12/30/2009

VOX RE-DUX

Everyone's favorite libertarian character (and evolutionary skeptic) Vox Dei is up to his old rhetorical tricks in a recent post, wherein he reviews an article at PhysicsBlog which is turn is based on a paper by Mark Pagel and his colleagues at the University of Reading which was published earlier this month in Nature.

Now, the original paper in question uses phylogenetic trees to compare different models for how speciation occurs, and concludes that : "....it is the natural selection element that has even less reliable scientific evidence to support it than speciation or the concept of evolution itself."

I find this latest offering from Vox to be delightfully 19th century. Vox, you are gloriously, spectacularly wrong in multiple ways on this post. Though, in your favor, you are taking reportage in an on-line 'physics' source at face value, reportage that does a clumsy and misleading job of conceptualizing another discipline.

The big problem here is in the opening paragraph: the Red Queen hypothesis proposed by Leigh Van Valen is not strictly speaking a hypothesis about how speciation occurs, but rather about the way evolutionary arms races shape evolution. Van Valen predicted that speciation should in fact be relatively constant, and that either stasis or anagenesis should be more common than cladogenesis. It is true that Darwin (not necessarily Van Valen) tended to think entirely in terms of gradual change, changes that (in his words) would be 'insensible'. As it turns out, Darwin was wrong on both points. That hardly counts as evidence that natural selection does not occur, or that the former can not lead to new species.

Now, Pagel and company have apparently applied this 'Red Queen hypothesis' rather broadly as a term for a model of speciation, but when I read the description of the model provided in the article, it doesn't sound like a description of Leigh Van Valen's 'evolutionary arms race' promoting stasis and anagenesis over cladogenesis. It sounds like a description of phyletic gradualism, which in its strictest form is not consistent with modern evolutionary theory. So, from my point of view, the article Vox cites has Pagel et al. are said to be making claims against a theoretical model, but one which is something of a 'Straw Man'.

In fairness to Pagel et al, I don't think the reportage is very good at the site Vox cites. Here's another discussion of the same paper. Would you be surprised to learn that they directly cite Pagel, and that Pagel comes to a slightly different conclusion than was previously reported? It turns out that Pagel's group actually endorses the Red Queen hypothesis of constant speciation rates, but proposes a novel reinterpretation of the data uncoupling the former from phyletic gradualism.

Anyway, rapid anagenesis within an isolated lineage can occur as a result of natural selection. As it so happens, just this evening I saw the latest episode of NOVA .


It featured a very straightforward demonstration of natural selection acting on a population of pocket mice, wherein a rare mutation conferring protective coloration in a lava field gradually becomes more widely distributed, as in the case of the peppered moth. You can watch OR download a video about the evolution of pocket mice from HHMI, here:


What makes this example particularly robust is that scientists who studied the pocket mouse populations in nature were able to determine precisely which coding genes (exons) were switched 'on' and 'off' by mutations affecting the action of regulatory genes (introns) to make the change, and how the different lineages flourished in response to different environmental pressures. In other words...natural selection!

The program ('What Darwin Never Knew') takes many of its clearest examples straight from Sean Carroll's book 'The Making of the Fittest'. I might add that the particular set of molecular 'switches' studied in pocket mice appear to be conserved across many taxa, and are typically employed in setting external coloration and pattern. A general discussion of genetic switches can be found in this interactive page from NOVA.

Want more? Check out this interactive series of pages, where you can look for start and stop codons, molecular switches, ancestral and hitchhiking segments of DNA, and other details as you 'Explore a Stretch of Code'. This is a bit older (2001-02) but still very 21st century.

Now, why do I refer joshingly to Vox's post as '19th century'? Because Vox is actually espousing a viewpoint similar to that of Darwin's contemporaries, who were persuaded by the Origin's presentation of evidence for 'descent with modification', but had serious reservations about Darwin's proposed mechanism, natural selection. Today, the situation is notably reversed amongst most creationists, who typically admit natural selection occurs and that evolution of a sort occurs within a lineage (anagenesis), but deny the large-scale 'macroevolutionary' pattern of cladogenesis and the descent from a common ancestor which is implied.

I've posted something similar in the comments section on Vox's blog and emailed him, so it would not be surprising to see the two of us go back and forth on this.

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