We are beguiled by the simple story line Behe rehearses. He invites us to consider the situation by supposing that the flagellum requires the introduc- tion of some number—20, say—of proteins that the ancestral bacterium doesn’t originally have. So Darwinians have to produce a sequence of 21 organisms, the first having none of the proteins, and each subsequent organ- ism having one more than its predecessor. Darwin is forlorn because however he tries to imagine the possible pathway along which genetic changes successively appeared, he appreciates the plight of numbers 2–20, each of which is clogged with proteins that can’t serve any function, proteins that interfere with important cellular processes. These organisms will be targets of selection, and will wither in the struggle for existence. Only number 1, and number 21, in which all the protein constituents come together to form the flagellum, have what it takes. Because of the dreadful plight of the inter- mediates, natural selection couldn’t have brought the bacterium from there to here.
The story is fantasy, and Darwinians should disavow any commitment to it. First, there is no good reason for supposing that the ancestral bacterium lacked all, or even any, of the proteins needed to build the flagellum. It’s a common theme of evolutionary biology that constituents of a cell, a tissue, or an organism are put to new uses because of a modification of the genome. Perhaps the immediate precursor of the bacterium with the flagellum is an organism in which all the protein constituents are already present, but are employed in different ways. Then, at the very last step there’s a change in the genome that removes whatever chemical barrier previously prevented the building of the flagellum. In this organism (the precursor), the function of one of the proteins is to increase the efficiency of a particular energy- transfer process. The precursor of the precursor lacked that protein, so that the genetic change that led to the precursor improved a process that was previously adequate. So it goes, back down a sequence of ancestors, all quite capable of functioning in their environments but all at a selective disadvan- tage to the bacteria that succeeded them.
Of course*—but it is no more the product of speculative imagination than Behe’s seemingly plausible assumption that the components of the flagellum would have had to be added one by one, and would have sat around idly (at best) until the culminating moment when all were present. Moreover, we were supposed to be offered a proof of impossibility, and that won’t be complete until Behe and his allies have shown that all the conceivable scenarios through which bacteria might ac- quire flagella are flawed. Really demonstrating impossibility—or even im- probability—here and in kindred instances, is extremely difficult, precisely because it would require a much more systematic survey of the molecular differences among bacteria.