What is natural selection and how does evolution use it to build new organisms? The fundamental difficulty for any undirected process of evolution is being able to see into the future and determine what functions that organism will need to survive. An unguided process like natural selection is incapable of doing that. What natural selection and other undirected natural mechanisms cannot achieve however, an intelligent agent can.
Philosopher of Biology Paul Nelson describes the amazing process by which the worm C. elegans is constructed and how it points toward intelligent design.