Nonetheless, the "argument from personal incredulity," as such sentiment has been appropriately described, has been a weapon of little value in the anti-evolution movement. Anyone can state at any time that they cannot imagine how evolutionary mechanisms might have produced a certain species, organ, structure. The hallmark of the intelligent design movement, however, is that it purports to rise above the level of personal skepticism.
It claims to have found a reason why evolution could not have produced a structure like the bacterial flagellum, a reason based on sound, solid scientific evidence. Why does the intelligent design movement regard the flagellum as unevolvable? Because it is said to possesses a quality known as "irreducible complexity.
They do exist, however, and therefore they must have been produced by something. That, simply stated, is the core of the new argument from design, and the intellectual basis of the intelligent design movement. The great irony of the flagellum's increasing acceptance as an icon of anti-evolution is that fact that research had demolished its status as an example of irreducible complexity almost at the very moment it was first proclaimed.
The purpose of this article is to explore the arguments by which the flagellum's notoriety has been achieved, and to review the research developments that have now undermined they very foundations of those arguments. The flagellum owes its status principally to Darwin's Black Box Behe a a book by Michael Behe that employed it in a carefully-crafted anti-evolution argument.
Building upon William Paley's well-known "argument from design," Behe sought to bring the argument two centuries forward into the realm of biochemistry. Like Paley, Behe appealed to his readers to appreciate the intricate complexity of living organisms as evidence for the work of a designer.
Unlike Paley, however, he raised the argument to a new level, claiming to have discovered a scientific principle that could be used to prove that certain structures could not have been produced by evolution. That principle goes by the name of "irreducible complexity. An irreducibly complex structure is defined as ". Because they could not possibly have been produced by the process of evolution:. Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on.
The phrase "numerous, successive, slight modifications" is not accidental. The very same words were used by Charles Darwin in The Origin of Species in describing the conditions that had to be met for his theory to be true. As Darwin wrote, if one could find an organ or structure that could not have been formed by "numerous, successive, slight modifications," his "theory would absolutely break down" Darwin , Living cells are filled, of course, with complex structures whose detailed evolutionary origins are not known.
Such arguments are easy to make, of course, but nature of scientific progress renders them far from compelling. The lack of a detailed current explanation for a structure, organ, or process does not mean that science will never come up with one. As an example, one might consider the question of how left-right asymmetry arises in vertebrate development, a question that was beyond explanation until the s Belmonte In one might have argued that the body's left-right asymmetry could just as well be explained by the intervention of a designer as by an unknown molecular mechanism.
Only a decade later, the actual molecular mechanism was identified Stern , and any claim one might have made for the intervention of a designer would have been discarded. The same point can be made, of course, regarding any structure or mechanism whose origins are not yet understood. The utility of the bacterial flagellum is that it seems to rise above this "argument from ignorance.
The existence of such a multipart machine therefore provides genuine scientific proof of the actions of an intelligent designer. In the case of the flagellum, the assertion of irreducible complexity means that a minimum number of protein components, perhaps 30, are required to produce a working biological function.
By the logic of irreducible complexity, these individual components should have no function until all 30 are put into place, at which point the function of motility appears. What this means, of course, is that evolution could not have fashioned those components a few at a time, since they do not have functions that could be favored by natural selection. As Behe wrote: ". The flagellum is irreducibly complex, and therefore, it must have been designed. Case closed. The assertion that cellular machines are irreducibly complex, and therefore provide proof of design, has not gone unnoticed by the scientific community.
A number of detailed rebuttals have appeared in the literature, and many have pointed out the poor reasoning of recasting the classic argument from design in the modern language of biochemistry Coyne ; Miller ; Depew ; Thornhill and Ussery I have suggested elsewhere that the scientific literature contains counter-examples to any assertion that evolution cannot explain biochemical complexity Miller , , and other workers have addressed the issue of how evolutionary mechanisms allow biological systems to increase in information content Schneider ; Adami, Ofria, and Collier The most powerful rebuttals to the flagellum story, however, have not come from direct attempts to answer the critics of evolution.
Rather, they have emerged from the steady progress of scientific work on the genes and proteins associated with the flagellum and other cellular structures. Remember the claim that "any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional? Functional enough, in some cases, to pose a serious threat to human life. Nonetheless, there are indeed bacteria that produce diseases, ranging from the mildly unpleasant to the truly dangerous. Pathogenic, or disease-causing, bacteria threaten the organisms they infect in a variety of ways, one of which is to produce poisons and inject them directly into the cells of the body.
Once inside, these toxins break down and destroy the host cells, producing illness, tissue damage, and sometimes even death. In order to carry out this diabolical work, bacteria must not only produce the protein toxins that bring about the demise of their hosts, but they must efficiently inject them across the cell membranes and into the cells of their hosts. They do this by means of any number of specialized protein secretory systems.
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One, known as the type III secretory system TTSS , allows gram negative bacteria to translocate proteins directly into the cytoplasm of a host cell Heuck At first glance, the existence of the TTSS, a nasty little device that allows bacteria to inject these toxins through the cell membranes of its unsuspecting hosts, would seem to have little to do with the flagellum. As figure 2 Heuck shows, these homologies extend to a cluster of closely-associated proteins found in both of these molecular "machines.
Extending such studies with a detailed comparison of the proteins associated with both systems, Aizawa has seconded this suggestion, noting that the two systems "consist of homologous component proteins with common physico-chemical properties" Aizawa , It is now clear, therefore, that a smaller subset of the full complement of proteins in the flagellum makes up the functional transmembrane portion of the TTSS. Figure 2: There are extensive homologies between type III secretory proteins and proteins involved in export in the basal region of the bacterial flagellum.
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These homologies demonstrate that the bacterial flagellum is not "irreducibly complex. Stated directly, the TTSS does its dirty work using a handful of proteins from the base of the flagellum. From the evolutionary point of view, this relationship is hardly surprising. In fact, it's to be expected that the opportunism of evolutionary processes would mix and match proteins to produce new and novel functions. According to the doctrine of irreducible complexity, however, this should not be possible.
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If the flagellum is indeed irreducibly complex, then removing just one part, let alone 10 or 15, should render what remains "by definition nonfunctional. The TTSS may be bad news for us, but for the bacteria that possess it, it is a truly valuable biochemical machine. The existence of the TTSS in a wide variety of bacteria demonstrates that a small portion of the "irreducibly complex" flagellum can indeed carry out an important biological function. Since such a function is clearly favored by natural selection, the contention that the flagellum must be fully-assembled before any of its component parts can be useful is obviously incorrect.
What this means is that the argument for intelligent design of the flagellum has failed. Classically, one of the most widely-repeated charges made by anti-evolutionists is that the fossil record contains wide "gaps" for which transitional fossils have never been found.
Therefore, the intervention of a creative agency, an intelligent designer, must be invoked to account for each gap.
Ironically, the response of anti-evolutionists to such discoveries is frequently to claim that things have only gotten worse for evolution. Where previously there had been just one gap, as a result of the transitional fossil, now there are two one on either side of the newly-discovered specimen. As word of the relationship between the eubacterial flagellum and the TTSS has begun to spread among the "design" community, the first hints of a remarkably similar reaction have emerged. The TTSS only makes problems worse for evolution, according to this response, because now there are two irreducibly-complex systems to deal with.
But now there are two systems for evolutionists to explain instead of just one.
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Unfortunately for this line of argument, the claim that one irreducibly-complex system might contain another is self-contradictory. To understand this, we need to remember that the entire point of the design argument, as exemplified by the flagellum, is that only the entire biochemical machine, with all of its parts, is functional. For the intelligent design argument to stand, this must be the case, since it provides the basis for their claim that only the complete flagellum can be favored by natural selection, not any its component parts. Since we now know that this is indeed the case, it is obviously true that the flagellum is not irreducibly complex.
A second reaction, which I have heard directly after describing the relationship between the secretory apparatus and the flagellum, is the objection that the TTSS does not tell us how either it or the flagellum evolved. This is certainly true, although Aizawa has suggested that the TTSS may indeed be an evolutionary precursor of the flagellum Aizawa Nonetheless, until we have produced a step-by-step account for the evolutionary derivation of the flagellum, one may indeed invoke the argument from ignorance for this and every other complex biochemical machine.
However, in agreeing to this, one must keep in mind that the doctrine of irreducible complexity was intended to go one step beyond the claim of ignorance. It was fashioned in order to provide a rationale for claiming that the bacterial flagellum couldn't have evolved, even in principle, because it is irreducibly complex. Now that a simpler, functional system the TTSS has been discovered among the protein components of the flagellum, the claim of irreducible complexity has collapsed, and with it any "evidence" that the flagellum was designed.
At first glance, William Dembski's case for intelligent design seems to follow a distinctly different strategy in dealing with biological complexity.
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His recent book, No Free Lunch Dembski a , lays out this case, using information theory and mathematics to show that life is the result of intelligent design. Dembski makes the assertion that living organisms contain what he calls "complex specified information" CSI , and claims to have shown that the evolutionary mechanism of natural selection cannot produce CSI.
Therefore, any instance of CSI in a living organism must be the result of intelligent design. And living organisms, according to Dembski, are chock-full of CSI. Dembski's arguments, couched in the language of information theory, are highly technical and are defended, almost exclusively, by reference to their utility in detecting information produced by human beings.
These include phone and credit card numbers, symphonies, and artistic woodcuts, to name just a few. One might then expect that Dembski, having shown how the presence of CSI can be demonstrated in man made objects, would then turn to a variety of biological objects.
Instead, he turns to just one such object, the bacterial flagellum. Dembski then offers his readers a calculation showing that the flagellum could not have possibly have evolved. Significantly, he begins that calculation by linking his arguments to those of Behe, writing: "I want therefore in this section to show how irreducible complexity is a special case of specified complexity, and in particular I want to sketch how one calculates the relevant probabilities needed to eliminate chance and infer design for such systems" Dembski a, Dembski then tells us that an irreducibly complex system, like the flagellum, is a "discrete combinatorial object.
Dembski refers to these three probabilities as P orig , P local , and P config , and he regards each of them as separate and independent Dembski a, This approach overlooks the fact that the last two probabilities are actually contained within the first. Localization and self-assembly of complex protein structures in prokaryotic cells are properties generally determined by signals built into the primary structures of the proteins themselves.
The same is likely true for the amino acid sequences of the 30 or so protein components of the flagellum and the approximately 20 proteins involved in the flagellum's assembly McNab ; Yonekura et al Therefore, if one gets the sequences of all the proteins right, localization and assembly will take care of themselves. To the ID enthusiast, however, this is a point of little concern.
According to Dembski, evolution could still not construct the 30 proteins needed for the flagellum. His reason is that the probability of their assembly falls below what he terms the "universal probability bound.
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