Wednesday, August 22, 2007
Stories of the Not So
I've been reading Philip Kitcher's Living with Darwin and he has a nice discussion of Darwin's preemptive (and prescient) strike on Intelligent Design advocates. One ploy -- hardly original with IDers -- is to try to identify evolutionary transitions that could not be managed by natural selection. Kitcher calls this the "concrete case" argument, which
... selects a collection of evolutionary changes, discusses them in detail, then endeavors to show that there's no conceivable process of natural selection that could have started from the original group of organisms and culminated in the finally modified group.But Darwin anticipated the argument:
Darwin's own consideration of the concrete case argument focused on some complex organs and structures that he rightly believed to be hard to understand in terms of natural selection. Two examples are prominent in the Origin, the eye and the electric organs found in some fish. ...This is, in fact, a cornerstone of the arguments of Michael Behe. He defines the intermediaries as being one protein added at a time to an organism that was lacking in any portion of the material that goes into the final product.
Darwin himself offered a tentative proposal about the evolution of the eye. He supposed that sensitivity to light might come in degrees, and that it might be possible to find, among existing organisms, some with a crude ability to respond to light, others with a more refined capacity and so on in something like a series. Perhaps, he speculated, research oh these creatures might expose reasons why the different levels of sensitivity provided an advantage over rival organisms who had less, thus providing a way of answering (or sidestepping) the creationist quip, "What use is half an eye?" ...
One feature of the story deserves emphasis. Darwin didn't start with a comparison between the fully formed eye -- in a human being or an octopus, say -- and then think of the component parts as being introduced, one at a time. He resisted the challenge to explain first the advantage of an eighth of an eye, then the advantage of a quarter of an eye, and so on, and focused instead on a function, light sensitivity, that might have been refined from an initial state of absence. To put it more bluntly, he didn't allow his envisaged challengers to define the sequence of "intermediates" for him.
We are beguiled by the simple story line Behe rehearses. He invites us to consider the situation by supposing that the flagellum requires the introduction 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 organism having one more than its predecessor. ...All Behe and the other IDers are doing is offering a "just not so" story, where, using a very limited and unrealistic version of evolutionary theory, a tale is told in such a way as to make the impossibility of some trait arising by natural selection seem more or less plausible. By any fair measure in logic or debate, all the response that is needed to refute such an argument is to give a plausible way for the trait to evolve. It is at that point that the IDers, who have never produced a scrap of positive evidence in favor of their argument -- merely conceptual claims -- then turn around and demand an impossible level of evidence to show every intermediate step or else, they assert, impossibility must be assumed.
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. ... So it goes, back down a sequence of ancestors, all quite capable of functioning in their environments but all at a selective disadvantage to the bacteria that succeeded them.
Isn't this all fantasy too? 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 acquire flagella are flawed. Really demonstrating impossibility -- or even improbability -- here and in kindred instances, is extremely difficult, precisely because it would require a much more systematic survey of the molecular differences among bacteria.
It's cheap rhetoric made no better by being dressed up in a two dollar tuxedo.
"VIII. Neither, lastly, would our observer be driven out of his conclusion [of a designer], or from his confidence in its truth, by being told that he knew nothing at all about the matter. He knows enough for his argument. He knows the utility of the end: he knows the subserviency and adaptation of the means to the end. These points being known, his ignorance of other points, his doubts concerning other points, affect not to certainty of his reasoning. The consciousness of knowing little, need not beget a distrust of that which he does know."
It's so much easier when you start from revealed "truth," isn't it?
A book you might be interested in is Neal C. Gillespie's Charles Darwin and the Problem of Creation, which tries to answer the question of why Darwin spent so much time in the Origin discussing creationism. He lists five versions of creationism or near-creationism in play among scientists of the time, ranging from flat-out miraculous special creation to idealism, all of which the IDers will crib from in a pinch.