Argument from Irreducible Complexity
Form
Michael Behe's argument (Darwin's Black Box, 1996), the flagship of the intelligent-design movement:
- An irreducibly complex system is one composed of several well-matched, interacting parts, where the removal of any one part causes the system to stop functioning. Behe's analogy is a mousetrap: take away the spring, the hammer, the catch, or the base and it catches nothing.
- Such a system cannot be produced by the gradual, step-by-step improvement of a precursor, because any precursor missing a part would be non-functional and so could not be selected for.
- The bacterial flagellum, the blood-clotting cascade, the cilium, and the adaptive immune system are irreducibly complex.
- Therefore they were designed.
Behe accepts common descent and an old Earth. The argument is that evolution cannot have built certain structures, not that it did not happen.
My Positions
- Premise 2 is false, and was shown to be false before it was made. Evolution is not limited to adding parts to a system that keeps the same function. Parts are co-opted from other jobs (exaptation), scaffolding parts are lost once the structure stands (as the centring is removed from a finished arch), and functions change along the way. Hermann Muller described exactly this in 1918 and 1939, under the name "interlocking complexity", and predicted it as an ordinary outcome of evolution. Irreducible complexity is what you expect selection to leave behind, not what it cannot produce.
- The examples fail on inspection. The flagellum contains a working subset, the type III secretion system, which bacteria use to inject toxins, and most of its proteins have homologues doing other work. The clotting cascade is missing factors in whales and puffer fish, which clot anyway. The immune system's evolution is documented in a literature that was stacked on the witness table in front of Behe at Kitzmiller v. Dover (2005) and that he conceded he had not read. The mousetrap works with parts removed, as a tie clip, a paperweight, or a cruder trap (John McDonald's reducible mousetrap).
- It is an argument from Personal Incredulity. "I cannot see how this evolved" is the premise Charles Darwin answered in the Origin (chapter 6, on the eye) by setting out the series of intermediates. The correct response to a structure whose history is unknown is to look for the history, and in every case where someone has looked, intermediates have turned up.
- Even granted, the conclusion is a gap, not a designer. "Could not have evolved" would show that we need another explanation. It does not show what it is. Intelligent design offers no mechanism, no time, no method, no prediction, and no research programme, and twenty-five years on it has produced none (Creationism and Intelligent Design).
- The argument was abandoned in court by its author. At Dover, Behe accepted that his definition of irreducible complexity did not rule out the indirect routes in point 1, that no peer-reviewed research supported design, and that a definition of science broad enough to admit intelligent design would admit astrology. The court found that intelligent design "cannot uncouple itself from its creationist, and thus religious, antecedents".
Sources
- Behe, M., Darwin's Black Box (1996)
- Muller, H. J., "Reversibility in Evolution Considered from the Standpoint of Genetics", Biological Reviews 14 (1939); and Genetics 3 (1918)
- Pallen, M. J. and Matzke, N. J., "From The Origin of Species to the origin of bacterial flagella", Nature Reviews Microbiology 4 (2006)
- Miller, K. R., "The Flagellum Unspun", in Dembski, W. and Ruse, M. (eds), Debating Design (2004)
- Bottaro, A., Inlay, M. A. and Matzke, N. J., "Immunology in the spotlight at the Dover 'Intelligent Design' trial", Nature Immunology 7 (2006)
- Kitzmiller v. Dover Area School District, 400 F. Supp. 2d 707 (M.D. Pa. 2005)
- Darwin, C., On the Origin of Species (1859), ch. 6, "Organs of extreme perfection and complication"