Re: [asa] Fw: book "By Design"

The issue of T3SS and the irreducible complexity of flagellum was addressed by Steve Meyer in his book "Signature in the Cell." I will copy the entire page and a half plus the endnote that he devotes to it. Perhaps the biochemists in this group, or others familiar with the work, can help us understand the implications. Is the T3SS argument a valid argument against Behe's claim?
Randy

Page 492-493:

    "The debate about the origin of irreducibly complex molecular ma­chines provides another example of this kind of prediction. In his book Darwin)s Black Box) biochemist Michael Behe argues that the many miniature machines and circuits that have been discovered in cells pro­vide strong evidence for intelligent design. A crucial part of Behe's argu­ment for intelligent design involves his attempt to show that miniature machines, such as the now infamous bacterial flagellar motor, could not have developed from simpler precursors in a gradual step-by-step fashion. In Behe's view, the coordinated interaction of the many parts of the flagellar motor resulted ultimately from an idea in the mind of a designing intelligence, not from a process of gradual step-by-step evolu­tion from a series of simpler material precursors.
    "Behe's critics, however, such as biologist Kenneth Miller of Brown University, have suggested that the flagellar motor might have arisen via a different causal pathway. They suggest that the flagellar motor might have arisen from the functional parts of other simpler systems or from simpler subsystems of the motor. They have pointed to a tiny molecular syringe called a type-3 secretory system (or T3SS). They note that the T3SS usually functions as part of the flagellar motor, but it is sometimes found in bacteria that do not have the other parts of the flagellar motor. Since the type-3 secretory system is made of approximately ten proteins that have close homologues in the thirty-protein flagellar motor, and since this tiny pump does perform a function, Miller intimates that the flagellar motor might have arisen from this smaller pump.[9] Behe, however, remains convinced that the flagellar motor is the aboriginal system. To defend Behe, other ID theorists have suggested that when the type-3 secretory system appears in isolation from the other parts of the flagellar motor, it does so as the result of degenerative evolution­, that is, as the result of a loss of the genetic information necessary to produce the other parts of the motor.
"Thus, these two different views of the T3SS imply something differ­ent about the relative age of the genes that produce the flagellar motor and the T3SS, respectively. The co-option theory predicts that the genes that produce the T3SS syringe should be older than the genes that produce the flagellar motor, since the syringe in this view is a pre­cursor system. The hypothesis from design and degenerative evolution predicts the opposite-that the genes of the flagellar motor should be older than those of the T3SS. Thus, the two theories posit causal histo­ries for these molecular machines that contradict each other, and both make testable predictions about features of the systems (the age of the genes) as a result.

"As it happens, phylogenetic analyses of the distribution of flagellar systems in bacteria now make it possible to assess the relative age of two suites of genes. These phylogenetic studies suggest that the flagellar motor genes are older than the T3SS genes, thus providing an initial confirmation of the design-theoretic hypothesis about the origin of the flagellar motor.[10]

    "There is an interesting twist to this story, however-one that under­scores how intelligent design leads to many new and unresolved research questions. There is another design hypothesis about the flagellar motor and the T3SS. It envisions both systems as the products of independent instances of design, despite the similarity of the T3SS to parts of the flagellar motor. It predicts that if the T3SS were designed indepen­dently of the flagellar motor, then we ought to find many unique (non­homologous) genes in the T3SS-genes that exhibit little similarity to those found in the flagellar system. It is now known that the T3SS does have several such nonhomologous genes, confirming this prediction of a separate ID hypothesis. So did the T3SS devolve from the flagellar motor, or did it arise independently by a separate act of design? This is another research question generated by the theory of intelligent design. Clearly, further experimental tests are needed to discriminate between these two competing design hypotheses.

10. " As microbiologist Milton H. Saier Jr. has noted: "In considering the logistics of the conclusions made by Gophna et al. [10], it should be noted that Fla[gellar] systems have been able to diverge in structure so as to span either one membrane or two in the envelopes of Gram-positive and Gram-negative bacteria, respectively. So why haven't T3SSs? The most plausible explanation considers that T3SSs arose late from preexisting Gram-negative bacterial Fla[gellar] systems" ("Evolution of Bacterial Type III Protein Secretion Systems"). Additionally, as Saier observes, mutation­density studies have shown that the T3SS and flagellar motor genes have roughly equivalent ages. Saier regards these studies as less definitive than phylogenetic studies because of their assumption of a constant rate of evolutionary change, i.e., that mutation rates function as a consistent "molecular clock." But even if these studies yield accurate conclusions, they are consistent with the possibility of independent design or with an extremely rapid devolution of the T3SS from the flagellar motor, both of which are possibilities that would challenge the co-option hypothesis. This is the case, because it would presumably take longer to build a new system than to offload parts of an old one. Saier himself has recently changed his mind about his 2004 claim that the flagellar motor is older than the T3SS. He now favors the view that both the T3SS and the flagellar motor share a common ancestor. Nevertheless, he offers little empirical justification for this new view."

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Received on Thu Sep 17 20:48:19 2009