What are some examples of irreducibly complex systems?

Answer 1

An eye and flagella Creationists commonly put forward the eye or the bacterial flagellum as examples of irreducibly complex systems that could not have evolved from simpler systems. However, neither the eye nor the bacterial flagellum is really irreducibly complex. Scientists can point to primitive eyes, going back to merely light-sensitive areas of skin. An evolutionary progression can readily be deduced. The bacterial flagellum is a complex structure that allows a bacterium to move around in a liquid, and indeed the flagellum would not work as such if any key component were removed. However, there are similar bacterial structures that contain most, but not all, the components of the flagellum and which perform useful functions. The type III secretory system, a molecular syringe which bacteria use to inject toxins into other cells, is one such structure. Whether the bacterial flagellum evolved from the type III secretory system or from another bacterial system, the existence of the type III secretory system proves that the bacterial flagellum is not irreducibly complex.