Irreducible complexity

Irreducible complexity is a concept developed by Lehigh University biochemist Michael Behe in support of his theory of intelligent design. The term assumes that some pathways within a living cell function only if all their parts are present in good working order at the outset. For example, a mousetrap consists of several parts which work together to catch a mouse; if any part is missing or defective, the mousetrap fails to work at all. Similarly, the biochemistry of light detection requires complex interactions among many different molecules, each performing a very specialized job.

According to Behe there is no imaginable way that the necessary combination of molecules could be built up piecemeal, as the theory of evolution requires; either they are all present, or the process does not work. Behe does accepts the exists of an [old earth]? and also accepts the existence of common descent.

Most biochemists do not believe that the concept is useful, because Behe ignores mechanisms by which complexity comes into being. One such mechanism is "scaffolding" as a set of biochemical reactions are used to build up a pathway and then are discarded, in much the same way that a building is built from the botton up even though removing any of the columns would cause the building to collapse. Another mechanism is that a set of biochemical reactions start being used with something completely unrelated. For example, the fact that any step of light detection would render the pathway useless for light detection would not be important if the original adaptive benefit of the pathway was something unrelated.

Behe has also been accused of falling into the argument by lack of imagination fallacy. Just because he does not see how a pathway can develop, or just because no one sees how a pathway can come into being does not mean that there is no possible way for this to occur.

See: intelligent design