The development of cumulative changes in the characteristics of populations of organisms from each generation to the next. Evolution occurs by mutations in the structure of the genetic material, and the passing on of these changes from ancestor to descendant. Evolution is a fact; it is the way in which it occurs that is a theory.
See:
Darwinism,
neo-Darwinism,
natural selection,
gradualist evolution,
punctuated equilibrium,
biochemical evolution,
macroevolution,
microevolution,
convergent evolution,
divergent evolution,
parallel evolution.
Evolution is as much a fact as the earth turning on its axis and going around the sun. At one time this was called the Copernican theory; but, when evidence for a theory becomes so overwhelming that no informed person can doubt it, it is customary for scientists to call it a fact. That all present life descended from earlier forms, over vast stretches of geologic time, is as firmly established as Copernican cosmology. Biologists differ
only with respect to theories about how the process operates. - Martin Gardner