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... evolution is not only a theory, it's also a fact. The key to understanding evolution is to recognize how it can be both:
As a fact: Evolution is simply genetic changes occurring through time in a group of individuals (a population, a species, and so on). Scientists know that these changes occur. They can see the changes, measure them; in many instances, figure out when they happened.
As a theory: Evolutionary theory seeks to explain what's responsible for the evolutionary process - in other words, what causes these changes. What scientists know today is that natural selection and genetic drift are two key forces driving these changes."
--- Evolution For Dummies, page 335-336, by Greg Krukonis, PhD, Tracy Barr,