"Natural selection" is not simply a magic word you can say that explains everything. If a system can be demonstrated to be "irreducibly complex", meaning that it cannot be built up from a sequence of small changes each of which is FUNCTIONAL, but rather requires several interacting components which are ALL essential or the system does nothing useful, then natural selection is ruled out.
The jury is still out on the IDers "irreducible complexity" arguments, but they're not unscientific. I just want you to admit the LOGICAL POSSIBILITY that deep analysis of a system could show that it was designed rather than evolving by a sequence of small changes to systems each of which had a function.
Natural selection can only provide a theory for upstepping evolution. It cant provide a theory for the basic organisms original existence and how it came to be.