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E. coli and its Rotary Propulsion System: Dembski’s Flagship Case for Design

Escherichia coli is a species of bacteria commonly found in the human intestinal tract and has been used extensively for studies of molecular genetics. Its single, rod-shaped, prokaryotic cell is surrounded by a rigid but porous cell wall. Immediately inside the cell wall is the plasma membrane, which effectively functions as the barrier between the interior of the cell and its external environment. The nucleoid within the cell contains the cell’s circular DNA molecule, which houses E. coli’s genetic information.

The genome of E. coli - the instructional information residing in the genetically relevant portion of its DNA - consists of about 4.7 million base pairs, representing approximately 4000 genes. These genes specify the particular character of the numerous RNA and protein molecules that carry out the multitude of diverse tasks that enable the cell to act and interact as it does. The formation, structure and functions of the cell and its component parts are expressions of the information coded in the base pair sequences that comprise the E. coli genome.

Protruding outward from the E. coli cell wall is a hair-like filament made of the protein flagellin. The base of the filament is attached via a bent “hook” structure to a miniature rotary drive mechanism embedded in the plasma membrane and constructed from several types of protein molecules. This configuration of motor, hook and filament constitutes the flagellum system. The energy for the flagellum’s rotary motion is derived from a proton gradient across the bacterial membrane.

A cutaway sketch of the flagellum, complete with its rotary motor system, appears prominently on the front cover of Dembski’s book, No Free Lunch. Dembski uses the bacterial flagellum as the principal example of what he considers to be an intelligently designed biotic structure.

The flagellum is an acid-powered rotary motor with a whip-like tail whose rotating motion enables a bacterium to navigate through its watery environment. Behe shows that the intricate machinery of this molecular motor - including a rotor, a stator, O-rings, bushings, and a drive shaft - requires the coordinated interactions of about thirty proteins and another twenty or so proteins to assist in their assembly.NFL, pp. 249-250.

On a Darwinian view, a bacterium with a flagellum evolved via the Darwinian selection mechanism from a bacterium without a flagellum. For this mechanism to produce the flagellum, chance modifications have to generate the various proteins that constitute the flagellum and then selection must preserve them, gather them to the right location in the bacterium, and then properly assemble them.NFL, p. 250.

With regard to a particular biochemical system like the bacterial flagellum, intelligent design asserts that “No undirected natural process could produce this system”NFL, p. 272.

In the eyes of design theorists like Behe and Dembski, the bacterial flagellum presents the Darwinian mechanism with an insurmountable problem. Employing some of the ID vocabulary that we have already examined, the nature of the problem as they see it can be stated as follows:

  1. The bacterial flagellum displays specified complexity.

  2. Specified complexity in biotic systems cannot be generated by the Darwinian mechanism, which relies on chance.

  3. Therefore the bacterial flagellum must have been intelligently designed - that is, it could have been actualized only with the assistance of form-conferring interventions by an unembodied intelligent agent.

Each of these three statements could, I believe, be successfully contested, but it should be clear that if the first of these fails, then Dembski’s whole system of design inferences built on the premise that specified complexity is demonstrably present but naturally impossible also fails. Therefore, let us examine statement 1) more closely.

To say that the bacterial flagellum exhibits specified complexity requires the demonstration that the flagellum is both complex and specified, where the meaning of each of these two terms must be taken from Dembski’s development of the complexity-specification criterion. We shall deal with each of these two requirements individually, beginning with complexity.

 Email link | Printer-friendly | Feedback | Contributed by: Dr. Howard Van Till

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