Charles Darwin's theory of evolution as written in his book On the Origin of Species determined that

On the Origin of Species (or more completely, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life), published on 24 November 1859, is a work of scientific literature by Charles Darwin which is considered to be the foundation of evolutionary biology.

Darwin's Theory of Evolution basically determined that Natural Selection is a key force in the small changes that occur over time in a population due to various causes of genetic variation in the gene pool. When these small changes are measured over a course of many years, they become examples of evolution.