Definition
Irreducible complexity is the claim that some biological systems are composed of several interacting parts such that the removal of any one part causes the whole system to stop functioning. The term was introduced by the biochemist Michael Behe, who argued that such systems could not have arisen through gradual, step-by-step evolution, since the intermediate stages would have been non-functional and so would not have been favoured by natural selection.
It is the central technical argument of the intelligent design movement, intended to identify structures whose origin, on this view, is better explained by design.
Why it matters
How it works
The argument assumes that evolution must build a system by adding parts one at a time, with each intermediate already serving the system's final function. If no part can be removed without breaking that function, the system appears unreachable by gradual steps.
Critics identify two flaws in that assumption. First, parts can be co-opted: a component that evolved for one purpose can later be recruited into a new system — a process called exaptation. Second, scaffolding can be removed: a system may have had extra parts that were later lost, so the present, tightly integrated form is not how it began. Dawkins also notes that a structure that is useless at, say, half its current form may still have been useful in a different role. Researchers have proposed detailed evolutionary routes for Behe's flagship examples. Behe and other ID proponents continue to contest these accounts.