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Friday, March 09, 2007

Happy Birthday PZ Myers

 
I don't remember exactly when I first encountered Paul Z. Myers, as he was known in those days. (Yes, I do know what the "Zed" stands for but I'm not telling.) I think it was in 1994, or possibly 1993. PZ was a brash youngster jumping into the maelstrom of talk.origins with both feet. It wasn't long before he established himself as one of the regular Howlers. I remember that he had a special fondness for Ed Conrad back in those days. Ed had just made an Earth shattering discovery—man was as old as coal—but Prof. Myers wasn't buying it.

I remember getting into a rather heated discussion about something or other in order to set PZ straight. That happened a lot back then. I suspect it was an attempt by me to correct his adaptationist tendencies. You wouldn't know it now but he was actually a fan of the Richard Dawkins version of evolution in the olden days.

Times have changed. And today he's 50 years young! Happy birthday PZ. (The photo was taken on the Sandwalk. That's not an outhouse we're sitting in. It's a small shelter at the end of the path leading directly from the base of the garden to just before it turns left into the woods.)

I looked for his earliest posting to talk.origins in order to present it to him and the blogger world, but I couldn't find it. The best I could do is this one from Oct. 1, 1997. You'll see that even though he was much younger then, he still had what it takes to explain complex ideas to the average reader—although he does seem to go on and on about this "pharyngula" thingy. It's a harbinger of what's coming when he sets up his blog.

... Raff is contradicting Richardson in a way. Raff says "all chordates ...pass through a pharyngula" while Richardson argues that "there is no highly conserved embryonic stage". My main beef with the Richardson article is not the bashing of Haeckel (he is actually quite reasonable in his comments on that matter), but that he is addressing a straw man. He disproves the *Haeckelian* version of a phylotypic stage, that is a stage when all vertebrates are virtually identical, but I don't consider that a viable possibility to begin with. In the quote above, Raff basically outlines the components of the phylotypic vertebrate: somites, pharyngeal arches, notochord, etc. It does not include specialized trophic structures, precise numbers of somites, or a specific size (when Collins published that nonsense claiming all vertebrates passed through a phylotypic stage when they reached the size of 7-8mm, I know of a few jaws that dropped...especially in the zebrafish community, where we work with an animal that doesn't exceed 3mm until it is well into post-hatching larval stages).

For a good example of the difference, look at Fig. 10 (the last one)in Richardson's paper. This is a set of drawings of the heads of various vertebrate embryos, drawn from photographs and far more accurate than Haeckel's stuff. Richardson's point with the figure is to say, "See how different all these animals are!", and yes, there are lots of superficial differences between the embryos.

When I saw that figure, though, my first thought was how much *ALIKE* they all were. All of them were unmistakeable chordates. All had pharyngeal arches, all had somites, all had that same general shape.

I've never seen Dr. Moran use subtlety in any way -- unless, perhaps, you consider public disembowelment a form of understatement?
Paul Z. Myers
July 10, 1997
Richardson was also blurring the boundaries a bit himself. I found the comments about my favorite animal, the zebrafish, a bit troubling. He arbitrarily decided that the criterion for the phylotypic stage in fish was the tailbud stage, the time when the tail first extends off the yolk. This is a criterion that excludes comparison with the chick, for instance, and also causes serious problems for comparisons with teleosts. Zebrafish develop remarkably rapidly, and have a relatively small yolk. The tail buds early, well before the pharyngula stage. That means he looked at zebrafish at this stage and claimed they lacked branchial arches altogether! Just six hours later, though, this animal would have a very nice collection of pharyngeal structures. He has done this for all of these animals: defined some specific criterion for the phylotypic stage, looked at the animals at this one time point, and found variation in certain features. This is fine for demolishing the accuracy of Haeckel's figures. It is *not* good for revealing fundamental homologies between these animals. Chicks do not have just two pharyngeal arches, except at the particular stage Richardson restricted himself to. Zebrafish do not lack arches: they have a very lovely set of six pairs...just not at that one particular time.

He is perfectly correct in showing that the explicit similarities implied in Haeckel's drawings are false. However, note that he does not call Haeckel a fraud in that paper -- what he suggests is that Haeckel was drawing a *stylized* (his word) rendering of the animals. Haeckel was blurring together stages and features to try and put together a stylized composite...admittedly exaggerating and deleting features to fit his thesis. That was wrong. He was caught in it, too, over a hundred years ago.

I think, though, that Richardson has gone too far the other way. He seems to be demanding near-photographic resemblance at a single narrowly defined point in development. That is just too restrictive a demand, and conceals the underlying similarity. I can't think of a single vertebrate embryo that lacks pharyngeal arches; that's too important and universal a feature to deny its significance by saying they don't all have precisely five arches at the same time that they have a tailbud.

There is one final irritation in that paper. Richardson claims that the persistence of the 'myth' of the phylotypic stage is in part due to the fact that so few people actually look at comparative embryology. I would turn that around and accuse him of a rather narrow-minded perspective on comparative embryology. I've done a fair amount of vertebrate embryology, but I've also got several years of work in arthropod development, and have spent a fair amount of time studying marine biology. One reason I can look at Figure 10 and see the huge similarities is that I've also looked at insect and crustacean and annelid and mollusc and echinoderm embryos -- and boy, they don't look nothin' like those chordates at *any* time.

2 comments :

  1. You guys make a very sweet couple.

    ReplyDelete
  2. [(Yes, I do know what the "Zed" stands for but I'm not telling.)]

    Um.....Zachary?

    ReplyDelete