Darwin described evolution as "descent with modification": the change in a lineage of populations between generations.
The theory of evolution has four main components:
• Population genetics provides the fundamental theory of the subject. If we know how any property of life is controlled genetically, population genetics can be applied to it directly.
• The theory of adaptation; how features of an organism evolve within its environment.
• The diversity of life; what a species is, how new species originate, and how to classify and reconstruct the history of life.
• Evolution on a grand scale; the fossil record is the main testing ground for large evolutionary events, which occur over the geologic time scale of tens or hundreds of millions of years.
John Maynard Smith gives his own definition of evolution.
Figure: the theory of evolution suggests that the history of life has proceeded as a branching tree.