This week's issue of Science has a book review (subscription required unfortunately) of Michael's Behe's latest effort to defend Intelligent Design Creationism. Michael Behe's latest book, The Edge of Evolution, contains Behe's latest incarnation of his idea of irreducible complexity. A few years ago he put forward this latest argument in a paper in Protein Science (a journal which one of my mentors dismissed, maybe a little unfairly, as a "junk journal"), and he elaborates on this argument more extensively in his new book. (See a response to Behe's Protein Science paper here.)
The argument is this: Any novel function in a protein that requires two simultaneous amino acid changes is so unlikely to occur by chance that the novel function must have been designed. Since most beneficial changes in protein function would require a change of two or more amino acids, most of the varied functional proteins we see in nature must have been designed and not evolved. So for example, if a receptor for a certain hormone were, over the course of evolution, to evolve a new specificity for a different hormone, and if that new specificity required at least two amino acid changes, then such change is incredibly unlikely to occur under just natural selection and random mutation.
In making this argument, Behe makes this explicit assumption: an 'intermediate' protein with only one amino acid change (so in other words, it's only halfway evolved towards a new function that requires two certain amino acid changes) is non-functional, and thus is not subject to selection for the new function. So in order for natural selection to act, the protein would need both amino acid changes simultaneously, arising by chance mutation in the same individual organism (an event which Behe calculates to be unlikely).
Sean Carroll takes on this argument by pointing out that Behe's assumption of non-functional intermediate proteins is contradicted by vast amounts evidence. Single amino acid changes in a protein do in fact cause beneficial changes that are favored by natural selection, and over time these single changes accumulate in a lineage to create a more robust novel function. This is the norm in evolution, not the exception as Behe would have it. The scientific literature supporting this is extensive.
Carroll goes on to make this important point: "Behe seems to lack any appreciation of the quantitative dimensions of molecular and trait evolution." This is because Behe, like me, is a biochemist - biochemists learn about the physics and chemistry of proteins. The kind of math we use to do our work consists primarily of differential equations that describe the kinetics and thermodynamics of proteins and nucleic acids. Biochemists generally do not study mutation rates, evolving populations, or the heavy statistics behind natural selection. That's a whole separate field, called quantitative genetics, founded primarily by the pioneering scientists Sewall Wright and Ronald Fisher, whose work is usually not that familiar to biochemists. (As someone who did a PhD in a biochemistry department, but now works in a genetics department, and in a lab that does serious quantitative genetics, I have acutely, even painfully, experienced this difference in training firsthand.)
Behe's problem is that he's tried to jump into this field without any serious background knowledge; it's like a chemist or engineer trying to tackle research problems in quantum gravity - the chances of producing anything worthwhile are essentially zero. Behe's efforts at modeling mutation and selection on protein function have thus been amateurish, and not taken seriously by people who work on these problems professionally.
On a different note, it's great to see Sean Carroll join the evolution/intelligent design fray. Many distinguished scientists write well and effectively against the weak claims of Intelligent Design, but few have the stature, as scientists, that Carroll has. He's a very big player in the field of evo-devo, a field which is directly relevant to the claims made by ID creationists. It's nice to see such a heavy-hitter get involved.