Evolutionary convergence in swimming fins

May 12, 2015 • 3:15 pm

In this video from the New York Time’s James Gorman, we see an evolutionary convergence: in unrelated creatures that swim with a single undulating fin, the length of the “wave” propagating along that fin is about 20 times longer than the side-to-side displacement of the fin. This ratio has evolved repeatedly. Why is that?

Gorman explains; click on the screenshot below to go to the video:

Screen Shot 2015-05-12 at 2.18.47 PM

Note that Gorman says, “Eight times evolution faced the same physics problem; eight times it got the right answer.”  That shows that there weren’t severe constraints on approaching the “adaptive peak”. That is, when we have some external criteria for judging how close an adaptation gets to “perfection,” in this case (and other cases, such as mimicry), it gets pretty damn close. (Note: you might contemplate what aspects of morphology, physiology, and behavior have to change to get to this ratio.)

Contra Simon Conway Morris, by the way, this kind of convergence is no evidence for God.

h/t: Bill

73 thoughts on “Evolutionary convergence in swimming fins”

    1. I guess that for a religious person, this may actually serve as an evidence for intelligent design: the designer used a template or something along this line.
      Maybe it’s because I am a layman, but I think that if I was otherwise a believer, similar solutions to similar challenges wouldn’t undermine my belief.

      1. Yes, a template. That is exactly the argument of Morris. A friend once told me she believed animals evolved, but she was not sure about humans. To me, that sums up the religious spirit. There is something about humans that sets them apart. Evolution proves beyond any doubt that humans are part of the animal kingdom. And that is something the religious just can’t bear.

        And even though there appears to be a niche for certain evolutionary paths, such a niche appears to be missing for humans because humans evolved only once, in Africa. To prove that there is a niche for humans means that Morris has to find human-like developments elsewhere. Which in turn would undermine the uniqueness of the human species. And THAT is the real horror for any believer.

        1. What undermines this thinking is the extensive fossil record for modern humans, human cousins, and their ancestors. Exactly what happened when to make us special, and what is the evidence for this?

          1. I’ve heard christian theologians say that God inserts the soul at some point, and the first person to receive a soul was Adam. But I’ve never seen or read any evidence of this event, nor have I seen or read any evidence of a soul.

          2. Where does this soul stuff come from. What is doing before bob puts it in.
            Can it run out?
            Does it start getting uppity before getting put in and start sinning or does it need a human to do that?

            If soul stuff needs a human to start acting badly and making wrong choices then there is already something special about humans.

            If it takes soul stuff to make humans special then the quality of good or bad is handed over by god.
            Maybe there is a special room where the naughty soul stuff gets held till god jokingly puts it in some unsuspecting shmo and then blames her/him for, gasp, thinking, and sends them to hell.

            God eh, who needs it?

          3. If humans are ensouled at conception, as many religious folks claim, what happens when the cells/blastocyst splits to form monozygotic twins? Do they split the soul, or share it, or is there a spare soul lying around?

          5. There are no good answers to these questions yet they persist with them as if it self explanatory.
            The whole ‘gave us free will’ nonsense too.

            Does the free will/free choice come with the soul?

            In which case twins should behave identically, at least with moral choices.
            Which I gather is not he case.

            If the soul doesn’t bring this quality with it, what does it bring.

          7. It’s an easy mistake to make. I’ve had doughnuts which were missing the custard.

      2. Seems to me the template argument works against the God hypothesis. An infinitely creative Designer would have no need to economize by reusing design templates. So reuse would seem to point more toward natural processes with constraints that reward economy of design.

        1. If that wasn’t clear enough, I don’t think that this is a convincing argument. I am just trying to see the issue through a religious person’s eyes.
          If there is only one optimal solution to a problem, why would a designer settle for a worse solution? It seems to me that your argument requires that there be more than one equally good solutions. This isn’t always true.
          Besides, who said that God is creative? Sure, we usually find creativity a good quality in people, but this doesn’t necessarily apply to God.

          1. But isn’t the whole argument from design based on the notion that natural processes aren’t sufficiently creative, and that’s why you need a Designer? That God is so creative he was able to just imagine the world into existence?

            Also, the idea that there’s just one optimal solution that God must perforce select implies that God is constrained by natural law, rather than being the author of those laws.

            Bottom line is that any argument along these lines seems to end up shooting itself in the foot by undermining one or more of God’s supposedly transcendent qualities.

          2. You’re mixing up two different meanings of ‘creative’, there.

            Meaning one: Producing a design to satisfy the conditions. There may very well be just one optimal design, in which case any competent Creator will settle on that design.

            Meaning two: Producing lots of original designs. That smacks more of the artist than the engineer. IF there is only one optimal design, then most of those designs will be sub-optimal and our Creator will have shown he’s not a very good designer.

            ALSO: I think your argument that “the idea that there’s just one optimal solution that God must perforce select implies that God is constrained by natural law, rather than being the author of those laws” is a bit of a straw man. One can imagine that God is the creator of natural laws, but such laws would have to be self-consistent, else the universe would fall apart. So even if God created natural laws, He must be constrained by His own laws. Like a law court citing precedents, consistency is essential.

            So I agree with Golan on this one, the convergence of solutions is not an argument against a Creator (though it is a powerful illustration of the power of evolution to give the appearance of ‘design’).

          3. But if the world is the product of God’s imagination, just a story he’s telling himself, or a dream he’s dreaming, then where does the need for consistency or natural law come from? Hollywood sci-fi scriptwriters clearly don’t feel bound by such constraints, so why should the Author of the Universe? The Ground Of All Being can sustain an inconsistent, whimsical universe as easily as a consistent, lawful one.

            What I’m saying is that from a Bayesian standpoint, the existence of natural law (and optimal solutions to those laws) lends more support to the naturalistic hypothesis than to the creation hypothesis, since we have no a priori reason to expect natural law from a creator. The claim that of course God would use design templates, natural law, evolution, fine tuning, etc. is just post-hoc special pleading.

          4. I disagree. Hollywood scriptwriters are bound by consistency if they want to produce a believable movie. Even (or maybe especially) in sci-fi, where legions of fans are ready to pounce on any inconsistency.

            “The Ground Of All Being can sustain an inconsistent, whimsical universe as easily as a consistent, lawful one.” I would absolutely disagree. If G*d creates a consistent universe then he can leave it to look after itself. Its existence is stable. If he creates an inconsistent universe, I think he’s going to have to continually attend to it, tinker with it, tweak little bits,all to stop it from collapsing. And don’t forget, we’re talking about hydrodynamics of different fishes in one and the same place – Earth. G*d might get away with making the viscosity of water half the regular value on Enceladus, which is conveniently isolated from other water bodies, but there’s no way he could make the viscosity for squid half of what it is for ribbonfish.

          5. I don’t want to drag this out, so three last points and then I’m done.

            First, you’re fooling yourself if you think Hollywood sci-fi movies are based on any kind of consistent physics. In the rare case when filmmakers bother to consult a scientist at all (such as Kip Thorne on Interstellar), it’s headline news. But if you want to try to explain the consistent physics behind, say, Star Wars or Age of Ultron, knock yourself out.

            Second, the Ground Of All Being is not the hands-off god of deism. Theistic theologians tell us that God sustains the world in existence by a continuous act of will. Without his attention, we all go poof.

            Third, viscosity is irrelevant to the argument I’m making. You’re still thinking in terms of a world necessarily driven by natural law. But I don’t see why God should be limited to that sort of world. Again, filmmakers are free to put on screen any sequence of images whatever that tells the story they want to tell, and physics be damned. (I suspect that if you did a hydrodynamic analysis of Finding Nemo, you’d be hard pressed to come up with a consistent viscosity value.)

            Suppose we lived in a world in which people are merely hollow shells, inhabited by immaterial spirits and moved by magic. The unfolding of events in that world might be rife with inconsistencies, continuity errors, lapses of logic, and so on (just like many films), and still tell a compelling story. If that’s what God wanted, if the celestial drama of sin, sacrifice, and salvation was his overriding priority, he could have done it that way, and physics be damned. Finding ourselves in such a world would be powerful evidence in favor of the God hypothesis and against naturalism of any sort.

            But that’s not the world we’re in. We’re in a world ruled by natural law, which (in my opinion) argues just as powerfully against the God hypothesis.

          6. OK. Leave out ‘Ground of all Being’ and what special properties theologians might have imputed to him. A God (any arbitrary creator god) could have chosen consistency, or arbitrary non-uniformity. It’s my view that consistency (i.e. ‘natural law’) would be much simpler.
            I don’t see that as an argument against a creator God.

          7. The fundamental essence of the god we are familiar with is that it is a creator god.
            A god that created us, that created the universe and created everything else.
            This is often the first thing I hear when I say I am an atheist, “well, who created you?”

            So in answer to you question, everybody.

            I would think any kind of god with the notion of infinitely associated with it could find more than one optimal solution.

            The template argument most certainly does undermine the god hypothesis, for the above reasons.

            Templates are an economy measure and to apply consistency across a range of participants. No god worth its salt would need templates.

            Lastly, again, creativity most certainly does necessarily apply to god. Where have you been?

          8. There may not *be* more than one optimal solution to be found. (Or there might actually be, I would think that point would be amenable to scientific investigation).

            But IF there is only one optimal solution, then any competent creator god, no matter how ‘creative’, would infallibly settle on it.

          9. Yes, I phrased it wrong. Not, be found within the laws, but found by any kind of creative act.
            A kind of deus ex machine for all and any aspect or being.

  4. This kind of convergent evolution makes me think alien life forms may be a lot more similar to Earths menagerie than many people think. If multicellular life evolved in the oceans of Titan, it might very well swim like this. I would think the ratio would be the same.

    1. I would think the ratio would be the same.

      I bet the ratio has something to do with the viscosity of water — and I was really hoping the video would explain the physics behind it and rather disappointed it didn’t. Anybody have any details?

      b&

      1. I think viscosity and density are the only properties of the water that could determine the ratio. Try to imagine a large creature moving through air. A big balloon air-fish. I think it would obviously have to have a much larger oscillating fin in air relative to the wight of the critter. The ratio might still be 20 to 1.
        An airplane’s propeller is more effective at sea level than at altitude due to the difference in density, so density might require reduced energy output. But I don’t know if that would have any effect on our hypothetical air-fish ratio.

      2. I was wondering that too. I guess it is something to do with the amount of parallel thrust generated by each undulation versus the amount of thrust that is blocked by the preceding wave. I don’t know whether this would be altered by viscosity/density or whether the relevant numbers would magically cancel out.

      4. It may have something to do with standing waves.
        I am not up on the details but I do know that modern sea vessels have a dolphin nose like protuberance on the bow, just below sea level.
        Once the bow had a sharpish edge all the way down.

        This is to help cancel standing waves.

          1. You mean the level the ship rides at? Yes. But while the ‘nose’ will give best results at some particular design depth, it will also give useful results over a range of depths.

          2. Yes, IIRC, the term I wanted was “draw.”

            Thanks for the answer, and that’s a very interesting mechanism. How long has it been around?

          4. Thanks again, very interesting. I’m embarrassed not to have known about this. My Dad was a captain in the merchant marines and I spent a lot of time around docks, sometimes aboard ships, and took a voyage once. I love anything about ships.

          5. And arrgh, I meant draft. But you had used sea level, so I just followed along…

          6. I used sea level cos I had a mental block.

            Draft, yes. Vessels have ballast tanks to even out and settle the list and draft, somewhat. It is interesting to see a ship unloaded, riding high above the Plimsoll line . Before it gets loaded again.
            Container ships obviously can’t count on and equivalent load and unload at any given port, hence ballast.

            The big bulbous nose I refer to didn’t really come in until the late 70’s – 80’s, there were early notions but they weren’t used much and weren’t as pronounced.

            I started work on the docks in the 70’s and have seen a lot of ships, including ones that still used rivets to hold them together and that used steam powered winches.

          7. My Dad retired in the mid 80’s, so maybe that’s why I hadn’t heard of this. He never lost touch with the waterfront, though, volunteering with a couple of maritime museums for years after. I believe most of his sailing experience was on liberty ships, as he came ashore a few years after marrying my Mom. (A late marriage for both, my Mom having been widowed when I was 1 year old. So my Dad was actually my adoptive Dad, but I never seem to remember that. 🙂 )

            Once ashore he worked for a series of US shippers (back when there were some), including Pope & Talbot and Statesline. He planned loading operations on pre-container ships, which was a science unto itself.

            Thanks for sparking some old memories. 🙂

          8. Here’s one for you: You are in a boat with a cannonball on a lake. You pitch the cannonball overboard. Does the level of the lake rise, fall, or remain the same?

          9. I’ll leave that for Diane but my answer’s at the bottom of this thread…

          10. Yes, planning vessel load was major endeavour. Planners had to have been a ships master, as the load distribution and many high level factors were involved.
            I think that requirement is lifting due to computers doing a lot of the work but most of our guys still were master mariners.

            I am not familiar with the line you mention as I am far far away.

            Ships now are amazing in size.

    1. Hi Diana

      Totally Off Topic, but I instantly thought of you –

      Sorry they’re all the worng orientation.

          1. The holders ought to be equipped with locks so the rolls can’t be tampered with.

          3. A commenter on the original post asked “How long before some smart guy swaps them around?”

            But I think he was referring to position, not orientation 😉

  7. We need an explanation for those animals which have a fin with the wrong ratio, if such exists, as I expect it does.

    1. That’s what I was trying to ask above – but badly. (It was time to go to work!) It would be interesting if there were animals with the “wrong” ratio to see what other constraints and trade-offs there are in play. It wasn’t clear to me from the video whether evolution has found the “right answer” 8/8 times, or 8/N times where N>8. Presumably fin anatomy has some bearing on how easy it is to control this ratio?

  8. I also wonder how much of this is hard-wired motion versus learnt behaviour. (Clearly, even if animals learn to optimise their fin motion to the best ratio, the anatomy that enables them to do so has evolved in response to Natural Selection.)

  9. I went down to the sea a while ago and saw some waves.
    Amazingly they looked just like waves I’ve seen in pictures on the other side of the world.

  10. I, too, was taken aback by the concluding comment in an otherwise unobjectional video post: something along the lines that ‘Eight times evolution did the experiment, and 8 times it got the right answer”. But is it really the “right” answer? Are there no other means of propulsion possible among extant species that yield superior efficiencies? What about hypothetical species whose body plans and undulating propulsion methods do not conform to those observed among extant species?

    I’d have much preferred that the narrator simply ended the piece by noting that, in repeated trials, convergent evolutionary processes arrived at the SAME answer, there i no need to assert that it is the RIGHT answer.

    1. Of course other propulsion methods are possible. High-speed swimmers such as tuna, dolphins, and ichthyosaurs use streamlining and tail fins. Squid and octopuses use jet propulsion. Some aquatic insects use their legs as oars.

      But the point of the video is that for a given propulsion method, such as the undulating-fin method, there’s an optimal configuration that all species that use that method converge on. I don’t think it’s out of line to call that the right answer for that method.

    2. I think they have done modelling and simulations with those configurations.

      That was one point of the piece, that till now they didn’t have tools to prove it. I think.

  11. “Eight times evolution faced the same physics problem; eight times it got the right answer.”

    Isn’t that a little misleading? Is it fair to assume that evolution face this problem more than eight times, and some of those it got right and some wrong?

  12. Breaking my duck on science(ish) posts here, so be gentle.

    If I’m right in thinking that all of the eight examples were either single fins (which seem to be on the underside of the creature) or two fins (on the sides), does this indicate that having fins of this sort operating close together wouldn’t be efficient due to some sort of interference?

    Just thought that more fins might be advantageous for speed or ease of manoeuvering.

  15. “Eight times evolution faced the same physics problem; eight times it got the right answer.”

    I’d disagree about the term ‘right answer’. That implies any other ratio would be ‘wrong’. But every ‘design’ is a compromise (no matter whether evolution of God did it). I’d rather say that, for the type of fin in question, in the regime which the animal occupies, that 1:20 ratio is probably near the optimum.

    This sort of consideration – a compromise between many factors – would apply to aircraft wings or ships hulls, I don’t see why it wouldn’t apply also to animals’ fins.

    1. evolution *or* God.

      ‘evolution of God’ paints an interesting picture… 🙁

  16. Sweet! And thanks a lot, I hadn’t dared put up “convergent evolution” as a tag because of lack of a measure.

    I found two papers in PLOS One who describe these results, the research and an editorial:

    “In aquatic animal swimming, one performance criterion is the Strouhal number (St), which specifies the frequency of fin movement for maximum propulsive efficiency in those animals that use the common “body/caudal fin” swimming mode, such as trout. … Our studies provide quantitative evidence for a complementary performance criterion, called the optimal specific wavelength (OSW), which determines the wavelength of fin movement required for maximum propulsive force or thrust.”

    [ http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002123 ]

    “The researchers posit that the OSW may be at the sweet spot of two competing mechanisms in undulatory fins. In the “friction mechanism,” steeply undulating fins move water backward more efficiently and so provide more thrust at a small specific wavelength. However, in the “velocity mechanism,” shallowly undulating fins move water fast and so provide more thrust at a large specific wavelength. Because a fin cannot be both steep and shallow, both mechanisms obviously cannot be optimized simultaneously. Indeed, the researchers found that swimming slows when fins are too steep or too shallow. Rather, an intermediate steepness yields the intermediate specific wavelength that maximizes thrust, which of course is the OSW.”

    ” This raises the question of why the slower OSW swimming has arisen repeatedly in species whose ancestors were tail-end swimmers.

    The answer may be that median/paired fin swimming takes less energy, which offers its own advantages. In addition, some ecological conditions may favor OSW swimming. The researchers speculate that this may be so for South American knifefish, which live in murky water and are nocturnal. They navigate this dark environment via electric fields generated in the trunks of their bodies, and these fields would be distorted if swimming entailed moving their trunks instead of keeping them OSW-style rigid.”

    [ http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002124 ]

  17. Does the amplitude-to-wavelength ratio vary with the speed of the fish through the water? A crude analogy with variable-pitch airscrews would suggest the fish might benefit (with increased acceleration) by using high-amplitude short-wavelength on starting, and then decreasing the amplitude/increasing the wavelength as speed builds up. (Like an airscrew going from fine to coarse pitch).

    1. It probably would given the strong coupling between the fin and the water, due the density of water.
      The variable pith propeller is to keep a similar optimal angle of attack to the relative airflow as the plane speeds up.

      I think the same angle ratio may obtain with the fish.
      Good question though, I’ll think more.

  18. (Answer to Matt G)

    “You are in a boat with a cannonball on a lake. You pitch the cannonball overboard. Does the level of the lake rise, fall, or remain the same?”

    Lake level falls. Because the cannonball is heavier than water. So while it was in the boat it effectively displaced its weight of water, i.e. several times its volume. When in the lake, it only displaces its own volume.

    1. Oh. A ship ties up alongside the wharf at low tide and someone throws a rope ladder over the side, 12 rungs long, it just reaches the water. The rungs are a foot apart. If the tidal range is six feet, how many rungs are exposed at high tide?

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