Irreducible complexity (IC) is a concept which, essentially, states that any operating system, when reduced to its lowest state of functional operation, cannot operate when any individual part of its system is removed. The common mousetrap is given as an example of IC; take away any of its parts (e.g., the base, the hammer, the spring, the catch) and it cannot function as a mousetrap. Michael Behe, in his book Darwin's Black Box, proposed that the bacterial flagellum is a biological example of irreducible complexity. His argument is, essentially, given that the neo-Darwinian model of evolutionary theory progresses through natural selection combined with genetic mutation, it is improbable that an irreducibly complex system, one that is composed of interrelated functional parts, arose by chance.
There are, of course, several arguments against Behe's claims. One argument essentially claims that it is logically possible for an irreducibly complex system to evolve by chance (see my response here and here). While certainly a true statement, it does not necessarily (or sufficiently) address the issue of probability. Another argument proposes intermediate steps, within the evolutionary process of the flagellum, which involve co-adapted function for the various parts of the system we now see - co-adapted function we are, conveniently, no longer privy to. And still another argument (aka the Boston argument) builds on the co-adapted function argument by proposing a scenario for building the flagellum through a series of functionally intermediate steps - surprisingly, or not, intentional steps.
I'd like to address the idea of past potential functional intermediates, and their relation to design intentions, with the reality of engineering and construction.
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