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Although definitions and usage vary immensely, they boil down to these:
- An irreducibly complex system has multiple required parts.
- An irreducibly complex system is one which natural evolutionary processes cannot produce.
"By irreducibly complex I mean a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning. An irreducibly complex system cannot be produced directly (that is, by continuously improving the initial function, which continues to work by the same mechanism) by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional. An irreducibly complex biological system, if there is such a thing, would be a powerful challenge to Darwinian evolution. (p. 39)"
Michael Behe claims that an irreducibly complex system cannot evolve through gradual accumulation of parts, and implicitly claims that it cannot evolve at all. From this, he concludes that any irreducibly complex system found in nature must have been designed by an intelligent being.
This argument rests on several faulty assumptions:
- Faulty assumption #1: Evolution can only proceed by adding parts, never by removing them. In fact, evolution can remove parts as easily as add new ones (perhaps more easily, even). If the system functions better without a given part, there will be selective pressure to remove it. Some species of bats, spiders and deep-water fish lack functioning eyes; it costs resources to grow eyes, for little or no benefit. Whales, although once quadrapeds, no longer have functioning hind legs. Humans no longer have decernable tails.
- Faulty assumption #2: Biological systems never change function. However, the components of an irreducibly complex system, individually or together, can serve a purpose other than that performed by the final system. As Kenneth Miller likes to demonstrate, a mousetrap with a missing trigger can be used as a tie clip; if the spring is missing, it can still be used as a key chain; and the base by itself can be used as a paperweight. See cooption and biological traps.
- Faulty assumption #3: Helpful parts cannot become required parts. But most "IC systems", when examined across many organisms, exhibit variability in what parts are required. See Variability in IC systems.
In addition, at the atomic and molecular level, the interaction of the building blocks may be thought of more as Complex adaptive systems than bowling balls. Another problem with irreducible complexity, as it is used by IDists, is that the implied intelligent designer would also probably be irreducibly complex.
In addition to its faulty assumptions, there is also a logical problem with the Irreducible Complexity argument for design in its current state. Irreducible Complexity purports to be a scientific method for consistently identifying biological systems which could not have evolved. Should it fail in any one case, it will be shown to be unreliable and all cases where irreducible complexity is used to argue for design would become suspect by extension. Thus, any example where a system identified as irreducibly complex is shown:
- A. to be reducibly complex or
- B. to have evolved
will indicate that irreducible complexity cannot be reliably identified and/or is not an indication of design. Depending on your point of view, the Blood Clotting system has been demonstrated to fit pattern A or B. As a result, no system that has been identified as Irreducibly Complex should be viewed as a product of design until a more accurate and testable definition of Irreducible Complexity has been proposed.
Furthermore, there are the problems of IC's inability to (scientifically) predict anything, and that it is, ultimately, unfalsifiable.
A natural bridge is a system which is no longer functional if any of its parts were removed. If such systems could not evolve in a natural process, we had to conclude, that there is a designer of natural bridges. But we know how natural bridges were formed, softer rock has been washed out from under harder rock. The predecessors did not have the same functionality.
Though the evolution of organisms is different from the processes shaping natural bridges, this counterexample shows that natural processes are perfectly capable of producing so-called "irreducible complexity" and there are in fact "clocks without clockmakers", or rather bridges without bridgebuilders.