Wednesday, April 02, 2014

How did the zebra get its stripes?

A couple of years ago I posted an article on How Did the Zebra Get Its Stripes?.

I pointed out that this wasn't a Rudyard Kipling story but that many of the explanations had strong ties to just-so stories. I noted that everyone just assumed that there had to be an adaptive explanation for zebra stripes so they kept looking and looking. Every time one of the adaptive explanations was ruled out, they invented another one.

The best explanation back in 2012 was that zebra stripes evolved to protect zebras from horseflies and there was some experimental support for the idea that horseflies tended to avoid stripes. However, there was no evidence that avoiding some horseflies actually conferred enough selective advantage to drive the evolution of stripes.

Now we have a press release that announces the final solution: Scientists solve the riddle of zebras' stripes.
Why zebras have black and white stripes is a question that has intrigued scientists and spectators for centuries. A research team led by the University of California, Davis, has now examined this riddle systematically. Their answer is published April 1 in the online journal Nature Communications.

The scientists found that biting flies, including horseflies and tsetse flies, are the evolutionary driver for zebra's stripes. Experimental work had previously shown that such flies tend to avoid black-and-white striped surfaces, but many other hypotheses for zebra stripes have been proposed since Alfred Russel Wallace and Charles Darwin debated the problem 120 years ago.
Sounds like old news to me. And I'm still not convinced that you need an adaptive explanation.

Here's the paper ...

Caro, T., Izzo, A., Reiner Jr, R.C., Walker, H. and Stankowich, T. (2014) The function of zebra stripes. Nature Communications 5, Article number: 3535 [doi: 10.1038/ncomms4535]
Despite over a century of interest, the function of zebra stripes has never been examined systematically. Here we match variation in striping of equid species and subspecies to geographic range overlap of environmental variables in multifactor models controlling for phylogeny to simultaneously test the five major explanations for this infamous colouration. For subspecies, there are significant associations between our proxy for tabanid biting fly annoyance and most striping measures (facial and neck stripe number, flank and rump striping, leg stripe intensity and shadow striping), and between belly stripe number and tsetse fly distribution, several of which are replicated at the species level. Conversely, there is no consistent support for camouflage, predator avoidance, heat management or social interaction hypotheses. Susceptibility to ectoparasite attack is discussed in relation to short coat hair, disease transmission and blood loss. A solution to the riddle of zebra stripes, discussed by Wallace and Darwin, is at hand.
I'm betting that in five years there will be papers examining the SIX major adaptive explanations and five of them will be ruled out in favor of the latest one.


53 comments :

  1. It is one thing to promote the idea that most changes of allele frequency are drift, but I do not understand why you feel the need to invoke drift to explain even cases like this or colourful bird bellies. These are striking patterns that need some "work" to get right. Surely if we assume something doing a random walk an outcome like regular, high contrast stripes would be vanishingly unlikely? It is a bit like trying to explain legs as the result of drift.

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    1. "These are striking patterns that need some "work" to get right"

      Hmm, do you have any evidence for that? I know nothing about the matter, except that the genetic basis of stripe formation has been studied, particularly in Drosophila. My impression was that it happens quite easily.

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    2. You are right, I don't have any understanding of the genetic basis here. It is just that to get a stripey pattern, you need some regulatory mechanism that says "put colour A here" and "put colour B here", and then you have to take into account the extremely contrasting colours of white and black.

      It just seems to me that neither the evolution of such a regulatory mechanism and of the strong contrast are easily explained by evolution of coat colour being a random walk; somehow I would expect a uniform colour across the entire body and either something bland or a complete loss of pigmentation.

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    3. Well the strong contrast is just on/off: express pigment gene here, don't express it there. The tricky part is repeatedly switching a gene on and off with regular spacing - but my impression is that this can be encoded in a surprisingly simple way (encoding the whole pattern at once rather than each stripe separately). Perhaps someone here knows more?

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    4. Sean Carroll (the evo devo one, not the physicist) has a nice explanation in more than one of his books of how Drosophila achieve alternating stripes of expression early in development to set up segmental boundaries. Endless Forms Most Beautiful, I think, but he's done it more than once.

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    5. This is not just alternating stripes on a pre-segmented insect body though...

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    6. True. Which makes it more applicable to the zebra stripes than otherwise.

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    7. I am reminded of the domestication of foxes in Russia, where in 40 generations of selecting for tame behavior, they got dog-like foxes, some of which also showed new mottled fur patterns. It must be pretty easy to tweak a regulatory protein somewhere in development to get variations in hair or fur pigmentation.

      I believe the zebra is considered to be black with white stripes, rather than the other way around. They are born with the stripes, although they are more brown and white at birth, and darken with age.

      I am curious about whether a population of zebras would lose their stripes over time if they lived in a different climate. What exactly is the cost of stripes, that there would be selection against having them if they had no benefit? If the stripes are free, then it doesn't seem like such a big deal that some species of equines have stripes, while others don't.

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    8. If they are black with white stripes then it isn't even simple on/off because black is also "pigments on", making randomness even less likely.

      Again, to me this is simply a question of the null hypothesis, of what a scientist would realistically expect if things were just noise. It appears that a few people here really do say, "yes, when I envision what the result of a random walk would most likely look like, I see sharply delineated black and white stripes".

      And I find that just astounding. I hope when you design a Real Life hypothesis test you take a different approach.

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    9. @Alex SL
      Alan Turing showed in 1952 that you can get quite elaborate patterns by the action of two diffusing morphogens. In addition, many complex patterns already exist cryptically in many organisms. It really isn't that hard to evolve a interesting pigmentation pattern. At the very least it is not unlikely enough to dismiss genetic drift as a viable alternative to adaptive explanations.

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    10. @Alex SL
      Surely it's possible that a simple recursive algorithm can generate these stripes. Perhaps a single change is enough and it doesn't require "work" to get them right at all?

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    11. @Corneel
      There are real proteins called morphogens and Turing's theoretical idea of morphogens but they aren't really the same thing. You can also get stripy patterns from cellular automata (similar to Conway's "Game of Life") as Wolfram went on and on about in his "A New Kind of of Science" a decade ago -- that doesn't mean that biological creatures have anything like a CA in them either.

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    12. @Jonathan Badger
      I wasn't trying to suggest that this was the actual mechanism that causes a zebra's stripes. Rather, I was trying to convey the point that "striking patterns" do not necessarily need a lot of work to get right, but might arise from very simple developmental processes.
      That being said, I agree that we don't really know what is going on yet.

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    13. This is not just alternating stripes on a pre-segmented insect body though...

      Neither are some of the phenomena Carroll describes (e.g., patterns of coloration on butterfly wings).

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    14. This is not just alternating stripes on a pre-segmented insect body though...

      Neither are some of the phenomena Carroll describes (e.g., patterns of coloration on butterfly wings).


      Neither, in fact are the stripes I first brought up Carroll in connection with. The stripes I was talking about aren't pigment stripes, but the spatial pattern of expression of certain developmental genes. They aren't stripes on a pre-segmented body. They're the stripes that begin the process of making segments.

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  2. Replies
    1. The date above says 2 April, at least on my screen, and LM has argued the same before in a case where some bird had a brightly coloured belly very different from the rest of its body although sadly I cannot find the post at this moment.

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    2. The Nature Communications paper was published on 1 April, but from a brief glance they appear to be serious. And I checked that the first author is a real person.

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    3. Haha, that was my first instinct too.

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  3. During the war, ships were painted with black and white stripes which were supposed to confuse Uboats making torpedo runs. Perhaps zebra stripes make predators miss.

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    1. supposed to... may be the key phrase.

      I haven't read about the theoretical basis for using this sort of disruptive painting for ships. However, its possible the military was merely mimicking what they thought was true in nature. In other words, the actions of humans should not be thought of as a natural analog of zebra stripes, but merely the result of copying from nature on the basis of what they thought was true about stripes. Alternatively, striping, if it has any effect at all, may have different effects in different contexts.

      Hey, who knows, maybe these ships were particularly resistant to biting flies ;-)

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    2. And tellingly, the stripy battleship seems to have gone extinct ;-)

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  4. Larry wrote: "The best explanation back in 2012 was that zebra stripes evolved to protect zebras from horseflies and there was some experimental support for the idea that horseflies tended to avoid stripes."

    I think that the term "the best explanation" is going to be your best friend in months to come if you continue to insist that YOUR explanation of the version of the origins of life which continues to lack any scientific evidence... I have been dealing with horse flies for decades and I didn't develop any stripes or even the outline of them....

    It must be difficult for a scientist like Diogenes (not for you Larry, I know the type) to claim to have scientific evidence for your claims, and then go on with your explanation of your beliefs without providing any scientific evidence...

    Well, that's what this blog is all about...

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    1. BTW: Troley is a moron too, but that doesn't change your beliefs....lol

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    2. "I have been dealing with horse flies for decades"

      So the 'Quest' you're on is to find something you can win an argument with?

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    3. I can think of a reason flies are so attracted to Quest.

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    4. "I have been dealing with horse flies for decades and I didn't develop any stripes or even the outline of them...."

      You will need to go back and read about natural selection and neutral theory to understand why change doesn't occur within the individual.

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    5. Piotr Gąsiorowski,

      "I can think of a reason flies are so attracted to Quest."

      I lived in Africa and Australia..... Have you ever been there you moron...?

      And you should know more about flies being attracted to you...I looked up your last name... It means "male goose"... Lol

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    6. For Quest's benefit, here is a snappy web page that informs you of the most common misconceptions about evolution. Today he/she pulled a #4.

      http://evolution.berkeley.edu/evolibrary/misconceptions_faq.php#a4

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  5. I am YEC but don't find Zebras strange or different then much of nature.
    The first conclusion should be predator interference. just like white tailed deer or bird flocks. The predator must select a target before they leap with their one chance.
    The stripes confuse, in a HERD, the target. its made into a blur or rather series of lines that don't comply with the lines showing the configuration of a creature.
    the zebra disappears in a moving herd. They are made for fast running to escapr and the stripes are just a part of helping a running escape.
    This came about after the flood when creatures exploded in numbers and types on the planet.
    The zebras would be in small groups and not hugh groups as today. tHere was more predators back then and this would force small groups to blur together.
    Possibly a minor selection took place to ensure results.
    By the way many horse types living in asia SOMETIMES have evidence on their legs that they also once had stripes. later they lost it because possibly selection was not affecting them. Yet its in the genes a little still.
    Wary of selection but possibly it maintained things and brought trivial colouration results.

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    1. Here is an excellent read on the concept of "hyperevolution" (The insane idea that all the species of a particular family that we see today came from a single breeding pair a few thousand years ago)

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    2. Take away the parts about a mythical flood and - my god! - Bob's makes a useful contribution! *slow clap*

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    3. Yes I was thinking the same thing, Robert sounds like he is on the verge of becoming a full-fledged Darwinist, what with his line of reasoning and all. And to think all this can happen in 6000 years...imagine the possibilities after millions of years.

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  6. I could see how someone might test the idea about stripes being a deterrent for biting flies. Have two pools of white horses. Paint the experimental group with black stripes, like a zebra. The second group of white horses is a control, which is painted with white stripes to take care of the variable of being painted. Then monitor the relative degree of attraction to biting flies.

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    1. You propose an interesting experiment. I can see a possible confounding factor you'd want to eliminate, or at least control for: The white paint and black paint aren't going to have the same pigment. How will the differing pigments affect the flies' behavior?

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    2. Could you do this exp in North America with our biting flies? Maybe it should be done in Africa.

      Could result in an interesting line in paper: "...earlier attempts at this experiment failed when lions ate both test groups".

      ...leave it to those pointy-headed scientists to come up with a more sophisticated version of the old "the dog ate my homework" excuse.

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  7. I see a lot of different patters of skin coloring in nature so I presume there exist mechanisms for that. One problem solved. Next problem: Why? Most likely because it has been selected for in reproductive cycle.

    Everything we see in nature have causes. The causes may not always be obvious to the eye, but that doesn't mean they are not there. No need to assume supernatural explanations. Experience shows supernature causes very unlikely on this planet.

    I also note that Quest is unable to shake off his obsession with the question of how life got started on this planet. Isn't the knowledge that somehow, no matter how, it did, sufficient for most purposes? We all are curious about how .- except that from a scientific pow, there seems to be no reasons why it cannot be of natural origins. The time is not ripe for a declaration of yes or no to the question. Except to the creationist mind, it already knows it is a loud NO. How does it know?

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  8. I agree with Larry that the null hypothesis should be neutral drift (and should be explicitly stated as such in these papers) and all the adaptation hypotheses should be tested against that. However I would also ask how much evidence do we need to show that a trait is not due to drift but to selection. Because it seems to me that a lot of circumstantial evidence has accumulated in recent years to show the adaptationist hypothesis more plausible than drift in the zebra stripe case. Of course we can never be 100% sure and can never do all the experiments (which Larry seems to expect), but this is the case in most scientific questions. Also there is something to be said for the researchers' preconceptions. Evolutionary biologists with an ecology/zoology/ethology background tend to prefer adaptationist explanations, whereas those with a biochemistry background see neutral hypotheses more plausible than others and require more evidence to rule them out.
    At any rate it is at least an interesting scientific debate that shows the scientific method in action. It is a pity that public evolution debates are almost exclusively about creatonism and not about real science issues like this.

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  9. If zebra stripes are due to neutral drift, what is maintaining the characteristic in the three subspecies; neutral drift?

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    1. It could be that the characteristic is not being maintained at all and that because the subspecies don't interbreed (perhaps due to geographic separation), the differences persist.

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    2. Drift would imply the features would eventually slowly degrade due to lack of purifying selection. So that would of course be an argument against drift.

      It could be a kind of spandrel in the sense that the stripe pattern exists and is maintained due to some pleiotropic effect where selection for something else inadvertently maintains the stripe pattern too. But why would that be any more plausible than a straight up direct selection pressure of some sort?

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    3. @Mikkel Rumraket Rasmussen:
      "maintained due to some pleiotropic effect"
      which I understand (and you appeared to say) would mean selection.
      Unless I'm missing something (my normal condition) suggesting zebra stripes result from drift stretches credulity.

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    4. Keith: if there is no non-neglible selective effect (i.e. the stripes arose due to neutral drift), then nothing is maintaining them - they just happen not to have drifted away again. Do we have reason to think enough time has passed to _expect_ them to have drifted away?

      Mikkel: yes, such an effect seems highly plausible and strikes me as the most obvious explanation. I could argue (even in the absence of a specific proposal for the secondary effect) that it seems more plausible than a straight up effect on the basis that no-one has come up with a sensible-sounding mechanism for the straight up effect, whereas we are sufficiently ignorant about possible secondary effects that many things could be plausible. But we don't need to establish that it is _more_ plausible, only that it is a possibility that shouldn't be rejected without evidence.

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    5. @Keith

      You are correct. The pleiotropic effect suggestion was meant as a kind of alternative selection theory, instead of selection directly for stripes(for whatever reason).

      In any case, I also disagree with Larry on this point even though I'm generally sympathetic to his views on the importance of drift.

      The question is how the underlying genetics of the stripes work. If that could be worked out I suppose it could be tested whether those genes seem to be evolving neutrally, which if they do, raises the question why or to what extend the Zebra's stripes nevertheless seems to persist. I suppose it is possible that the "Zebra's stripes allele" has only temporarily and relatively recently been fixed in the Zebra population through drift, and will eventually be replaced by another.

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    6. In any case, the authors here didn't just merely suggest a new, or blindly declare and adherence to an already existing adaptive hypothesis. They actually did some comparative data analysis which said signficantly came out agreeing with one of the extant adaptive hypotheses. I'm not qualified to asses rigours of their methodology, but at least that raises the quality of their suggestion beyond mere ad-hoc, adaptive speculation, contrary to what Larry seems to imply.

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    7. My biggest complaint is the assumption that zebra stripes must have a purpose. If you start with the assumption that stripes must be an adaptation then it's not surprising that you will find data to support at least one explanation.

      Personally, I don't think this is one of the most important problems in biology.

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    8. Nobody has mentioned this so far, but I think it should be pointed out that "zebras" seem to be a paraphyletic grouping. According to Orlando et al. (2009), Grévy's zebra is nested within the hemionid clade (with onagers, kiangs and their kin), the mountain zebra is a close relative od African wild asses and domestic donkeys, and the various subspecies of the plain zebra (including the quagga and the extinct giant Cape zebra) form a separate clade.

      If the authors are right, stripes either evolved separately more than once, or, alternatively, were present in the common ancestor of non-caballine equids and have been lost (again separately, more than once) in Asian and African asses and donkeys. In other words, asses are ex-zebras that have lost their stripes.

      Actually, Somali wild asses have zebra-like stripes on their legs -- possibly a retention rather than a separate innovation.

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    9. Oops, I meant "polyphyletic" rather than "paraphyletic" (unless zebra stripes are indeed an ancestral feature of non-caballine horses).

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    10. I have nothing to add that isn't obvious, so I suppose I should say nothing, but since my comment generated thoughtful replies ... .
      My intuition favours sexual selection as the force retaining stripes, leaving open the possibility that the character arose by drift. Since I hike a lot in summer, I have no trouble believing that striping as horse fly repellent (if it actually has an effect) would also be much more than sufficient to assure that retention.

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  10. If zebras were living in tall grasslands, or in a mixed forest and grassland environment, we would not be wondering why they have stripes. Of course then their stripes would likely be for camouflage against predators. To any question about why they are black and white and not light brown with black stripes, one would need to consider how they would look to their predators. Since predators have limited color vision, a black and white striped animal would be camouflaged.
    In any case, we often hear that the African plains were more forested in the past. It could be that the zebra stripes were an adaptation from that time, and the stripes have persisted b/c it also has other benefits (confusion of predators and biting flies...).

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  11. Wow. The pervasiveness of the 'striking morphology' therefore 'adaptation bias on *this* blog's comments is surprising (and, I can imagine', frustrating for the author).

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  13. I was intrigued at Piotr's suggestion that zebras are polyphyletic which would suggest to me convergent evolution hinting (to my mind at least) greater support for adaptation than Neutrality. (forgive my bias)

    However, it would appear according to a variety of sources that zebras are in fact monophyletic
    http://en.wikipedia.org/wiki/Zebra

    No matter, I am reminded of how the military history co-opted Biology in WW II.

    From wikipedia:

    The British zoologist John Graham Kerr, who first applied dazzle camouflage to British warships in WWI, outlined the principle in a letter to Winston Churchill in 1914 explaining that disruptive camouflage sought to confuse, not to conceal, "It is essential to break up the regularity of outline and this can be easily effected by strongly contrasting shades ... a giraffe or zebra or jaguar looks extraordinarily conspicuous in a museum but in nature, especially when moving, is wonderfully difficult to pick up."[1]

    While dazzle did not conceal a ship, it made it difficult for the enemy to estimate its type, size, speed, and heading. The idea was to disrupt the visual rangefinders used for naval artillery. Its purpose was confusion rather than concealment.[2] An observer would find it difficult to know exactly whether the stern or the bow was in view; and it would be equally difficult to estimate whether the observed vessel was moving towards or away from the observer's position.[3]
    http://en.wikipedia.org/wiki/Dazzle_camouflage

    So are zebra stripes as camouflage an advantage? The answer is an unequivocal yes!

    Unless I am missing something: "score 1" for the "adaptationists".

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