Evolution is the process by which organisms change over time. Mutations produce genetic variation in populations, and the environment interacts with this variation to select those individuals best adapted to their surroundings. The best-adapted individuals leave behind more offspring than less well-adapted individuals. Given enough time, one species may evolve into many others.
Evolution, as Darwin described it in "The Origin of Species" in 1859, had several components; three to be exact. First of all, evolution proposes that all organisms are descended from a common ancestor, or at least a very small set of common ancestors--still some debate about that, but probably a single common ancestor. Evolution says that there is gradual change over time, which we know is due to mutations in DNA. And then third, and very important, natural selection operates upon, over those long time periods, the changes that occur, to result in emergence of species that have particular abilities to survive in a niche that the environment has provided. So put those three things together...you've got evolution.- Francis S. Collins, M.D., Ph.D.