Pharyngula

Flap those gills and fly!

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I am going mildly nuts right now—somehow, I managed to arrange things so multiple deadlines hit me on one day: tomorrow. I’ve got a new lecture to polish up for our introductory biology course, a small grant proposal due, and of course, tomorrow evening is our second Café Scientifique. Let’s not forget that I also have a neurobiology lecture to give this afternoon, and I owe them a stack of grading which is not finished yet. I’m really looking forward to Wednesday.

Anyway, so my new lecture for our introductory biology course is on…creationism, yuck. What I’m planning to do is to describe some of the most common creationist arguments and then give a biologist’s rebuttal. Creationism is really a waste of our class time, but using it to explain some general concepts that any informed biologist should understand (and that the creationists, including Mike Behe, are astonishingly clueless about) will make it a little more productive, I hope. We’ll find out tomorrow.

One of the common creationist claims I plan to shoot down is the whole idea of “irreducible complexity” as an obstacle to evolution. I was going to bring up two ideas that invalidate it: the principle of scaffolding (which I discussed here), and exaptation, in which features evolved for some other purpose than the one that they play in an organism we observe today. I was looking for a good example, and then John Wilkins fortuitously sent me a paper that filled the bill (we evilutionists, you know, are sneakily sending each other data behind the scenes to help in our assault on ignorance. We’re devious that way.)

The question is how insect wings evolved. Wings are a classic issue in evolution, because they aren’t going to function for flight at all until they’ve achieved a certain minimal size—half a wing isn’t any good at all for getting an animal in the air, so any explanation for their selective evolution has to incorporate alternative functions: as stabilizers for cursorial animals, for instance, or traps for catching small prey on the run.

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In insects, we have an interesting origin explanation for wings: they’re modified gills. It makes sense. For gills, you want to have an increased surface area for gas exchange, and you want them exposed to the external environment. Most animals evolved sophisticated gills with convoluted surfaces and tucked them away in a protective chamber, with a mechanism to pump water over them, but others took a simpler path. Mayflies, for instance, have flat vanes on each segment in the larval stage as respiratory surfaces—they even look like wings. Arthropods evolved a recipe for flat, cuticular structures to serve as gills, and perhaps one explanation for the evolution of wings is that they simply re-evoked that recipe as adults, used it for gliding, and then expanded and elaborated on the formula incrementally to generate flapping, powered flight.

More evidence for this hypothesis comes from an analysis of non-flying arthropods, the crustaceans. The arthropod limb is primitively complex with multiple branches, shown below, while insects have stripped it down to a simpler jointed stalk. Many crustaceans have retained the tripartite branching structure of the limb, with an endopod (the foot), an exopod, and of most interest to us right now, a dorsal epipod.

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The insect arrangement is illustrated at the top. They have wings and legs, diagrammed as simple discs (appropriately; they form from imaginal discs in the larva). We also have a lot of information about patterns of gene expression in these structures in insects. A gene called engrailed (en) is expressed in just the posterior half of each segment, and this gene has the same pattern in crustaceans. There is also a gene called Distal-less (Dll) that is expressed in all appendages; that one isn’t quite as interesting for this study. The genes that are particularly provocative are pdm, which is expressed only in the insect wing and not in the leg, and apterous (ap), which is expressed only in the dorsal half of the wing and in a narrow ring on the leg. The question is whether a) crustaceans also have pdm and ap, and b) if they do, are they expressed in the epipod, which would suggest that wings and epipods are homologous structures.

And the answer is yes to both. Genes homologous to the Drosophila ap and pdm genes were identified in Artemia, and they are active in just the epipods of the crustacean limb. Pdm is similarly active in the epipods of the crayfish.

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Artemia thoracic limb. b)Expression of Dll in all outgrowing regions of the Artemia limb. c-e) Expression of pdm in Artemia. f-h) Expression of ap. i-k) pdm expression in the thoracic limbs of Pacifastacus leniusculus.

What it implies in the evolution of the arthropods is that the wings of pterygote insects are derived from epipod gills, or alternatively, have coopted a molecular pathway that first arose in epipods. While most of the terrestrial arthropods have been simplifying their limbs, the winged insects retained one element that gave them the power of flight.

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It’s this kind of history that invalidates Behe’s notion of irreducible complexity. Sure, it’s hard to imagine why an aquatic arthropod would begin the stepwise Darwinian process of assembling a set of wings for flight, but what this work shows is that there is an incremental pathway for expanding epipods as aqueous respiratory surfaces. The mistake creationists make, which seems intrinsic to their nature, is to assign functions erroneously to adaptations, when the simpler idea that structures have only local and immediate functions is far more productive over the long term. It’s the same with Behe’s favorite example, the flagellum: if it evolved as a secretory pump first, it wouldn’t have required every feature of an “outboard motor” to function. His mistake is to assume that every step in its evolution was part of a drive to make a motor.


Averof M, Cohen SM (1997) Evolutionary origin of insect wings from ancestral gills. Nature 385:627-630.

Comments

  1. #1 ScottM
    July 27, 2006

    Thanks so much for suggesting Endless Forms Most Beautiful: The New Science of Evo Devo. You were right about it being well written– there were a few times when he wandered deeper into the details than I expected. On the plus side, after reading it, Distal-less (Dll) is immediately familiar.

  2. #2 Bob
    July 27, 2006

    Dawkins actually talks about insect wings in one of his books. I read it awhile ago and I am going to get this wrong. I think the book was Climbing Mount Improbable. dAVE’s mention of even small wings being useful is part of what Dawkin’s talked about. IANAEB, but I think is argument it involved the fact that at the incredibly small size of an insect, it really doesn’t take much to get airborne at all. For some of the smallest insects, a mere stub is enough to provide steering because the slightest breeze gets you in the air. So a slight stub, less than the proverbial half a wing, gets you flight and steering. Then the wing can gradually grow and get you increased mobility and you can grow and get bigger at the same time.

  3. #3 procyon
    July 27, 2006

    Some cool movies showing how insect flight could have evolved from surface “skimming” insects can be viewed here: http://www.bio.psu.edu/People/Faculty/Marden/skim.html

  4. #4 Chris Ho-Stuart
    July 27, 2006

    In discussing Irreducible Complexity, I often cite the 1918 paper by Herman Muller, Genetic Variablity, Twin Hybrids and Constant Hybrids, in a Case of Balanced Lethal Factors, (linked) which appeared in Genetics, Vol 3, No 5, Sept 1918, pp 422-499. Muller describes irreducible complexity as prediction of evolutionary processes. The term he used was “Interlocking Complexity“, which I think is a much more sensible term capturing the essential aspect of IC. It is plainly the same concept as Behe’s IC.

    Here is an extract (italics from original):

    … Most present day animals are the result of a long process of evolution, in which at least thousands of mutations must have taken place. Each new mutant in turn must have derived its survival value from the effect which it produced upon the “reaction system” that had been brought into being by the many previously formed factors in cooperation; thus a complicated machine was gradually built up whose effective working was dependent upon the interlocking action of very numerous elementary parts or factors, and many of the characters are factors which, when new, where originally merely an asset finally become necessary because other necessary characters and factors had subsequently become changed so as to be dependent on the former. It must result, in consequence, that a dropping out of, or even a slight change in any one of these parts is very likely to disturb fatally the whole machinery; …

    Cheers — Chris

  5. #5 Steviepinhead
    July 28, 2006

    No later than the Cambrian, certainly!

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