Co-option An Introduction
Co-option is a proposed solution to the problem of irreducibly complex machines inherent in living cells. Irreducible complexity relates to a characteristic of common complex systems whereby they need all of their individual components parts in place in order to impart functionality to the system as a whole. In other words, it is impossible to reduce the complexity of an irreducibly complex system by removing any of its component parts and still maintain its functionality.
The concept was popularized by Lehigh University Professor Michael Behe in his breakthrough book Darwins black Box. Behe uses the mousetrap as an illustrative example of this concept. A mousetrap consists of five interacting pieces -- the base, the catch, the spring, the hammer, and the hold-down bar. All of these must be in place for the mousetrap to work, as the removal of any one piece destroys the function of the mousetrap. In like manner, biological systems require multiple parts working together in order to function. Removal of one of the constituent parts renders the entire system non-functional.
Behes best known example of an irreducibly complex system from biological systems is the bacterial flagellum. An ensemble of over forty different kinds of proteins makes up the typical bacterial flagellum. These proteins function in concert as a literal rotary motor. The bacterial flagellums components stand as direct analogs to the parts of a man-made motor, including a rotor, stator, drive shaft, bushing, universal joint, and propeller.
Critics of the notion of Irreducible complexity have responded with reference to the concept of co-option. According to this hypothesis, apparently irreducibly complex systems can evolve from simpler precursors which serve other unrelated functions. One such argument that Kenneth Miller has advanced is that the basal body of the flagella is similar in a number of respects to the Type III secretion system, a needle-like structure that pathogenic germs use to inject toxins into living eukaryotic cells. The needles base has ten elements in common with the flagellum, but it is missing forty of the proteins that make a flagellum work. Hence, Kenneth Miller concludes that, The parts of this supposedly irreducibly complex system actually have functions of their own. Critics of Millers co-optive idea as it relates to the flagellar motor point out that analysis of the gene sequences of the two systems suggest that the flagellar motor arose first and then the pump came later. In other words, if anything, the pump evolved from the motor, not the motor from the pump.
Co-option The challenges
In order for co-option to produce a bacterial flagellum, all of the component parts would need to be present simultaneously and in roughly the same place, and all of them must have had other naturally-selectable, useful functions. The evidence is lacking, however, that such has ever been the case.
Moreover, co-option necessitates the compatibility of the components in relation to each other. A bolt that is too large or small, or that has threads that are too fine or too coarse to match those of the nut cannot be combined with the nut to make a fastener. Again, there is a substantial lack of evidence that this interface compatibility has ever existed.
Even if all the parts are available at the same time and in the correct location, and are functionally compatible, an assemble mechanism is still required, and that mechanism must be complete in every detail -- otherwise incomplete or improper assembly will result. The assembly mechanism thus represents yet another irreducibly complex hurdle. Assembly must be timed and coordinated perfectly -- and the assembly instructions must be complete in every detail, otherwise it is inconceivable that any function will result. To date, no materialistic mechanism has been documented that is able to produce the complex and highly specified assembly instructions which would be necessary to achieve such a fete.
While the concept of co-option is certainly an interesting idea, more research is required to establish its tenability as a plausible explanation for the presence of apparent irreducibly complex systems which so permeate lifes biochemistry. The co-option argument tacitly presupposes the need for the very thing it seeks to explain -- a functionally interdependent system of proteins. Until a detailed analysis is undertaken, and a plausible model constructed, the hypothesis stands as a conjecture, based not upon empirical data, but upon an a-priori commitment to a Darwinistic paradigm.