A talk for the San Francisco Atheists' “Evolutionpalooza 2009” Darwin Day celebration. In 1859, Charles Darwin convincingly presented “transmutation” of species to other scientists and the public, and suggested that the major mechanism of that process was his theory of natural selection. Since his time, evolution, the inference that living things have descended with modification from common ancestors, has been supported by every development in the biological sciences, from the discovery of Mendelian to molecular genetics. Evolution is the key to understanding virtually every area in biology from biochemistry, to cell biology, to organismic biology, to population biology, to ecology. Evolution is the glue that holds biology together as a coherent science, and tells us why biology is the way it is.