On November 24, 2009--the 150th anniversary of Charles Darwin's On the Origin of Species--the theory that new species can arise from old ones through natural selection is still met with some resistance.

Some of evolution's most vocal critics are proponents of "intelligent design," arguing that many structures in plants and animals bear the unmistakable signature of design by a supernatural intelligence.

Intelligent design proponents say the eyes of vertebrates--including humans and the common snapping turtle seen above--could not have evolved in a stepwise fashion. That's because the eye is made of several interacting parts, and the removal of any one part will cause the entire system to cease functioning. Thus, the argument goes, the eye must have been produced in one fell