Species, not arbitrary, but not clear & distinct

By Razib Khan | February 8, 2011 1:48 am

John Hawks and Jerry Coyne are mooting the ‘species concepts’ debate, with particular focus on recent human origins (specifically, the relationship of modern humans to Neandertals and Denisovans). Coyne, who coauthored the book Speciation and remains preoccupied with the issue in his academic work, knows of what he speaks. And of course he wouldn’t think that the discussion of species, how to delineate them, and what they are, is a sterile exercise. He has chosen to allocate a significant portion of his life to the topic. I think very few would disagree with Coyne when he contends that “Species are not arbitrary divisions of an organic continuum.” If there is one taxonomic category which has a concrete basis in reality, that would seem to be species. But, I would observe that I’m not sure that species are necessarily so clear and distinct. After all, we know that there is here and there, but where does here end, and there begin?

I’m of a reminded of the classic Zeno’s paradox:

In the paradox of Achilles and the Tortoise, Achilles is in a footrace with the tortoise. Achilles allows the tortoise a head start of 100 metres. If we suppose that each racer starts running at some constant speed (one very fast and one very slow), then after some finite time, Achilles will have run 100 metres, bringing him to the tortoise’s starting point. During this time, the tortoise has run a much shorter distance, say, 10 metres. It will then take Achilles some further time to run that distance, by which time the tortoise will have advanced farther; and then more time still to reach this third point, while the tortoise moves ahead. Thus, whenever Achilles reaches somewhere the tortoise has been, he still has farther to go. Therefore, because there are an infinite number of points Achilles must reach where the tortoise has already been, he can never overtake the tortoise.

The Greeks had a fascination with paradoxes because they perceived that they illuminated deep truths about the true nature of reality which we may have been blind to via sense perception. But sometimes I think that a fixation on species as the taxonomic category to rule them all confuses and calcifies the understanding of evolutionary genetic processes in the eyes of the public. Just as it is difficult to communicate that science is not a collection of facts, that it is a process and a method, so many people seem to take species categorizations as reflecting the true order of the universe. Historically this goes back to the pre-evolutionary taxonomists whose aim was to catalog all of God’s creation. It persists today explicitly among Creationists, who bandy about terms like “kinds,” but are really talking about immutable ideal entities with particular essences. Species on steroids if you will.

A fixation on the species has also confused the public on the issue of microevolution vs. macroevolution. Creationists regularly accept the former and reject the latter, despite the fact that the majority of biologists would probably assert that evolutionary processes are a unified whole, with no difference of scale. The constant usage of the term “microevolution” by the enemies of evolution seems to have even cast that term into some disrepute, I’ll admit to be shocked when a reader was confused as to the non-Creationist usage when I recommended Alan Templeton’s Population Genetics and Microevolutionary Theory. One could argue that the subject of modern population genetics is fundamentally microevolution.

Species are obviously abstractions. But I think an analogy can be made between them and physical objects. At the end of the day we know that the solidity and boundedness of physical objects are perceptions and interpretations filtered through our brains. Fundamentally they’re a bundle of particles and forces, interacting with other particles and forces. We don’t need to deny this deep reality, all the while instrumentally acknowledging the usefulness of categories of physical objections.

CATEGORIZED UNDER: Biology, philosophy

Comments (9)

  1. I like the “physical objects” analogy. “Species” remains a mirage of a term for many. For that matter even “evolution” is a far more ambiguous, amorphous term than some would imply. Their lack of precision doesn’t take away from their scientific applicability, but it does lead to very prolonged, nuanced (and maybe unresolvable) debates/discussion.

  2. Vincent Vizachero

    Nicely done. I, too, liked the analogy to physicality.

  3. bob sykes

    Thanks for the reference to Hawkes and Coyne.

    As a retired environmental engineer/scientist, I have long followed the debate about species. It is very confusing indeed. The classic Dobzhansky/Mayer definition (breeding population) works very well for animals. It is largely irrelevant for Bacteria/Archaea, which are mostly clones with occasional DNA transfer.

    But what is most interesting is that conservationists, especially those engaged in species preservation, will have none of the Dobzhansky/Mayer definition. Instead, they rely entirely on morphology, and often trivial bits of morphology at that. E.g., the Florida panthers bent tail. This makes all sorts of decisions required by the Endangered Species Act to be unavoidably arbitrary and capricious. So much for science-based policy.

    Most working biologists also depend on morphology over breeding, in large part because applying the Dobzhansky/Mayer theory to long-lived animals is all but impossible. Of course, if your studies cover a restricted geographic region, morphology is a very good species criterion.

  4. Markk

    Look at Oak trees of various kinds in North America for a failure of the species idea to match up the genetics and morphology of “species”. It is a total smash. Gene frequencies do not match up well with what plants look like, and gene “transfer happens all over the “species” map and looks quite different to a morphological map.

  5. J. Wong

    Just a note that Zeno’s Paradox is not a paradox at all. Zeno was trying to prove (by contradiction) that nothing moved. His paradox assumes that with a converging series in length you cannot have a converging series in time. This is clearly false: Both series must agree so for Zeno then the series in length was also not converging (i.e., infinite), and movement is an illusion. The reality of course is that both series converge, hence no paradox.

  6. Andrew Lancaster

    I agree with others that you’ve balanced this well. I think a lot of people who think about this subject intelligently come down to having to balance pretty much the same fixed points you mention: a species is not normally just arbitrary; but then again there are reasons to say that species are not perfectly well defined. Thanks.

  7. Chris T

    Even animals can get weird. What do we make of two animals that could potentially interbreed, but if left alone in the wild, would not?

  8. As biologist Joan Roughgarden puts it, Nature abhors a category. In a universe of multitudinous patterns and structures, working from fundamental particles upwards, our abstractions cannot be definitive “all the way down” but they can usefully pick up aspects of reality, provided we do not expect too much of them. (Which natural law theory generally does, for example. And racism really does.)

  9. #3 Bob: the legal definition of species under the ESA for vertebrates doesn’t match a biological definition. That doesn’t make the legal definition wrong: preserving the existence and genetic diversity of subspecies or even less-isolated subcategories has biological value, even if biologists might not like how “their” word is being used by lawyers.

    In any event, it’s only for vertebrates, not for invertebrates or plants.


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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com


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