British biological theorist Richard Dawkins is perhaps the world's best known atheist. He is certain that we have evolution to thank for life on earth, not a creator. Evolution is the topic of his new book, "The Greatest Show On Earth." Dawkins says the book is his "personal summary of the evidence that the 'theory' of evolution is actually a fact - as incontrovertible a fact as any in science." He joins Doug on Tuesday to discuss the evidence for evolution.