Irreducible complexity

Irreducible complexity is a property of any system whose internal workings are so complex that the removal or alteration of any part would prevent the system from functioning. For example, a car will not go without an engine. Indeed, the engine will not run without fuel and a starter motor. The designer must ensure that all parts work together to produce the intended function.

This concept has been used by astronomer Fred Hoyle and biochemistry professor Michael Behe to argue against the idea of "evolution through natural selection". Hoyle compares the probability of all the parts of a cell coming together through natural forces to the probability of a cyclone in a hanger assembling all the parts of a Boeing 747 aircraft.

Intelligent Design theorists argue that a biological system could not make a gradual transition via small successive steps, unless each step provided an advantage to the species.

Behe presents several examples of organs and cells which could not have evolved through natural forces alone due to their irreducible complexity.

Behe introduced and defined this term on page 9 of his work, "Darwin's Black Box":

Irreducible complexity is a "single system which is composed of several interacting parts that contribute to the basic function, and where the removal of any one of the parts causes the system to effectively cease functioning."

It is therefore a state of interacting components that could not function without all its parts. In particular, the term is used by proponents of Intelligent Design to argue that irreducibly complex biological systems cannot evolve.

The most commonly used examples of irreducibly complex biological systems are the human compound eye, the blood clotting cascade, and the bacterial flagellum.