Irreducible complexity, as defined by Michael Behe in Darwin's Black Box is a property of a system such that if any part is removed, the system ceases to function. Irreducible complexity is often used as an argument for Intelligent design.
The classic illustration of an irreducibly complex system is a mousetrap: it consists of a base, hammer, spring, catch (or trigger), and fasteners to hold the pieces together. If any of those parts is removed, the mousetrap no longer works.
The argument, then, is that since evolution proceeds by adding parts to an existing system one by one, the precursors of an irreducibly complex system would have been useless, and would not have been selected for. Ergo, all of the pieces had to be put together by an intelligent designer.
Construction by Addition, Modification and Removal
The argument that evolution always proceeds by adding parts is false. Natural selection can remove parts as well as add them. For instance, whales have no hind legs, but retain vestigial pelvises where their ancestors' legs were attached.
Another example of an irreducibly complex system are naturally occurring rock arches formed by the sea or wind: if any section is removed, the arch collapses. They are formed by the gradual removal of material. Similarly, biological mechanisms often do not exist in the surroundings that allowed them to initially evolve. There is therefore no reason to accept the claim that if a system is irreducibly complex that it cannot be built by natural processes.
Systems incomplete for one function can serve a different function
While it is true that an irreducibly complex system with a missing part loses its nominal function, it may still have some other function. For instance, a mousetrap without a catch is no longer a working mousetrap, but can still work as a tie clip, or a paperweight. A mousetrap without a base can be nailed to the floor. Such a mousetrap would not be as useful, but would still function.
For a biological example, consider the bacterial flagellum, a long spinning hair that functions as an "outboard motor" for bacteria. It is often cited as an example of an irreducibly complex system. But if some of its parts are removed, the resulting system bears a striking similarity to the Type Three Secretory System, a "syringe" that allows bacteria to infect other cells.
Argumentum ad Ignorantiam
No potential example of a supposed irreducibly complex system can, even in theory, demonstrate that it did not evolve from less complex components. One can only demonstrate how a system can be reduced, or claim ignorance as to how it can be. Irreducible complexity is therefore an argument from ignorance and, more specifically, a God of the gaps argument.
A claim that a system is irreducibly complex is not a falsifiable claim. Demonstrating how a complex system can be reduced to less complex components only shows the apologist to be wrong on that particular example. Each 'reduced' component is, in turn, another system susceptible to the same claim of being irreducibly complex, ad infinitum. This lack of falsifiability makes such claims unscientific.
Irreducible complexity is problematic because some natural structures exhibit irreducible complexity (such as a sea or wind arch). The earth has a solid core, which if "removed" would break the Earth's magnetic field which has the "function" of shielding us. The moon stabilises the spin of the Earth, and if removed, would break the spin axis of the Earth which has the "function" of regulating the seasons. The solar system contains the sun but if this were removed, it would break the solar system's "function" of giving the Earth a stable environment. Based on these examples, irreducible complexity concept is an invalid indicator of design in living things.