Biology as a science made its move from an Arisotitlean stage to a Newtonian one with the development of the theory of evolution.
Evolution is a change in the gene pool of a population over time.
The English moth, Biston betularia , is a frequently cited example of observed evolution. Then, the frequency of the dark morph began to increase. The moth population changed from mostly light colored moths to mostly dark colored moths. The moths' color was primarily determined by a single gene. So, the change in frequency of dark colored moths represented a change in the gene pool.