How the tapir got his spots III

by Greg Mayer

The two great classes of phenomena that Darwin set out to explain were those of adaptation– the fit between an organism’s features (structure, behavior, etc.) and its conditions of existence; and unity of type — the similarities of basic structure among organisms in diverse conditions of existence (e.g., the one bone-two bones-many bones pattern of tetrapod forelimbs, whether they be burrowers, swimmers, climbers, runners, etc.). The unified explanation that Darwin provided for these phenomena was descent with modification: the similarities were due to inheritance from a common ancestor (i.e. descent), while adaptation arose from the process of modification (i.e. natural selection).

The methods of studying adaptation are thus crucial for biology.  How can we tell what (if anything) the spots of the baby tapir are adaptive for?

There are three basic ways of studying adaptation, in the sense of determining what a trait is an adaptation to. The first is engineering: does the feature conform to what we would expect if it is performing some adaptive function?  Study of hydrodynamics enables us to understand the shapes of the bodies, flippers, and fins in fish, dolphins, icthyosaurs, etc. as adaptations to movement within a fluid environment.  The dorsal fin of an ichthyosaur, for example, stabilizes the reptile in its forward movement through water, preventing unwanted roll (for recent discussions of ichthyosaur aquatic adaptations, see here, here, and here). For another example of the engineering approach, see Richard Dawkins’ delightful account of bat sonar in chap. 2 of The Blind Watchmaker.

Second, there is the method of correlation (also called the comparative method): does the feature evolve repeatedly in particular environmental circumstances? Thus even if we were wholly ignorant of hydrodynamics, the repeated evolution of dorsal fins in aquatic fish, reptiles, and mammals provides evidence that dorsal fins are adaptations to an aquatic existence.

250px-Tigershark3800px-Ickthyosaur_MNHOrca_dorsalfin_NOAA

Third, we can study the effects on survival and reproduction of variations in the trait of interest.  This can be done either by altering the features of the character experimentally (as in this neat experiment on sexual selection in widowbirds) or by studying naturally occurring variants (as was done with peppered moths by  H.B.D. Kettlewell).

The evidence for the adaptiveness of spotting/striping in mammals is primarily of the first sort (Hugh B. Cott, in his classic Adaptive Coloration in Animals, has a lot about optical principles, and what makes things hard to see), the second sort (pacas, bongos, deer, tapirs all have spots and/or stripes [and note that pacas are rodents, and that tapirs, which are perissodactyls, are not at all closely related to the artiodactyl deer and bongo, so it would be hard to argue it's a retained ancestral feature]), and very little of the third sort– no one’s painted baby tapirs’ spots over to see what happens to them (at least as far as I know). I’ll touch on all three sorts as they relate to tapirs in later posts.

(For other examples of camouflage, see Matthew Cobb’s earlier post on the subject.)

6 Comments

  1. NewEnglandBob
    Posted August 13, 2009 at 1:35 pm | Permalink

    I am a bit confused by the difference from the first sort to the third sort.

    Conform to what we would expect if it is performing some adaptive function.

    VS

    Effects on survival and reproduction of variations in the trait of interest.

    Aren’t these two basically the same thing?

    • Posted August 13, 2009 at 9:27 pm | Permalink

      The difference is that the first sort involves design considerations only, while the third involves actual measurement of survival and reproduction (‘fitness’ in the technical sense). As an example, I could hypothesize, on the basis of background color matching, that the white color of the winter coat of short-tailed weasels (Mustela erminea)is an adaptation for crypsis in its snowy environment. This is evidence of the first sort. If I now measure the survival and reproduction of brown vs. white weasels in winter (this would require some way of getting brown weasels, either by experimentally altering white ones to brown, or obtaining naturally occurring brown ones), this would be evidence of the third sort. If the white ones did better, this would support that their white color is an adaptation for crypsis.

      GCM

      • NewEnglandBob
        Posted August 14, 2009 at 4:05 am | Permalink

        Thanks.

  2. Posted August 13, 2009 at 1:43 pm | Permalink

    The unified explanation that Darwin provided for these phenomena was descent with modification: the similarities were due to inheritance from a common ancestor (i.e. descent), while adaptation arose from the process of modification (i.e. natural selection).

    As important in the battles over science vs. anti-science is that there was nothing new about such explanations, just in the application of this through time.

    In a sense, this is just an application of organisms “reproducing after their kind.” Similar animals are similar because they share the same ancestors, so that red roosters have sons who are red (usually, though genetics is complicated by factors like recessive genes).

    Your creationist believes that similar traits come from common ancestry, and mostly did so in Darwin’s time. Extending this principle down through time appears the most sensible course available, at least once the source of difference is also accounted for (today, through mutations).

    By analogy, much the same is true for languages and texts as well. Commonalities are explained through common ancestry (including horizontal transmission where appropriate), and copyright violations are decided based on the principles used by evolution.

    Then we get creationism/ID, and all of a sudden commonality means nothing, even when it explains what are weird adaptations by design principles (sure, a “designer” will choose a terrestrial leg to change into bat wings, rather than pterodactyl or bird wings). If you’re Behe, though apparently ancestry is the cause, which is just as miraculous as all of the transformations, since there is absolutely no reason for any “designer” to follow evolutionary constraints–it’s as if someone is sure that the similarities between the Biblical flood story and those of Babylonians and others were due to god’s intervention, rather than to common ancestry of the texts.

    Which means that it’s the anti-evolutionists who either deny the ancient observation that similarities are due to common ancestry, or who make god into the magical agent who chooses both to conserve the past however idiotically, or to change things without precedent.

    It’s a parallel intellectual world they inhabit, one which follows none of the rules of thought, inference, and logic that are used in the world of sense that we inhabit.

    Glen Davidson

    http://tinyurl.com/mxaa3p

  3. Sili
    Posted August 14, 2009 at 12:48 pm | Permalink

    He linked to that other book! Kill the heretic!

    How endangered are tapirs? I kinda like the idea of painting them …

    • whyevolutionistrue
      Posted August 15, 2009 at 8:51 pm | Permalink

      While I am unaware of significant naturally occurring variation in color pattern of young tapirs, all-black Malay tapirs have, on very rare occasions, been observed in the wild. Their rarity has precluded study of fitness differences. As to their endangerment, “All four species of tapir are either endangered or threatened”; see the Tapir Specialist Group website for details.

      GCM


3 Trackbacks/Pingbacks

  1. [...] as they mature. Jerry and I disputed whether such patterns in young animals are atavistic or adaptive, but my concern today is not with whether spots are adaptive, but whether there are spotted [...]

  2. [...] make them non-arbitrary), while clearly adaptive similarities might arise through convergence (see whales and icthyosaurs). About this time we all realized we had other things we were supposed to be doing– meetings, [...]

  3. [...] Would Aliens Realize We Were Intelligent? Greg Mayer, as part of a charming larger piece, relates a back and forth he had with some students a few days ago on whether or to what extent we and alien intelligent life forms would be able to identify each other as intelligent life forms: I mentioned to the students that I’d seen a great web post that morning about first contact with aliens (h/t: PZ), which stressed the likely lack of similarity and extreme technological disparity between us and interstellar travelers (“nuclear weapons [used by them] vs. sponges [which would be us]“), and how a binary code would be the way to communicate, although PZ noted they’d probably collect several specimens for the interstellar natural history museum before they figured out the sponges [that would be us] were sentient. The grad student suggested that it wouldn’t be that bad, since convergent evolution would insure that they had some basic similarities to us. I said I’m not so sure, and noted that George Gaylord Simpson, in his famous essay on the nonprevalence of humanoids (link might require subscription), had argued strongly that life elsewhere is decidedly unlikely to be familiar to us. We discussed what basic similarities there might be among life forms evolved completely independently.  Bilateral symmetry?  Common on earth, but how many times had it evolved independently here? Cephalization? There were some interesting cases of it evolving in primitively radial urchins. Carbon based?  It beats silicon, but there was the Horta on Star Trek. We went on to note that there were ways of trying to distinguish independent from convergent origins. Shared, yet arbitrary, characteristics, such as the genetic code, suggest a single origin (unless of course there are functional differences among possible codes, which would make them non-arbitrary), while clearly adaptive similarities might arise through convergence (see whales and icthyosaurs). [...]

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