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.
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.
With regard to a particular biochemical system like the bacterial
flagellum, intelligent design asserts that “No undirected natural process could
produce this system”
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:
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The bacterial flagellum displays specified complexity.
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Specified complexity in biotic systems
cannot be generated by the Darwinian mechanism,
which relies on chance.
-
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.
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| Contributed by: Dr. Howard Van
Till
|
