12 October 2007
Michael J Behe
This is the third in a series of responses I’m posting this week.
In “Evolution of the Bacterial Flagellum” (Microbe Magazine, July 2007), Wong et al seek to counter arguments of intelligent design proponents such as myself that the flagellum did not evolve by random mutation and natural selection. Unfortunately, their otherwise-fine review misunderstands design reasoning and so fails to engage that issue. The critical passage from Wong et al is the first paragraph:
Proponents of the intelligent design (ID) explanation for how organisms developed claim that the bacterial flagellum (BF) is irreducibly complex. They argue that this structure is so complicated that it could not have emerged through random selection but had to be designed by an intelligent entity. One part of this claim is that each flagellar component is used solely for the purpose of making a flagellum that, in turn, is used only for motility. Further, each flagellar protein is assumed to have appeared independently of the other component proteins.
Although the first two sentences are correct, the last two sentences are quite wrong. (The authors cite no references for these latter claims.) It is no part of the design argument that each component of an irreducibly complex structure must be used solely for that purpose, nor that each part must arise independently. In my 1996 book Darwin’s Black Box, which brought the concept of irreducible complexity to wide public attention, I pointed out the fact that, for example, proteins of the blood clotting cascade share sequence homology with each other and with other serine proteases, and the fact that ciliary proteins such as tubulin are involved in other tasks in the cell. Yet I explained that neither sequence homology nor multiple functions showed how integrated systems containing many parts could be put together by small random steps. Unfortunately, Wong et al spend their efforts addressing their own erroneous assertions. They fail to address the only pertinent question, the question of whether random, unintelligent processes — even when filtered by natural selection — could plausibly build a structure such as the flagellum.
To address the adequacy of random processes plus selection would require rigorous experiments or calculations showing that the intricate, functional structures are not too improbable given the evolutionary resources available. Recent work bears negatively on this difficult question. In long term laboratory evolution experiments over tens of thousands of generations (Lenski, R.E. 2004. Phenotypic and genomic evolution during a 20,000-generation experiment with the bacterium Escherichia coli. Plant Breeding Reviews 24:225-265), cultures of E. coli were repeatedly seen to lose the ability to make ribose and maltose, and to repair their DNA. Some mutations shut down expression of their flagellar genes, apparently to conserve energy. No selected mutations were observed which could plausibly be argued to be the incipient stages of some new, complex functional system. Similar kinds of results are seen in other well-studied evolutionary systems. For example, in response to strong pressure from the malarial parasite, the human genome has suffered a handful of positively-selected-yet-degradative mutations (Carter, R. and Mendis, K.N. 2002. Evolutionary and historical aspects of the burden of malaria. Clin. Microbiol. Rev. 15:564-594), including ones that render nonfunctional the genes for glucose-6-phosphate dehydrogenase, the alpha and beta chains of hemoglobin, band 3 protein, and others. Again, no selected mutations were observed which could plausibly be argued to be the incipient stages of some new, complex functional system.
To a skeptic such as myself, this does not look like the sort of process which could build complex molecular machinery.