Evolution means change over time. In Biology, evolution refers to populations of organisms changing over time in response to changes in their environment. At the most basic level, biological evolution refers to measurable changes in allele frequencies within a population from one generation to the next. As these changes build up over time, they have been observed to result in the formation of new species and greater biological diversity. The observed formation of new species leads to the inference that all living things share common ancestry back through time. This inference has been confirmed through repeated scientific tests to such a degree that it is considered an established scientific fact.

When biologists refer to the "theory of evolution," they are usually referring to the proposed mechanisms by which evolutionary change has occurred over time, including natural selection and genetic drift. These mechanisms are tested and refined through laboratory and field experiments.