Irreducible complexity is the foundational argument for intelligent design (ID) and its critique of Darwinism. The argument was popularized by Michael Behe's Darwin's Black Box. Darwin's black box, the thing he couldn't penetrate, was the cell. Today we can see into the cell and know it's more complex than Darwin dreamed, even more complex than our most complex machines. Not only is it complex but it's "irreducibly complex," as Behe argues.
Darwinists assume, for good reason, that nature selects mutations that have survival qualities to be passed on to the next generation. As these mutations build up complexity increases. This is not really controversial. Plague survivors of the Middle Ages passed on their plague resistant genetic mutations to future generations who were naturally more resistant to the plague. When they came into contact with nonEuropeans whose populations had never been exposed to the viruses, the natives suffered plagues of their own. These plagues decimated their populations until nature selected the fit among them to survive and pass on there survival qualities to future generations. This is why the plagues eventually faded.
The problem occurs when Darwinists use the engines of mutation and natural selection to explain all of life in all of its species diversity. They even try to explain the origin of life in Darwinistic terms. It seems however that Darwinism has major trouble explaining the cell and even parts of certain simple cells like the bacterial flagellum. Some bacteria swim because they have a flagellum that propels them like a little outboard motor. The flagellum has multiple moving parts that work together in the propulsion system. None of these parts are favorable or have survival value by themselves but only as parts of the more complex whole. See a diagram here. They could not have evolved in a step by step selection process. The bacterial flagellum is an irreducibly complex system. The rotor doesn't make sense without the stator, bearing, hook, multiple membranes, and flagellar filament that whips like a propeller.
Even more problematic is the origin of life itself. No clever arrangement of inorganic matter will ever produce a living cell. No effect can be greater than its cause. The appearance of higher taxa in what has been called the cambrian explosion is also problematic, because Darwinists are committed to gradualism. Mutations build up step by step until a new species slowly emerges on the tree of life. But life is not a tree with a missing root, but a forest in a miraculous ecosystem.
"In him all things hold together" (Colossians 1:17).
