P. Z. gets a column, and more on what counts as evidence for evolution

April 16, 2009 • 9:54 am

P. Z. Myers, the beloved (and also despiséd) author of the popular science blog Pharyngula, has started producing a column on the Guardian website.  His first column is on asymmetry in animals — in particular the gene nodal, which sets up directional (left-right) asymmetries in animals.  P. Z. points out recent research (reference below) showing that snails, who have directionally coiled shells, lose the directionality when nodal is inactivated.   The asymmetry of the human body is also generated in ways similar to that of snails, and again nodal plays a key role.  The gene is somehow involved in determining the directionality of the way cilia (small hairs) beat in the early embryo, which sets up a concentration gradient that can make an embryo left- or right-handed.

I’ve always been fascinated by directional asymmetries — those traits that occur on a consistent side (right or left) in a species.  These include the side of the body that harbors the narwhal’s tusk. and our own internal organs. Other such traits include the direction in which the ears of an owl are turned, and what side of its body a flounder comes to rest on when it changes into a bottom-dweller.  Directional asymmetries are not rare in animals. But how are they formed? How does a gene “know” it’s on the right or the left?  The finding that the direction of cilia movement can tell a gene which side it’s on goes a long way to solving this question, but still leaves open the final question:  why do cilia spin in a given direction? How do they know whether to go clockwise or counterclockwise?  This may, ultimately, reside in the asymmetry of molecules that make up cilia.

At any rate, P. Z.’s column is good, but his explanation of nodal on Pharyngula is even better.  P. Z. has a real talent for explaining science clearly and engagingly, and too often this is overlooked by the hordes of people who visit his blog for the controversy, atheism, and trenchant attacks on religion.  (One thing I’ve found from writing this blog is that visits are much more numerous when I’m attacking something than when I’m talking about science, a fact that’s a little bit sad.)
But the point I wanted to make relates to an earlier post I made about what counts as evidence for evolution.  P. Z.’s  elegant description of how nodal works was hijacked by the Guardian editors by putting it under the title:

Lopsided gene that proves

humans are distant cousins

of the humble snail

A gene shared by birds, fish, reptiles, people – and snails – reveals the fundamental relatedness of all living creatures

Well, we already knew, of course, that we were distant cousins of the humble snail.   We don’t need nodal to tell us that.  And the observation that the gene has similar functions in humans and snails is not, to me, dispositive evidence that humans and snails are related. After all, creationists could always say, “Well of course the gene does the same thing in humans and snails! That’s just the way the Creator decided to make asymmetries!  It doesn’t say anything about common ancestry.”  As I’ve mentioned before, the fact that related creatures use similar genes to do similar things does not count as strong evidence for evolution as opposed to a creationist/intelligent-design alternative.  We might as well say that snails have a gene producing cytochrome c as part of their metabolic pathway, and proclaim that this “proves that humans are distant cousins of the humble snail.”  We share hundreds of genes with the humble snail.

The choice of what to emphasize in a headline is the editors’, not P. Z.’s. And I suppose anything touting evolution is a good thing for readers.  Still, the Guardian editors should realize that hundreds and hundreds of genes already testify to common ancestry — if you choose to use genic similarity as evidence.  I prefer to look at dead genes that are active in relatives as far stronger evidence for evolution against the creationist alternative.

Anyway, congrats to P. Z. for his new gig and a good inaugural column.

Reference: Grande, C., and N. H. Patel. 2009. Nodal signalling is involved in left–right asymmetry in snails. Nature 457:1008-1011.

5 thoughts on “P. Z. gets a column, and more on what counts as evidence for evolution

  1. PZ says that humans and snails had a common ancestor 600M years ago. It is known that the common ancestor had nodal? If so, that would be interesting evidence for evolution. If not, then why does nodal have anything to do with humans and snails being related?

  2. I have several times encountered in your writing your saying that genes may not directly “prove” evolution. I appreciate your use of pseudogenes, as better. But I wonder if you are not selling the molecular evidence a little short. The layperson may not realize some facts that I suspect we might agree on to some degree and, at least, to my mind provide a great molecular support for evolution.

    I suppose you are formally correct, but I think there is a very strong second conformation of evolution from genes, namely their genomic and protein sequence. This is more than than just shared gene content of homologs; sequence analysis seems better support. If you essentially blindly look for patterns in diverse types of biological sequences, you essentially recreate the basic patterns of descent that evolutionists have suggested from non-molecular evidence. Some of this “pattern” is in parts of the gene, promoter, protein, intergenic region, etc. that like your pseudogene have no known function. Even in the cases of using molecular evidence to further refine the descent, these are compatible with fossils and related approaches. The patterns do not seems to correlate that well with just functional changes of proteins, for example. I may have it wrong, but it seems to me that one of the drivers for a neutral theory was the feeling that there was perhaps more neutral divergence in first protein sequences found in 60’s and early 70’s than could be explained by divergence in function across proteins in species.

    Just curious if you agree or not. IF one focuses on genes or genomes, as sequences, then it seems the molecular support for evolution is much better.

    (I have to admit, have bought your book and looking forward to reading all of it but too many grants to write to read now–so if you addressed this maybe just point out location in book.)

  3. “One thing I’ve found from writing this blog is that visits are much more numerous when I’m attacking something than when I’m talking about science, a fact that’s a little bit sad.”

    Is it possible that could be because many or most people don’t feel informed enough to say as much about a science article as about some other topics? I know I enjoy reading science articles, but I seldom feel I have the depth of knowledge or expertise to comment on them.

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