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A series of primate skulls.

Photo © Christopher Walsh / Harvard Medical School.
Definition: Evolution is change over time. Under this broad definition, evolution can refer to a variety of changes that occur over time—the uplifting of mountains, the erosion of riverbeds, or the creation of new species. The term biological evolution is a more specific type of evolution.

Biological evolution refers to the changes over time that occur in living organisms. Biological evolution is defined as descent with modification.

Biological evolution occurs at different scales. These include small-scale evolution and broad-scale evolution. Small-scale evolution, also referred to as microevolution, is the change in gene frequencies within a population of organisms changes from one generation to the next. Broad-scale evolution, also referred to as macroevolution, refers to evolution at a grander scale. It focuses on the progression of species or entire clades from a common ancestor to descendent clades over the course of numerous generations.

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