Biological evolution, defined as genetic change in species over time, is an observable fact. It is a fact that insects evolve resistance to pesticides, that new diseases arise when viruses evolve the ability to invade new hosts, and that humans have created new species using the same mechanisms that produce species naturally. Furthermore, the evidence based on facts from molecular biology and geology (i.e. gene sequences, dated fossils) clearly indicates that all living species, including our own, share a common ancestor, which is over 3 billion years old.
The theory of evolution explains the mechanisms (e.g. non-random natural selection acting on random mutation) by which organisms change over time (microevolution), become more complex, and diversify into new species (macroevolution). Evolution is the central unifying theory of biology, supported by independent evidence from paleontology, geology, genetics, molecular biology and genomics, developmental biology, biogeography and behavioral ecology. Even though new information from nearly every field of science has been applied, attempts to falsify evolutionary theory using the scientific method have failed. As is true for any active science, the details of the theory are continually debated as new data are collected. However, there is no controversy in the scientific community about the fact of evolution.
Although in popular speech the word 'theory' means 'a guess', in science 'theory' refers to an explanation so well supported by facts that it is as close to the truth as science can come. Although even the most successful theory can never be proven, any scientific theory can be refuted by facts that are at odds with its predictions. In fact, the most useful theories are those that generate many testable predictions and thus leave themselves particularly susceptible to being proven wrong. It is this quality that most distinguishes a scientific concept from a non-scientific one.
In science, not all explanations are equal. By the rigorous criteria of science, supernatural mechanisms, including Intelligent Design creationism, are not scientific because they do not generate testable predictions about how species change or diversify. To argue that supernatural explanations merit discussion in science classrooms so that 'both sides' of the issue are taught is to advocate that non-science be legitimized as science. In an era where scientific solutions to complex problems are of first priority, this is dangerous logic.
We thus oppose any attempt to weaken scientific standards with respect to evolution, or to broaden the science curriculum to include the supernatural. In this, we stand with our colleagues in the National Academy of Sciences, the American Association for the Advancement of Science, and other scientific organizations. We urge all citizens to learn about science and work to assure that our children receive a first-class science education.