Evolution
Life Sciences
Revisiting Those Early Developmental Stages: A Response to PZ Myers
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� Part 1: Last week, I published a review and critique of a lecture which I attended by developmental biologist PZ Myers, of the University of Minnesota Morris. Since that time, PZ Myers has published a response on his blog. This is my rebuttal to that response.
For those who want the bottom line, here it is. Myers thinks I’m worried about Haeckelian recapitulation. But that’s completely wrong. Neo-Darwinism itself predicts that early development, starting with fertilization, should be conserved. But early development is NOT conserved, a fact Myers readily admits. If common descent is true, however, early development must somehow evolve via mutations.
Yet mutations to early developmental stages are not tolerated by animals. Not surprisingly, Myers provides no evidence that early development can be heritably modified by mutations.
Myers contests my suggestion that common descent would predict the conservation of early developmental stages. The logic of my position takes a modus tollendo tollens form of argument:
A
1 If P then Q
2 ~ Q
3 ~ P
By instantiation in A
B
1 If the theory of common descent is true then early developmental stages should be conserved.
2 Early developmental stages are not conserved.
3 The theory of common descent is not true.
The argument is impeccable: Whence the disagreement?
Myers offers no objections to Premise 2. The conclusion at 3 follows logically from the premises at 1 and 2. Myers must (and does) therefore reject that 1 is true. In this he has set his face against very many opinions to the contrary.
The process of early development from the egg to the phylotypic stage should be at least as conserved as the pattern of the phylotypic stage. One might reasonably expect mechanisms of early development to be especially resistant to modification because all subsequent development derives from early processes. Traditionally, features of early development and conserved larval stages, even between phyla, have been regarded as strong homologous characters for the inference of phylogeny. The division of animals into protostome and deuterosome superphyla is based on the idea that embryonic similarities are homologous and have been largely immutable over hundreds of millions of years. [emphasis added]
Raff is an evolutionary biologist; he is hardly alone in his views. Myers’ disagreement is with them. I have reported their opinions, and since they are both obvious and persuasive, I have endorsed them.
Furthermore, Kenneth Miller and Joseph Levine, in their textbook, Biology: Discovering Life pg. 162 (2nd Ed., D.C. Heath, 1994), weigh in on this subject:
Why, then, should the embryos of related organisms retain similar features when adults of their species look quite different? The cells and tissues of the earliest embryological stages of any organism are like the bottom levels in a house of cards. The final form of the organism is built upon them, and even a small change in their character can result in disaster later. It would hardly be adaptive for a bird to grow a longer beak, for example, if it lost its tongue in the process.
The earliest stages of the embryo’s life, therefore, are essentially “locked in,” whereas cells and tissues that are produced later can change more freely without harming the organism. As species with common ancestors evolve over time, divergent sets of successful evolutionary changes accumulate as development proceeds, but early embryos stick more closely to their original appearance.
Let’s take an illustrative example. Anurans and urodeles are both modern amphibian groups which we would consider to be closely related. However, there is significant difference in the source of their primordial germ cells. For instance, in urodeles, they arise from unspecific ectodermal cells at the blastula stage; whereas, in anurans, they arise from specific cells of endodermal origin, the cells possessing cytoplasmic granules that originated in the unfertilized egg. Now, here’s the conundrum. The difference relates to organs of extreme importance — i.e. the germ cells. The difference is not only substantial, but it occurs extremely early in development. From the standpoint of evolutionary rationale, there seems to be two possible explanations for this:
1) A very radical change has occurred in the relevant developmental mechanisms.
2) An early bifurcation has occurred in the phylogeny of amphibians — a remarkable case of convergent evolution, perhaps from different groups of fish.
The second hypothesis seems to be implausible, for it would be a remarkable coincidence for two groups to have evolved independently for such a length of time and yet share so many distinctive features. The first hypothesis seems to be even more implausible still because, as Rudolf Raff says, all subsequent development derives from those early processes. As such, modifying these early embryonic stages is likely to cause catastrophic detriment to the organism. This is the type of conundrum that I was getting at in question 6 of my list.
Myers also accuses me of quote-mining the Alberch paper, noting,
First, it’s a slightly odd quote: the two phrases are from two different paragraphs, and are in the reverse order from how they’re written here. He doesn’t substantially change the meaning, though, so it’s not quite as nasty as the usual scrambling. (However, it is peculiar that this same exact cut & flip quote can also be found in the works of Harun Yahya, and who knows where he got it; it’s just another example of creationists copying each other.)
For the record, I had never heard of Harun Yahya (whom, I have now learned, is an Islamic creationist) prior to reading Myers’ blog entry. Second, as Myers acknowledges, I have not altered the meaning of the text — actually, the meaning of the text is not altered one iota. The paper says,
The opposite situation, the case where homologous structures form from distinctly dissimilar sites is found, for example, in neural tube morphogenesis … [elaborates and gives another example] … These two examples are more than unusual oddities. They are the rule rather than the exception [emphasis added].
So I don’t think that stating that the authors claim it is “the rule rather than the exception” that “homologous structures form from distinctly dissimilar sites” misrepresents the paper at all.
Myers then goes on to write that this paper is “a criticism of naive interpretations of developmental processes that are built on Haeckelian assumptions that the sequence of stages will be evolutionarily conserved.” So it is. The important point lies less with Alberch’s intentions and more with the plain facts that he acknowledges: Early conservation is not observed. And, at any rate, Myers’ argument seems somewhat circular here — since the stages are not highly conserved, they must not be so resilient to evolutionary change after all: and thus their conservation must not be a prediction of Darwinism.
PZ Myers subsequently alludes to my comments regarding Jonathan Wells and The Politically Incorrect Guide to Darwinism and Intelligent Design, claiming:
Why, yes, it’s part of Wells’ game. He declares that Haeckel’s theory has been thoroughly rubbished, and therefore the foundations of ‘Darwinism’ have been destroyed. Note the sneaky substitution: Haeckel’s theory is not the foundation of evolution. We can kill it, kick it when it’s down, run it through a woodchipper, and it just doesn’t matter — it’s not part of evolutionary theory. I’ve dealt with this subterfuge at length, so I don’t really need to go into it again, do I?
I don’t think Jonathan Wells is claiming that modern evolutionary biology is built on a foundation of recapitulation (at least in its original Haeckelian sense). Rather, Jonathan Wells takes objection to the repeated use of Haeckel’s discredited embryo diagrams in science textbooks to support Darwin’s theory. That these diagrams have been used in such a manner for much of the twentieth (and even, in lesser measure, in the twenty-first) century cannot be denied.
Wells actually devotes quite a bit of discussion to the developmental hourglass model (which describes the observation that embryogenesis within a phylum diverges most extensively during early and late development, while converging in the middle).
PZ Myers subsequently turns his attention to the remaining three of my questions.
Regarding my fourth question, Myers writes,
This is just like the standard creationist claim that there are no transitional fossils: there are no transitional mutations, either! When we see variations in morphology between populations of organisms, how did those changes get there, were they implanted by angels? As clear examples of “transitional mutations”, I’d point to polyphenisms, cases where there are discrete differences between genetically identical individuals based entirely on their environment.
The relevance of this remark is doubtful. There is no evidence that beneficial mutations to early development have occurred or could have occurred. Such mutations are uniformly destructive, and normally lethal. Evolutionary biologists have championed such mutations, not because of the evidence, but because they believe in undirected common ancestry which demands it. Actually, it is precisely by inducing mutations to normal developmental sequences that we discover the workings of development.
For further discussion of this topic, I refer readers to this lecture presented by Paul Nelson.
Thus by a trivial elaboration of B (above), there is:
C
1 If the theory of common descent is true then mutations to early developmental stages should be beneficial.
2 Early developmental mutations are not beneficial.
3 The theory of common descent is not true.
From which it follows that either Premise 1 is false or that a great many evolutionary biologists are unwilling to believe evidence that is directly underneath their noses. The evidence for Premise 1 is considerable, and so, too, Premise 2. The pertinent conclusion is obvious.
Myers then claims I misrepresented his article on “Jonathan Wells’ weird notions about development.” He writes,
First sentence: he claims I advocate a central dogmatist (gene-centric) view that an organism’s DNA sequence contains both the necessary and sufficient information needed to actualise an embryo’s final morphology, and to support that, links to one of my articles where I supposedly get all totalitarian for dogmatic genecentrism. Go ahead, follow the link. I say exactly the opposite.
Well, yes, like virtually every other Darwinist, PZ Myers believes that development proceeds by virtue of “the interdependency of genes, cytoplasm, and environment.” But Myers rejects the notion espoused by Wells that there is a further layer of ontogenetic information which lies beyond the remit of the DNA sequence. It is this point which I was getting at.
Myers closes his reflections by commenting on the supposedly circuitous route taken by the vas deferens, noting, “Sure, you can put together physiological explanations for why each of those organs is in its particular place, but it doesn’t change the fact that the whole assemblage is a contingent kluge stuck together opportunistically.” As with most of the other cases involving apparently suboptimal design, however, the argument rests on the claim that the route taken by the vas deferens is one that PZ Myers would not have chosen. Given that there are indeed reasonable physiological reasons for the system being built in the manner in which it is, this type of argument is substantially undercut.
To conclude, PZ Myers has given us another rant about the alleged stupidity of those with whom he disagrees. This climaxed with his lament that he was not rude enough to me at the talk! Myers claimed that the point of his talk was that recapitulation (in its Haeckelian sense) is incorrect. But to claim that this is what I was arguing against is to fundamentally misunderstand what I wrote. The key point is that common descent predicts shared early embryonic pathways. The fact that there exists tremendous variation requires an explanation. The question which must be addressed is this: Are the early embryonic stages similar enough between organisms such that minor tweaking could allow for the differences between organisms? It seems to me that the answer is clearly no.