Irreducible complexity
Irreducible complexity is one of the ideas I've heard used to attempt to dispute the theory of evolution many times. Someone's even brought it up here. Here's how the argument goes. Suppose a biological system consists of multiple parts and the only way the system is benefitial to the organism is if all the parts of the system are present and none of the parts are benefitial to the organism unless all are present. The creationist's argument is that such a biological system is not able to have evolved from a simpler predecessor, and thus must have been created by an "intelligent designer" AKA god.


