"Evolution" refers both to a set of scientific facts and to a theory explaining such facts. "Evolution" refers to the scientific fact that biological organisms have changed through time, and that all life, including humanity, has descended with modification from common ancestors. Evolution is as well documented as are other currently accepted scientific facts. The theory of evolution is a comprehensive and well-established scientific explanation, based on natural processes, of the fact of biological evolution.
Evolutionary theory should be taught in public schools because it is one of the most important scientific theories ever generated, and because it is the accepted scientific explanation for the diversity of life. As a scientific theory, it is testable and has been extensively tested. As stated by the great geneticist and evolutionist Theodosius Dobzhansky, "Nothing in biology makes sense except in the light of evolution." The theory of evolution is subject to refinements and revisions, but this is no different from any other major scientific theory, such as the those providing the explanatory frameworks of geology, physics, or chemistry. There is no pedagogical or scientific reason to treat evolutionary theory any differently than any other well-accepted scientific theory, and it should be taught in public schools as the firmly established, accepted unifying scientific principle that it is.