PZ Myers. 2005 Oct 24. Flap those gills and fly!. <http://pharyngula.org/index/weblog/flap_those_gills_and_fly/>. Accessed 2008 Aug 20.
Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Monday, October 24, 2005
Flap those gills and fly!
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.
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.
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.
a) The branched morphology of an 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.
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.
Science • EvoDevo • Organisms • 2 Trackbacks • Other weblogs • Permalink
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It's interesting to juxtapose this with the walking stick species that apparently evolved flight and then lost it and then re-evolved it, etc., multiple times of hundreds of millions of years:
http://news-info.wustl.edu/tips/2003/science-tech/walkingstick.html#: Posted by on 10/24 at 10:49 AM -
Paul wrote:
"Creationism is really a waste of our class time,..."
Agreed.
They're paying you to teach them biology, not proselytize your own ideology. I would have thought that you would be against "teaching the controversy"?
LIBERAL
Ten degrees to the left of center in good times,
ten degrees to the right when it affects you personally.#: Posted by charlie wagner on 10/24 at 11:02 AM -
Damned if you do, damned if you don't.
They are going to learn some real biology. One slide to illustrate a bogus problem brought up by Behe, a dozen slides after explaining how reality works. - But its not, "teaching the contraversy", its burying the contraversy in the back yard, along with the rest of the fertilizer. "Teaching the contraversy", would require not discrediting the whole of IDs arguements, but rather implying that they have some validity and should be considered an alternative. Ask yourself, as a believer in this nonsense, how PZ telling his students, "This is bullshit and here is why.", is somehow positive for ID? Then again, you haven't let logic get in the way of believing in it, so it wouldn't be a suprised that logic also fails to express to you how PZ's method of talking about it constitutes a rebuttal, if not out right viceration, of ID, not treatment of it as an equal concept.
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You can bring in a box of 'Sea Monkeys' from Toys'R'Us to show them Artemia, too.
Until Wednesday, we'll play in the sandbox...ahem, open thread here.... - I have lots of Artemia--it's what I feed my fish. I've always got a couple of flasks of saltwater bubbling away in the lab.
- I love Artemia. I always make my students do the running-tube experiments with it - that is such a great starting point for all sorts of discussion topics, from physiology and behavior to ecology and evolution, not to mention conservation biology....
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But its not, "teaching the contraversy", its burying the contraversy in the back yard, along with the rest of the fertilizer.
Not only that, but this is NOT a required High School biology course. This is a fully advertised course where the students should know what they are getting into.
Imagine a mediocre biology teacher trying to 'splain the above post to a group of uninterested high school biology students. You end up with people who don't understand biology, and will buy whatever snake-oil is pedaled their way... -
< i>Creationism is really a waste of our class time, but using it to explain some general concepts [...] will make it a little more productive, I hope.</i>
For what it's worth, reading the rebuttals of creationism here and on Panda's Thumb has taught me a lot, and directed me towards other places where I can learn yet more. It's one reason I keep on coming here. Any time you feel discouraged at having to explain it all yet again, bear in mind it's a public service...
Re His mistake is to assume that every step in its evolution was part of a drive to make a motor.
It seems to me this end-directed thinking is the absolute central reason why creationists don't take on board what scientists say. The creationists are starting from the premise that life (and possibly the whole universe) was brought into being in order that humanity should be able to exist, so they're rationalising backwards. No argument that starts from a different perspective is going to give them that same sense of validation.#: Posted by on 10/24 at 11:47 AM -
Paul wrote:
"They are going to learn some real biology."
I'm certain they will.
As teachers, we both understand the difference between "teaching" and "discussing", a point that is often overlooked by many.
Sometimes it's impossible to draw a discrete line between the two. A history professor who is a zionist could discuss the Palestine issue fairly in his classroom without going over the line and proselytizing his zionism. Similarly, an evolutionist (neo-darwinian) could discuss the debate between ID and neo-darwinism fairly and factually without going over the line. So could a proponent of ID.
What it comes down to is the integrity of the teacher to treat the discussion fairly and not influence the students right to have their own opinions in areas that are not empirically supported and are open to interpretation.
I believe that you are a person of such integrity, therefore I trust that you will handle this in a totally professional manner.#: Posted by charlie wagner on 10/24 at 11:49 AM -
There was a really nice paper on stoneflies that came out a few years after the Averof and Cohen paper that you summarize. It complements the gill --> wing hypothesis by showing how functional intermediates could lead from a swimming insect to a flying one. So much for the "half-a-wing" problem.
Marden et al (2000) Surface-skimming stoneflies and mayflies: the taxonomic and mechanical diversity of two-dimensional aerodynamic locomotion.
Physiol Biochem Zool 73:751-64
also http://www.bio.psu.edu/People/Faculty/Marden/project2.html#: Posted by on 10/24 at 11:57 AM -
I'm not a biologist, but I read once that the problem small insects have is not "flying," but "not flying." That is, at that scale, air is almost like water is for us. I know imigination isn't proof of anything, but I have no problem imagining that wings initially evolved just to help navigate the air currents, and, essentially, not get totally blown somewhere the bug didn't "want" to go. (OK, I've given natural selection more intentionality that I intended, but you get the idea). In that context, I don't see wings on an insect as being any more (or less) suprising than fins on a fish, leg-like things on a halucigenia, or flagella on a bacterium.
#: Posted by on 10/24 at 02:06 PM
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There is nothing wrong in teaching kids some philoposphy of science and using ID as an example of non-science. If more kids were taught some basic skeptism, the USA would not be in the throws of this ridiculous trial. But the trial does make you think. If we woke up one morning to find a shiny metallic space ship on the lawn, we could deduce it was the result of intelligent design. This would immediately bring in a host of other questions - where did it come from? who designed it? Similarly, it is unlikely but concievable we could be faced with something in nature which indicated the intevention of some form of unkown intelligence. Similar questions would be also be forthcoming. Is this the only thing it has designed? what are the parameters for its iterfacing with us? What can we say about it?
What is so unscientfic about Behe and his group, is that once they decide on ID, they don't want to ask any more questions. As far as they are concerned it is all over. That is clearly religion.#: Posted by on 10/24 at 03:25 PM -
That's wonderful. Literally. Thanks PZ.
#: Posted by on 10/24 at 03:32 PM
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Thanks, PZ; great post and cool information. But nobody is going to be fooled by facts and interconnectedness and coherent arguements when there is a controversy brewing!
#: Posted by on 10/24 at 04:00 PM
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It seems that Behe and Dembski both suffer this bassackwards thinking problem: Behe in his inability to look beyond the final function as being the "intention", and Dembski in assuming that evolution can be modeled as a search algorithm with a prespecified target.
When something is described as irreducibly complex, it means we can't understand how evolution could have produced it within the creationist paradigm.
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one o' these mornin's
yo' gonna rise up singin'
then you'll spread your gills
and take to the sky... - Speaking of insects and evoluiton, check out this cool new paper.
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Multiple deadlines tomorrow - so you write a detailed explanation of wing evolution on your blog. Not that I didn't enjoy it, but how _do_ you keep up with your job?
#: Posted by on 10/24 at 06:47 PM
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Nick, that Marden site is amazing. Did you check out the movies of these things using wings to surface skim? I know some skiers who'd kill to be able to do that.
#: Posted by John Wilkins on 10/24 at 06:48 PM
- This is something I'll be using in the talk, so it was a useful exercise.
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Typical type A personality...
First you lists the Tuesday's challenges (so we know there is no way you have time for...), then get to the meat and potatoes of debunking creationist's claims, but wait... not done yet! You treat us to details on wing evolution in insects.
Whew!#: Posted by on 10/24 at 09:02 PM -
#45211: charlie wagner — 10/24 at 11:49 AM
...A history professor who is a zionist could discuss the Palestine issue fairly in his classroom without going over the line and proselytizing his zionism
What the hell does that mean. Zionist?? Who in their right mind would use THAT example in a discussion of evolutionary biology? Sorry for being petty and off-topic, but what a way to show YOUR bias, charlie. It's the funniest thing I've seen in a while. Said with such sincerity too.#: Posted by on 10/24 at 11:25 PM -
John,
Yeah, the movies are nifty. I first noticed Marden's group when they had a paper on the sailing stoneflies in Nature (1995) 377:332. I think it may have had a cover photo, too. What I like is that they can show function for every intermediate step: sailing with non flapping wings (or big gill plates), increasing the size of the "sails", flapping for a controlled skim, and finally flying.#: Posted by on 10/25 at 08:05 AM -
Do all or most insects have larval stage and metamorphosis? And is this true of arthropoda in general?
#: Posted by Jeremy Osner on 10/25 at 11:16 AM
- I’m tickled to see that my work on stonefly flight has come up in this discussion. One thing that is worth adding here is that a gills-to-wings transition would require a simultaneous change in gas exchange, since a sophisticated wing is unlikely to also be an effective gill, and the physics and physiology of gas exchange are very different in an aquatic versus a terrestrial environment. My research shows that modern stoneflies may have retained intermediate forms of flight that date back to an evolutionary transition from gills to wings, and therefore perhaps they have retained other traits related to a transition in gas exchange physiology. This line of thinking led me to suggest to Thorsten Burmester, an expert on arthropod gas exchange proteins, that he should check to see if stoneflies have hemocyanin in their blood. This was a pretty far out idea, since blood-based gas exchange is what other arthropods use (including aquatic ones) but was previously thought to be completely absent in insects, which deliver air directly to their tissues via tracheae. Burmester found that stoneflies do indeed have hemocyanin in their blood (Proceedings of the National Academy of Sciences 101: 871-874) that reversibly binds oxygen, and it appears that no other pterygote insects possess this trait. In summary, the developmental evidence that you have presented for a gills-to-wings transition is supported by both a set of mechanically intermediate forms of winged locomotion in stoneflies and molecular evidence that a simultaneous transition occurred in gas exchange physiology.
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But don't dipterans, at least, use hemoglobin as an oxygen carrier? I realize that this doesn't really matter to the stoneflies discussion, except to suggest that other flying insects use oxygen transporters in the hemolymph, which might conceivably weaken the proposed derivation.
#: Posted by on 10/31 at 03:01 PM
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PZ
Well, let’s see. First you say that “In insects, we have an interesting origin explanation for wings: they're modified gills. It makes sense.”
OK, clearly this is just a hypothesis – as you said – "an interesting…explanation”. But then your verbiage changes to: “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”
Whoa.
First it’s speculation, now it’s HISTORY? That’s quite a jump – care to explain how managed to make that jump?
First, such speculations should NEVER be characterized as “history”, since you don’t really KNOW if alleged wing evolution happened in the manner you described or not. It is, as you said, just an 'interesting explanation'. So, while your argument might challenge Behe’s, it in NO WAY ‘invalidates’ his notions. BOTH of you are positing speculations, and as such, there is no possible way either view can be said to be better than the other (scientifically). One speculation does not cancel out or invalidate another - that's wholly irrational.
Though you admit that it’s “hard to imagine why an aquatic arthropod would begin the stepwise Darwinian process of assembling a set of wings”, this obviously has not given you much pause in your ingenious and spectacular speculations.
But – they do remain just that: speculations only.
NOT history.#: Posted by Kevin Wirth on 11/01 at 05:08 PM -
Kevin,
You need to understand that Behe's "irreducible complexity" argument is based on the contention that there are, provably, no paths via which evolution could have produced the features he claims to be designed.
The existence of any plausible evolutionary path for an apparently irreducibly complex feature shows how Behe's whole concept is flawed. It shows the foolishness of jumping from "I can't see any way in which that could happen" to "a designer must have done it."
Can you tell us which of Behe's irreducible complexities have stood the test of time since he wrote about them in the '90s? -
Virge,
I say with all humility that we might wish to view Behe's irreducable complexity argument as a starting point for the development of ID theory. Behe has simply rolled out the concept. Let's not jump to the conclustion that his early dabbling is a failure. We're WAY too early in the game to go there. Darwin himself, as you know, took 20 years of thinking and revising before he published his "Origin of Species", and it underwent several revisions after that.
Yet, I don't see folks today suggesting that those revisions indicate that his theory was 'flawed'. It was just being revised, like any other theory. Let's cut Behe some slack on this point, shall we?
And as for what is considered 'plausible' for many Darwinians... that is simply a matter of the limits of one's imagination. IF by 'plausible evolutionary path for an apparently irreducibly complex feature' you mean anything that sounds good or likely or possible to the Darwinian with a great sense of creativity - I would have to say sure - anyone with half a brain could come up with an evolutionary scenario that is 'plausible'. That's nothing new, in fact, it's what evolutionists do all the time.
But, that's also a fatal flaw of Darwinism. That it claims to be able to propose valid evolutionary explanations (which are nothing more than conjecture) for almost any critter when everyone knows full well that we can NEVER conclusively refute ANY of those conjectures, is, to be blunt, a rather ominous fault.
And, largely because of this, the basic concept that Behe has rolled out should be viewed as at least EQUALLY plausible. Intelligent Design is a conjecture, in the same sense that the evolutionary pathway of almost any ancient critter is a conjecture. We don't know HOW critter A evolved - but we ASSUME that it DID. We can never PROVE those conjectures, so I hardly see how they could negate Behe's assumptions.
If there is no willingness to examine the possiblity that ID might be what actually occurred, then we're not doing science, we're strapped to dogmatism.
Also not a good idea.#: Posted by Kevin Wirth on 11/02 at 02:39 AM -
Kevin wrote:
If there is no willingness to examine the possiblity that ID might be what actually occurred, then we're not doing science, we're strapped to dogmatism.
At least here is something that we can agree on, Kevin.
Please show how this examination will occur. Scientists have been begging Behe for how to do this for the last decade. Show us where he even suggests experiments. What do we examine?
When you admit that all Behe's work so far has produced nothing, and ask for more slack, please don't presume to compare him with Darwin. If Behe spent 20 years gathering evidence and produced a body of work that opened up the very essence of biology, he'd still have controversy, but at least there'd be something to look at. Darwin's work survived because the evidence was compelling, not because he'd produced an elegant idea.
Behe's already been cut enough slack to hang himself. What he proposes can at very best be described as philosophy, not science. If you think it's science, tell us what experiments are proposed; tell us what predictions can be made from his ideas.
But, before you do, you may want to familiarise yourself with the evidence for evolution, the specific predictions that have been made and have been put to the test. That old "evolution can fit anything" argument is patently false for anyone who's taken the time to study evolution (as opposed to swallowing the religious and political spin of the DI and other creationists).
I'd suggest some extensive reading at Talk.Origins and a regular dose of the Panda's Thumb. If you're unfamiliar with the material you're arguing against, you're not doing yourself any favors.