Monday, June 04, 2012

A Platypus Describes Humans

Ryan Gregory invents The Platypus Fallacy to illustrate an important concept in evolution. Here's how a platypus describes humans. See if you can spot the problem.
The lineage of which humans are a part is a very ancient offshoot of our mammalian family tree, so it was 166 million years ago that we last shared a common ancestor with humans, and that puts them somewhere between mammals and reptiles, because they lack a lot of specialized characters that we have gained but the ancestral amniote also lacked; for instance, they have no electroreception, no bills, no webbed feet, and no venom. So we can use them to trace the changes that have occurred as we went from being a reptile, to having fur to making milk to having our specialized features.


24 comments :

  1. How nicely put, every lineage is 'ancient', or else it's extinct.
    It also seems to be nicely illustrate how selection of an outgroup can influence polarization of synapomorphies/plesiomorphies!

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  2. Venom may actually be a primitive retention, like egg-laying.

    Hurum, J.H., Luo, Z−X., and Kielan−Jaworowska, Z. 2006. Were mammals originally venomous? Acta Palaeontologica Polonica 51 (1): 1–11.

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    1. The electroreception is probably an ancestral trait too. All echidna species are weakly electroreceptive.

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  3. Prof Moran, you regularly put up posts of this type. I presume you have a problem with what Prof Gregory has said but you've not explained and have left it to us to spot the problem.

    I am not a highfaluting academic versed in the intricacies of evolution, but a layman fascinated by the complexity of life. I do not have the confidence to say where Mr Gregory has made a mistake and find you sadly do not give enough Socratic guidance for me to be lead to an understanding. As a result I am left (as I am with slightly too many of your posts) feeling left out from the academic smirking at other people's mistakes.

    When it comes to creationists I can usually in an uncertain way get your point, and my self education has confirmed the IDiotic nature of their ideas, but when it comes to genuine controversies in science education I am left wanting more.

    I genuinely hope you will take the time to explain where you think Prof Gregory has a problem. I feel it would be educational and I would learn some of the subtle issues concerning the evolution of characteristics and populations. But at the moment I am at a loss and without some guidance merely know one academic has a problem with what another has written, which without the details isn't much use to me. What a shame.

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    1. I believe Prof. Moran's problem is that he thinks that there is indeed a pinnacle of evolution and that it is, in fact, the platypus.

      Is that right, Larry? :-)

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    2. They win from the standpoint of sex chromosomes alone. They kick sand in the faces of puny XY males like you.

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    3. The answer is possibly in the original post by Ryan Gregory:

      "The fallacy here is to assume that the non-human species is “primitive”, such that it can be used as a proxy for a distant human ancestor. This is a fallacy because “primitive” and “derived” refer to individual traits, not entire species, and because the comparison being made is between two modern species, not an ancestor and a descendant. Both lineages have, by definition, been evolving for exactly the same amount of time since diverging from a common ancestor."

      Seems like a good point.
      Or it may be something arcane.

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    4. T R Gregory: I believe Prof. Moran's problem is that…

      I think Prof. Moran's might be fooling us with his query, which should have been: See if you can spot the problems.

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  4. Chinahand, Larry is almost always in agreement with TR Gregory, and this article is no exception. In fact the only thing to take exception with is mentioned in the first comment.

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  5. @Chinahand:
    Prof. Moran is not disagreeing with Prof. Gregory. The "problem" is the actual fallacy. Follow the link to Prof. Gregory's original post for a full description of that fallacy.

    As for your frustration: Smile and bear. From your curiosity alone I can tell you are capable of understanding intricacies of evolution. All you need is a little more tenacity. That's a lesson my child taught me, btw.

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  6. I too don't see anything wrong with Gregory's platypus argument. It is an amusing parody of linear progress arguments which points out how dependent they are on which species is chosen to be the pinnacle of evolution. Nothing wrong with a good parody.

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  7. Think of this next time you refer to monotremes as "basal" mammals

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  8. Neither modern humans nor modern platypuses are somewhere between mammals and reptiles, as they are both on the mammalian family tree and equally distanced phylogenetically from the root of this tree.

    However, things are more complex, and the Fallacy might not be as bad as it might seem if we asses phenotypic or genotypic characters from a different perspective. In this particular case, although both humans and platypuses have had the same period of time to diverge from their common ancestor, because of differential rate of mutation/selection, generation time, and population size, one of them might have more common phenotypic or even genotypic features with the their ancestor (and possibly with reptiles) than the other one. And, therefore, one of them, I would think in this particular case is the platypus, might better represent or serve as model for the common ancestor.

    Let me bring up an even stronger case, which I call if evolution could be quantified, which compares humans to bacteria:

    if evolution could be quantified, based on population size, generation time, and rate of mutation/selection, bacteria are more 'evolved' than humans by an enormous factor

    Whether they are or not, I’ll bet again that modern bacteria have more in common with LUCA (the hypothetical common ancestor) than humans and, therefore, they are a better model for LUCA.

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    1. "Ancestral" and "derived" refer to traits, not to species -- the rest is just different expressions, some more obvious than others, of the same fallacy.

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    2. Ryan: Can you be more explicit/specific on what are you referring to?

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    3. Ryan addressed this issue more thorough at his blog Genomicron, where I made additional comments and clarified my perspective on his Platypus Fallacy:

      “The Platypus Fallacy is a welcomed concept as it points to common errors/abuses made, particularly when assessing the evolution of humans and their traits. However, we need to remain open to the likely possibility that certain traits of the modern species are more likely to reflect those of their ancestors than the homologous traits of other related modern species with which they share the ancestor.”

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  9. I forgot to mention: great picture Larry! Do I see some human knuckles on platypus’ right hand?

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  10. Good luck on quantifying "evolutionary advancement". I don't think this depends all that much on choice of outgroups, which are a bunch of non-mammalian synapsids. It depends on definition of a "main line" of evolution, which we and the platypuses consider to be their own. (Just kidding; the platypuses don't care, so we win by default.)

    Perhaps this would work a bit better if we just talked about individual characters. Monotremes are primitive in many ways compared to placentals, just as some placentals are primitive in many ways compared to monotremes. We lose interclavicles, they lose teeth. Why can't we all just get along?

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    1. john harshman: Good luck on quantifying "evolutionary advancement"…

      What about trying to quantify evolution based on the rate of mutation/selection, population size, generation time, etc?

      Evidently, viruses will beat us and all the other life forms, but are you aware of any attempts by anyone, including our mathematician fiends?

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    2. What about it? How are you going to relate the rate of mutation, population size, etc. to anything to value-tinged as "evolutionary advancement". Viruses certainly evolve quickly, but does that make them advanced? On what grounds?

      We can of course quantify the rate of evolution, at least in terms of the number of fixations along a lineage. I do that all the time. And it's interesting to test hypotheses about the reasons for differences in rates. But if, say, aldolase B introns have lots more fixations per unit time in chickens than in alligators, what have we learned about evolutionary advancement? Nothing, I think.

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    3. The point was to quantify evolution, not "evolutionary advancement" which might be a no-starter because of problems defining ‘advancement’ in context of evolution.

      However, we might be able to quantify evolution, possibly expressed as the rate of evolution, based on the rate of mutation/selection, population size, generation time, etc.

      Is anybody aware of proposal or discussions on the potential relevance/limitations of such a concept or approach?

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    4. Oh. Then sure, evolution can be quantified easily. Like I said, I do that all the time. It's pretty easy. All you need is a data set, a phylogenetic tree to map it onto, and a calibration to real time. I'm not sure why population size or generation time should enter into it, though. Those are factors that could influence the rate, but they aren't the rate.

      I'm aware of a huge literature on this. Google "The time trees of life", for example.

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  11. A late thought.

    That platypus' description of humans reminds me of Professor Ichthyosaurus.

    http://en.wikipedia.org/wiki/File:Awfulchanges.jpg

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