assertion that synapomorphies are the traits to use when constructing phylogenies is a statement about process: those synapomorphies are shared and derived because they’re passed on from ancestors to descendants during speciation! If that’s not a process, I’ll eat my boots.)

I have reached the end of my two-part rant.  I won’t convince everyone, of course, but I hope I’ve convinced at least some readers of the utility of the BSC for answering a most important problem of evolutionary biology: why is nature discontinuous rather than continuous?  For those who wish to go further, have a look at Speciation by Coyne and Orr, especially chapter 1 and the appendix.

Enough about species. Now on to something really important: boots!

37 Comments

  1. Clemmie
    Posted January 4, 2011 at 7:47 am | Permalink

    Jerry, I find this from the first paragraph confusing:

    “…I outlined what I see as “the species problem” (the existence of discontinuities in nature in one area), outlined the “biological species concept” (BSC), and suggested that the BSC was the best species concept to use when studying that species problem. Let me reiterate that I don’t think there’s a single “right” or “best” species concept—…”

    So is it the best or not? Did you mean something like “most useful” or “simplest” or even “most easily understood” rather than “best” in the first of those sentences. Yes, I know I used a lot of inverted commas – sorry. And sorry for the nitpick, but I think a clarification might help. Thanks.

    • Posted January 4, 2011 at 9:22 am | Permalink

      I saw that too, but think the point is he says “single right or best” with the emphasis is on “single”.

    • Gregory Kusnick
      Posted January 4, 2011 at 12:32 pm | Permalink

      I thought he was pretty clear. Since there’s more than one species problem, there isn’t any single best species concept for addressing them all. But for the particular species problem that Jerry considers most important — Dobzhansky’s problem of discrete morphological populations — the BSC is the best answer.

  2. Posted January 4, 2011 at 7:51 am | Permalink

    Thanks again for this rant!

    Your argument for the BSC is most persuasive…though I still think quantification of some sort would be quite useful. Being able to measure something, to put a number to it, goes an awfully long way to answering a great many questions.

    Cheers,

    b&

    • abb3w
      Posted January 5, 2011 at 2:07 pm | Permalink

      Probability of A and B having fertile descendants would seem to give a measure. Probability p=1, same; p=0, different. In between… well, you might care to look at the Ship of Theseus problem; the concept of mutual information can resolve that similarly.

  3. Posted January 4, 2011 at 7:57 am | Permalink

    “If that’s not a process, I’ll eat my boots.”

    If you mean all of your boots, then that’s a pretty serious wager.

    To this layperson, the BSC sounds hard to beat.

    • NewEnglandBob
      Posted January 4, 2011 at 9:42 am | Permalink

      This layperson concurs.

  4. Posted January 4, 2011 at 8:24 am | Permalink

    But we now know that both mtDNA and cpDNA move between species a lot more readily than the vastly greater amount of nuclear DNA, so claiming pervasive gene exchange based on observations of mtDNA or cpDNA alone is a perilous claim, and is known to be wrong in oaks.

    The forest-savanna elephant case is another one where the mtDNA argues for recent gene flow, and the nuclear genome argues for an ancient divergence.

    That said, I think it is too early to generalize about nuclear gene flow, we really have very few species pairs where the data are strong enough to test genome-wide. The reason we recognize frequent introgression in mtDNA is that we have so many large samples now.

    • whyevolutionistrue
      Posted January 4, 2011 at 8:34 am | Permalink

      I have a list of at least two dozen papers where they compare mtDNA with nuclear DNA and show that the mtDNA introgresses far more readily. In contrast, I have only one or two papers that shows the opposite result. And these papers involve many organisms. I think we’re really dealing with a consistent biological phenomenon here, though I have no explanation for it!

      • AJ
        Posted January 6, 2011 at 3:18 am | Permalink

        Couldn’t this perceived phenomenon simply be due to detection bias? I imagine that for mtDNA, introgression would be easier to demonstrate than for nuclear DNA, since there is no recombination in the former case.

      • qqq5qqq6
        Posted January 6, 2011 at 3:26 am | Permalink

        The pattern that seems to be present in most but not all cases of interspecies hybridization is that the genetic markers that cannot be transmitted by the dispersing sex (e.g., in mammals mtDNA for males; Y-chromosome for females) is a poor guide to species boundaries:

        “Clearly, the results presented here caution against the use of uniparentally inherited markers for species delimitation when they are inherited only from the least-dispersing sex.”

        This based on a survey of the literature on introgression of genetic markers in between-species hybridization events among dozens of pairs of taxa.

        Source:
        Gene flow and species delimitation.
        Petit RJ, Excoffier L.
        Trends Ecol Evol. 2009 Jul;24(7):386-93. Epub 2009 May 4.

  5. Tulse
    Posted January 4, 2011 at 9:50 am | Permalink

    Jerry, to what extent do you feel such species discontinuity is essential to the theory of natural selection? There seem to be a fair number of microbially-inclined researchers (such as Woese) who question even the notion of species in the world of single-celled organisms, and some (like the science historian Jan Sapp) have argues that things like lateral gene transfer threaten the standard Darwinian notion of natural selection, since it makes a hash of the species notion. It seems to me that tossing the baby out with the bathwater, since at its core natural selection can be described in terms of changes in gene frequency, and not necessarily species frequency. In your view, how critical is the species notion to the concept of natural selection?

    • whyevolutionistrue
      Posted January 4, 2011 at 9:56 am | Permalink

      Species discontinuity has little to do with natural selection per se, although it’s a product of natural selection. Clearly the notion of natural selection is largely independent of species concepts or discontinuity, because selection occurs in every species but perhaps asexuals aren’t discontinuous.

      In our book we consider whether asexuals like bacteria do form species in the sense of discrete entities with few intermediates. We consider this issue unresolved, but if they do form species it wouldn’t have anything to do with reproductive isolation. Really, not many people are studying what I see as an important question: discontinuity in asexuals.

      As for Jan Sapp and his lucubrations, they sound insane to me. Clearly lateral gene transfer doesn’t overturn the concept of natural selection: l.g.t. is more or less like a mutation: a new source of genetic variation that can be acted on by selection. (Not all transferred genes will confer a reproductive advantage). So to say that l.g.t. makes threatens the Darwinian notion of selection is to betray one’s ignorance. And l.g.t. doesn’t make a hash of the species notion, either. Really, do you think that the idea that humans and chimps are different species is threatened by lateral gene transfer, which is almost nonexistent in most metazoans? That notion seems silly to me.

      • Posted January 4, 2011 at 10:11 am | Permalink

        I think it is important to recognize that the issue is problematic if we are talking about evolution and life as a whole, instead of a relatively small corner of biological diversity.

        The biological species concept is great, with relatively minor problems, if we are talking about Metazoans and requires only a few adjustments for multicellular plants.

        For the vast bulk of life on earth though (Bacteria, Archea, and the hordes of single-celled eukaryotes) things do get much more problematic.

        In terms of LGT and classic Darwinian evolution yes, in one sense it is a type of mutation, but it DOES seriously impact the species question. Depending on just how rampant LGT is (or more appropriately was over evolutionary time) the more reticulated and net-like the majority of the “tree of life” becomes. And that makes a big difference in terms of what we are talking about with species, lineages, and ancestry in microbes.

  6. Posted January 4, 2011 at 10:10 am | Permalink

    I must say, not being a scientist, that your rants have been fascinating. I remember when I first read the Origin and realised that, while entitled the Origin of Species, Darwin didn’t have a clear definition of what a species is. Now I see a bit more clearly why. The one case that seems so fascinating to me is the one concerning the benthic and limnetic sticklebacks. They were geographically separated only in the way that they specialised in the areas they foraged for food. I suppose the same thing applies to the enormous diversity of cichlid species in Lake Victoria — though I speak from ignorance.

    • Frank
      Posted January 4, 2011 at 11:04 am | Permalink

      I am not sure it is fair to criticize Darwin for not having a “clear definition” of what a species is – this thread illustrates that no one does, even thought the BSC makes the most sense in terms of evolution.

      I think Darwin recognized that species boundaries were clear in many cases, but fuzzy in others (including putative cases of incipient speciation). With no real understanding of genetic mechanisms, he certainly could not address the speciation process as one could today. But I think the following quote from the Origin illustrates that he was well aware of the sloppiness in nature, a sloppiness that is glossed over by our emphasis on Latin binomials and clearly defined species:

      “Certainly no clear line of demarcation has as yet been drawn between species and sub-species —
      that is, the forms which … come very near to, but do not quite arrive at, the rank of species. … A
      well-marked variety may therefore be called an incipient species. … From these remarks it will be seen that I look at the term species as one arbitrarily given.”

  7. gillt
    Posted January 4, 2011 at 11:09 am | Permalink

    I had a great time accompanying a friend on a three-spine stickleback cataloging expedition in the lakes and ponds of the San Juan islands in Washington. These bodies of water are only as old as the last ice age (15-20k) so it’s interesting to see how distinct the populations are among the lakes and islands.

    Differences in male morphology/behavior can lead to reproductive isolation in these fish. Genome-wide linkage mapping is then used to characterize molecular pathways involved in speciation.

  8. Jollyreaper
    Posted January 4, 2011 at 1:20 pm | Permalink

    I’m still kind of disappointed that we have so few human varieties around. It seems like there were several promising lines of talking apes and they all died out in favor of homo sapiens sapiens, the ape so nice we named it twice. I take a look at the range of breeds within a single species like dog and wonder just what we could see among humans if we had geographic isolation for a few million years.

    My first thought was that humans got around, often far from where they were from, and so populations were never truly isolated for long. Humans only made it to the Americas after the last ice age and vikings and chinese were still able to make visits long before Columbus. With all that, the native americans didn’t manage to become all that different from the asians they descended from.

    But then if we consider the dog, all of the crazy-ass breeds are the direct result of human intervention. Purse dogs and Great Danes are all allegedly the same species. So how were they able to become so divergent in such a short time while humans remain so similar?

    Maybe it’s just the old fantasy fan in me but I miss that we don’t have lots of different hominid species running about, hobbits in their holes, shaggy-furred frost giants in the great forests of the north, seal-skinned waterfolk of the southern isles, the clannish dwarves who live deep in their delvings. And we mustn’t forget the elves, poncing about and making catty remarks about the other hominids.

    I suppose one argument against this kind of specific speciation is that humans don’t mold their bodies into tools, they make tools to fit their bodies. No need to grow shaggy coat when you can take one from an animal that won’t be needing it anymore, no need to develop fangs and claws when arrows and knives will take down prey. A crab develops a claw strong enough to crack a coconut and the finch who lacks the claw does without. The human just bashes the thing with a rock and is satisfied until he gets around to inventing the machete.

    • Posted January 4, 2011 at 1:37 pm | Permalink

      But then if we consider the dog, all of the crazy-ass breeds are the direct result of human intervention. Purse dogs and Great Danes are all allegedly the same species. So how were they able to become so divergent in such a short time while humans remain so similar?

      I think you know the answer to that question. Human intervention or, dare I say, “intelligent design” allowed for the changes in dogs. Human beings weren’t designed, things happened naturally and naturally humans do all the things you mention.

      I’m sure we could get a similar level of divergence in humans if someone completely controlled all breeding. :-)

      • Jollyreaper
        Posted January 4, 2011 at 2:23 pm | Permalink

        We’ve had just as much divergent interest in idealized human aesthetics. Look at the Africans trying to stretch necks, the Chinese angling for tiny feet, Americans looking for breasts large enough to be called gravity wells, etc. Granted, our boob fetish is somewhat recent but there have been cultures with continuity for a number of years and you’d think there would be enough selection pressure to see more divergence. Or are human generations too long, pressures too transient?

        • Thanny
          Posted January 5, 2011 at 4:46 am | Permalink

          First, such acquired characteristics (from foot binding or neck stretching) cannot be inherited.

          Second, even if you assume some kind of Baldwin effect, where those born with longer necks or smaller feet somehow gain a reproductive advantage (less work to be done with the stretching or binding?), you’re looking at most a couple thousand years of consistent cultural pressure on any of these traits. Likely quite a bit less.

          Maintaining lactase production after weening would be an example of culturally-driven human evolution. I don’t think any culture has been extant long enough to drive gross morphological evolution.

          • Jollyreaper
            Posted January 5, 2011 at 10:01 am | Permalink

            I’m not being Lamarckian, I would just think that if long necks are attractive, men would try to find wives who had long necks to begin with. But yeah, I was wondering if we just didn’t have enough time scale to work with here. In the west, the fascination with the rubenesque gave way to Twiggy in short order. We’ll likely be back to that in another decade or three.

  9. Andrew
    Posted January 4, 2011 at 2:30 pm | Permalink

    I have to read Speciation now! We learned about different species definitions in Evolution class as an undergrad. Its damn interesting! Carl Zimmer wrote an article about species concept too.

  10. Posted January 4, 2011 at 3:31 pm | Permalink

    Jerry said:

     In that sense, the ESC really puts the phylogenetic cart before the genetic horse..To explain why lineages evolve independently in sympatry, you must explain why there are barriers to gene exchange between those lineages—and that’s the species problem addressed by the BSC.

    I find it funny you should put this difference like that, because I think you’re doing the very same thing when you discuss “extrinsic postzygotic isolation”:

    Regardless of whether we call these forms “species,” though, the point is that these two forms do interbreed in the wild.  Why, then, do they still remain distinct? Because, as Dolph and others have shown, the hybrids are at an ecological disadvantage.  They cannot forage as efficiently as either the pure benthic or limnetic forms and so leave few offspring.  This is a type of reproductive isolating barrier (we call it “extrinsic postzygotic isolation”) that helps keep the benthic and limnetic forms distinct.

    Here you are putting the genetic cart before the ecological horse. To explain why there is “extrinsic postzygotic isolation” and a barrier to genetic exchange, you are really saying that it’s the ecology that is responsible. Here, hybrid depression is solely due to the particular environment, and that could change immediately, e.g., by a new resource appearing that supports the hybrids. The better explanation is therefore the Ecological Species Concept (not to be confused with the Evolutionary Species Concept).

    Why is nature discontinuous rather than continuous? Because resources are discontinuous.

    • whyevolutionistrue
      Posted January 4, 2011 at 4:56 pm | Permalink

      You are aware, aren’t you, Bjorn, of all the recent work showing that you can get discontinuous species arising from continuous distributions of resources? That’s one of the most popular models of sympatric speciation, and shows that organic discontinuity need not reflect resource discontinuity. You shouldn’t be so dogmatic, putting statements in bold and stuff, unless you can refute those studies.

      • Posted January 5, 2011 at 8:41 am | Permalink

        Hey, I’m not being dogmatic. At all. Not sure why you say that. (Bold for emphasis, but I can discontinue that if it seems trolly).

        Yes, I am aware of adaptive dynamics by Doebeli and Dieckmann, for example, and you’re right you can get sympatric speciation based on continuous resources (and I don’t refute them – I just think they are very limited in their applicability, because resources aren’t always continuous – think various sugars). But my point was that in the example you give of extrinsic postzygotic isolation, hybrid depression is driven by discontinuous resources. That can equally well lead to sympatric speciation. But you are saying the cause is really, in this case, reproductive isolation, when I think it’s much more accurate to say that the resource distribution is doing it, particularly since those hybrids are totally viable and presumably fertile. And since the ecology is driving the speciation, it is more appropriate to apply the ecological species concept.

        As for species definitions, I am of the opinion that we should accept two species of they conform to any of the, how to say, “major” definitions. I would count BSC (reproductive), EvoSC (phylogenetic), and EcoSP (resource), for starters.

    • whyevolutionistrue
      Posted January 8, 2011 at 12:54 pm | Permalink

      At the end of chapter 1 of Speciation, Allen Orr and I discuss the reasons why nature is discontinuous. That’s a different problem from simply constructing a species concept, which encapsulates that discontinuity in words. What is it about organisms—at least sexually reproducing organisms—that gives them an inherent tendency to divide up into discontinuous groups rather than ones that intergrade?

      Bjørn suggests above that the answer is that resources are discontinuous. While that may be the answer for asexually reproducing taxa, like bacteria, I don’t think it’s a general explanation for sexually-reproducing one, or for all taxa, for several reasons.

      1. Taxa that have a mixture of sexual and asexual reproduction, like dandelions, don’t form discontinuous groups, but a quasi-continuous mess. Yet the resources for dandelions must be as discontinuous as the resources for other plants. This suggests that the mode of reproduction plays some role in discontinuities, and that discontinuous resources can’t be the overwhelming explanation.

      2. As I mentioned above, sympatric speciation models show that one can, under many circumstances, get discontinuous species evolving when they’re competing on a continuous resource. If that can happen in sympatry, it can also happen in allopatry if species evolved to use a given resource come back into sympatry and compete for that resource.

      3. In some cases where species use identical resources, as in the even- and odd-year salmon I talk about in Speciation, they nevertheless can evolve reproductive barriers that can lead to speciation.

      4. Most ecologists I know no longer wholly accept the competitive exclusion principle—that species using the same resources cannot coexist. There are many circumstances, at least in theory, where they can. If that’s the case, then discontinuity of species can arise and persist without their having any difference in resource use. (The discontinuity could arise for reasons other than resource use–see next points.

      5. Sexual selection can lead to speciation and it really has nothing to do with discontinuous resources, but with female choice for various male traits. It is hard to conceive of discontinuous species isolated largely by mate preference having anything to do with “discontinuous resources.” Cichlid fish in African rift lakes are an example of this, and in many cases studies of diet have shown largely identical resource use, perhaps with only small differences in the proportion of various foods ingested.

      6. Finally, differential use of resources is only one of many evolutionary forces that can lead to the evolution of reproductive isolation and speciation. Here are a few: meiotic drive, genetic drift, sexual selection (mentioned above), adaptation to different climates (climates are neither “resources” nor “discontinuous”), polyploidy (responsible for much plant speciation), and so on. Therefore there is no necessary connection between discontinuous species and discontinuous resources.

      That said, Orr and I do posit that in taxa that reproduce almost completely by asexual means, their discontinuity may reflect resource discontinuity—or at least some theories suggest that that could happen. However, I don’t think we’re at the point where we can say with assurance that groups like bacteria really do form clusters in nature that are as distinct objectively as are sexual taxa.

      • Posted January 9, 2011 at 12:24 pm | Permalink

        Bjørn suggests above that the answer is that resources are discontinuous.

        Hold on. Not in all cases. I was talking about the case you discussed in this post, where hybrids between the limnetic and benthic sticklebacks are unfit only because they aren’t well adapted to resource use. I did not mean in general.

        1) Yes, the mode of reproduction plays some role in speciation.
        2) Yes, competition for continuous resources can lead to speciation.
        3) Yes, reproductive barriers can arise when resources are shared.
        4) Yes, sexual selection can lead to speciation.
        5) Yes, there are many other mechanisms which can lead to speciation.

        It would seem you have missed my point.

        My point is this. The case of extrinsic postzygotic isolation has everything to do with discontinuous resources, and nothing to do with questions of reproduction. That the hybrids are of low fitness is a result of discontinuous resources, in this case. And it is because of this that I think the BSC is not a good definition to use in this case. The take-home message is that, since the mechanism by which two species originate and are maintained is ecological, the EcSC is the better one to use in this case. And lastly, since no one definition always works, the following meta-definition is preferable (and perhaps the one that people really live by):

        Species are groups of populations that can be distinguished by at least one of the following criteria: BSC, GCSC, EvSC, EcSC, PSC…”

  11. Achrachno
    Posted January 4, 2011 at 7:24 pm | Permalink

    JC: I have reached the end of my two-part rant. I won’t convince everyone, of course, but I hope I’ve convinced at least some readers of the utility of the BSC for answering a most important problem of evolutionary biology: why is nature discontinuous rather than continuous? For those who wish to go further, have a look at Speciation by Coyne and Orr, especially chapter 1 and the appendix.

    I’ve ordered it, but it’s obvious this discussion is going to be extinct before it even arrives! Can we reopen this topic again in a couple of weeks?

  12. Insightful Ape
    Posted January 5, 2011 at 10:24 am | Permalink

    Dr Coyne, you write with absolute clarity. It is a pleasure reading your posts. If anything, Mooney could learn a thing or two from YOU about communicating science.

  13. abb3w
    Posted January 5, 2011 at 1:36 pm | Permalink

    As a quibble: reproductive isolating barriers (RIBs) so far as I understand develop probabilistically (99.9% chance of fertile descendants, 90%, 50%, 5%, 0.001%… somewhere in there, it starts to be considered two species). Therefore, it would seem if you look closely enough, the apparent discreteness actually breaks down into a fuzzy continuum again.

    Of course, this corresponds to the degree that the BSC breaks down as well. Any problem is not in the math or the science, but concepts that human brains develop to try and wrap around the idea. Non-binary logics make people’s heads hurt.

    • whyevolutionistrue
      Posted January 5, 2011 at 1:48 pm | Permalink

      We do take up that “quibble” (which isn’t a quibble but a serious point) in our book. I don’t want to reiterate what we said at length there, but the gist was that things become more and more “species like” as barriers become stronger to the point where they’re essentially complete, in which case things are good species. And we have many cases of closely related sister species that show NO evidence of gene flow, so it’s not the case that BSC status is always a judgment call.

      • abb3w
        Posted January 7, 2011 at 10:45 am | Permalink

        Oh, I’m not arguing the division is a subjective judgement call, nor even that a modified BSC model using “fuzzy” membership concepts might be day-to-day useful; I’m just indicating that the mathematical tools for a more comprehensive model exist.

        Nice to know this has been addressed. Are you aware of any work on what numerical function describes how fast a genetic (as opposed to environmental) strength-a probability barrier goes to a strength-b probability barrier?

  14. Alex SL
    Posted January 5, 2011 at 6:22 pm | Permalink

    Okay, I now find that we agree nearly completely. In essence, you are also saying that you do not mind a low degree of gene flow between species.

    The thing is just, this is not what I or most people I know would call the BSC. Mayr, who supposedly formulated it, even wrote of only potentially interbreeding populations as being in one species. And the “reproductively isolated” part of the definition kinda sounds like it was meant to be 100%.

    For what you now seem to describe, I have learned the term Phylogenetic Species Concept: yes, there may be a bit of gene flow, but it is not enough to produce a continuum, either because it is too little in comparison to the dominating “pure species” gene pool, or because the ecological and morphological characters that make the pure species distinct are selected for, but markers that are not under selection, like plastid or non-coding DNA, still allow us to see the introgession (the latter being the point of the genic view of species).

  15. Jim Thomerson
    Posted January 19, 2011 at 6:46 pm | Permalink

    I’m a bit surprised that the term “niche” didn’t come up in the discussion of the nature of species.

    As I understand it, “sympatric” and “allopatric” have to do with ranges as drawn on range maps. A couple of useful terms were created by the late Luis Rivas, an ichthyologist, back about 1953. “Syntopic” means you catch them together in the same seine haul. i. e. they normally encounter each other and could hybridize if they wanted to. Allotopic means they do not ordinarily encounter each other (not occurring in the same seine haul). A salmon and a sequoia might be sympatric, but they probably would not be considered syntopic.

  16. Jim Thomerson
    Posted January 20, 2011 at 10:34 am | Permalink

    It seems to me that the Phylogenetic Species Concept means that a species originates from a speciation event and terminates with extinction. It is a definition which adds a through time historical element. However, if one examines the species at some moment in time, as we do, what is found is a species conforming to the Biological Species Concept, or perhaps some other.


5 Trackbacks/Pingbacks

  1. [...] Article one: elephants and the recent article on the two “species” of African elephants. Article two on speciation. Article three on speciation. [...]

  2. [...] of Why Evolution is True, posted a couple of thought-provoking essays (he calls them rants, Part 1, Part 2) on the Biological Species Concept (BSC). There are more than a few species concepts, something [...]

  3. [...] addressed with respect to how many species of elephants there are (with follow-ups here and here). Ernst Mayr defined species in 1942 as Species are groups of actually or potentially interbreeding [...]

  4. [...] http://whyevolutionistrue.wordpress.com/2011/01/04/more-on-species-part-2/ [...]

  5. [...] http://whyevolutionistrue.wordpress.com/2011/01/03/more-on-species-part-1/                           http://whyevolutionistrue.wordpress.com/2011/01/04/more-on-species-part-2/ [...]

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