Darwinist
Out of context: Reply #462
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DS: One of the big claims made by IDCists involves a concept called Irreducible Complexity which means, as I understand it, that there are structures at many levels in living organisms in which each component is critical. Remove any component and the entire system fails catastrophically. And so, the claim goes that such a system could not develop in a step by step fashion required by natural selection because the transitory stages would have either have no adaptive value or result in the death of the owner. What's wrong with that?
PZ: This is the same logic that would say it is impossible to build an arch, because removing one piece would cause the whole thing to tumble down. Yet arches are built every day -- bridges must be miracles!
The answer, of course, is that arches are supported by a scaffold during their assembly, and similarly, "irreducibly complex" pathways were supported by duplications and redundancy during their evolution. I've explained this in a little detail here. Simply put, there are two broad explanations for how IC systems could evolve. One is that intermediate steps can be added by gene duplication that do not interfere and can even enhance the effectiveness of the pathway, and subsequent loss of redundancy makes them essential and unremovable. The other explanation is that it is a mistake to assume loss of a piece would cause failure; it may not function for the role you think it should, but it may function in some other capacity. Biological systems tend to be highly multifunctional and rich with redundancies, so none of this is surprising.
You asked earlier why people should think me more credible than Behe. One reason is that he has rested his career on this untenable nonsense of "irreducible complexity", which is so trivially false that it implies a deep misunderstanding of basic concepts of molecular evolution. "