Defining Irreducible Complexity

So many IDiots, so little time ....

Granville Sewell has posted a message on Uncommon Descent asking What if we DID find irreducibly complex biological features?. He writes,

In any debate on Intelligent Design, there is a question I have long wished to see posed to ID opponents: “If we DID discover some biological feature that was irreducibly complex, to your satisfication and to the satisfaction of all reasonable observers, would that justify the design inference?” (Of course, I believe we have found thousands of such features, but never mind that.)

If the answer is yes, we just haven’t found any such thing yet, then all the constantly-repeated philosophical arguments that “ID is not science” immediately fall. If the answer is no, then at least the lay observer will be able to understand what is going on here, that Darwinism is not grounded on empirical evidence but a philosophy.

Here's how Michael Behe defines irreducible complexity in Darwin's Black Box (p. 39).

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 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.

There are many irreducibly complex systems in biology. One of my favorites is the citric acid cycle or Krebs cycle. This is a circular pathway of enzymes that oxidize acetate groups to two molecules of CO₂.


If you remove any one of the enzymes then there is no cycle and it will be impossible to oxidize acetyl groups to CO₂ and regenerate oxaloacetate. You cannot evolve a cycle for the complete net oxidation of acetate by starting with a more simple circular pathway then adding additional enzymes to improve the initial function; namely, the cyclic pathway of oxidation. Thus, by Behe's definition this is an irreducibly complex system whose function is to oxidize acetyl groups and regenerate the original precursor.

We have a damn good idea how the citric acid cycle evolved so the answer to Granville Sewell's original question is: no, the discovery of an irreducibly complex system does not justify the design inference. There are many ways of evolving irreducibly complex systems. This is the same answer that we've been giving for over ten years. Please try and keep up.

Now I have a question for the IDiots. If we can prove to your satisfaction that a particular system is irreducibly complex and demonstrate how it could easily have evolved, will you stop claiming that irreducibly complex systems can't evolve?