How does natural selection explain goo-to-you evolution?

Why is natural selection, a principle recognized by creationists, taught as ‘evolution’, AS IF it explains the origin of the diversity of life? By definition it is a selective process (selecting from already existing information), so is not a creative process. It might explain why certain genes benefit creatures more in certain environments, but not where the genes and creatures came from in the first place. The death of individuals not adapted to an environment and the survival of those that are suited does not explain the origin of the traits that make an organism adapted to an environment. E.g., how do minor back-and-forth variations in finch beaks explain the origin of beaks or finches?