 It is essential to understand that biologists recognize many ways that evolution can occur, evolution by  natural selection  being just one of them, although it is often held to be the most prevalent one. Evolution can also occur through genetic drift, mutation, or migration. It can also occur through sexual selection, which some consider to be a form of natural selection and others consider to be distinct from natural selection (the latter having been Darwin’s 1859, 1874 view). Evolutionary theory, then, can be taken to be the study (including, but not limited to, mathematical models) of these and other modes of evolution.