“With the criterion of two protein-protein binding sites, we can quickly see why stupendously complex structures such as the cilium, the flagellum, and the machinery that builds them are beyond Darwinian evolution. The flagellum has dozens of protein parts that specifically bind to each other; the cilium has hundreds. The IFT particle itself has sixteen proteins; even complex A, the smaller subset of IFT, has half a dozen protein parts, enormously beyond the reach of Darwinian processes. In fact, drawing the edge of evolution at complexes of three different kinds of cellular proteins means that the great majority of functional cellular features are across that line, not just the most intricate ones that command our attention such as the cilium and flagellum. Most proteins in the cell work as teams of a half dozen or more.” (The Edge of Evolution, p. 146)
This assumes that what Behe considers a “selectable path” is how such things arise.
It isn’t.
Then a selectable path is unlikely, do we agree?
The hypothesis is that an irreducibly complex system can not evolve through pathways that involve neutral mutations. How do we falsify that hypothesis?
Is that Behe’s hypothesis? This is a genuine question, I don’t know that he has ever worded it in quite that manner.
To my understanding (and according to how @lee_merrill keeps presenting it) Behe argues that a system of a given level of complexity cannot arise without a number of selectable steps, and sometimes (e.g. for a “CCC”) a “selectable step” would require that two or more mutations occur at the same time. He then argues that the number of “selectable steps” required exceeds the number that could be expected to arise thru unguided natural processes.
I must not have been speaking clearly, Behe’s edge is at a “double-CCC” (CCC: chloroquine-complexity cluster, 10^20 organisms needed for chloroquine resistance to arise). Since a CCC evidently requires 2 deleterious mutations, and a new protein-protein binding site (at 4-6 deleterious mutations) is on the order of a CCC of difficulty, Behe places his edge at a “double-CCC”, or two new protein-protein binding sites.