Evolution is both a scientific fact and a scientific theory. Evolution is a fact in the sense that life has changed through time. In nature today, the characteristics of species are changing, and new species are arising. The fossil record is the primary factual evidence for evolution in times past, and evolution is well documented by further evidence from other scientific disciplines, including comparative anatomy, biogeography, genetics, molecular biology, and studies of viral and bacterial diseases.
Evolution is also a theory — an explanation for the observed changes in life through Earth history that has been tested numerous times and repeatedly confirmed. Evolution is an elegant theory that explains the history of life through geologic time; the diversity of living organisms, including their genetic, molecular, and physical similarities and differences; and the geographic distribution of organisms. Evolutionary principles are the foundation of all basic and applied biology and paleontology, from biodiversity studies to studies on the control of emerging diseases.
Because evolution is fundamental to understanding both living and extinct organisms, it must be taught in public school science classes. In contrast, creationism is religion rather than science, as ruled in recent court cases, because it invokes supernatural explanations that cannot be tested. Consequently, creationism in any form (including "scientific creationism," "creation science," and "intelligent design" must be excluded from public school science classes. Because science involves testing hypotheses, scientific explanations are restricted to natural causes.
This difference between science and religion does not mean that the two fields are incompatible. Many scientists who study evolution are religious, and many religious denominations have issued statements supporting evolution. Science and religion address different questions and employ different ways of knowing.
The evolution paradigm has withstood nearly 150 years of scrutiny. Although the existence of evolution has been confirmed many times, as a science evolutionary theory must continue to be open to testing. At this time, however, more fruitful inquiries address the tempo and mode of evolution, various processes involved in evolution, and driving factors for evolution. Through such inquiry, the unifying theory of evolution will become an even more powerful explanation for the history of life on Earth.