This is a slightly modified version of a post from January, 2007 which, in turn, is a moified version of an essay that appears here. An even earlier version is on the TalkOrigins Archive.One of the most respected evolutionary biologists has defined biological evolution as follows:
Biological (or organic) evolution is change in the properties of populations of organisms ..., over the course of generations. The development, or ontogeny, of an individual organism is not considered evolution: individual organisms do not evolve. The changes in populations that are considered evolutionary are those that are ‘heritable' via the genetic material from one generation to the next. Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportions of different forms of a gene within a population, such as the alleles that determine the different human blood types, to the alterations that led from the earliest organisms to dinosaurs, bees, snapdragons, and humans.
Douglas J. Futuyma (1998) Evolutionary Biology 3rd ed., Sinauer Associates Inc. Sunderland MA p.4Read more »