Factors of evolution observed today, such as natural selection, migration, and the origin and extinction of species, have operated throughout the history of life, but the characteristic rates of these processes have varied greatly over time and from one group of organisms to another. Because of this variation, evolutionary patterns seen in the fossil record are highly episodic and irregular. Paleontological studies of biological diversity seek to document this nonuniformity and to understand it in the context of geological, environmental, and ecological change.