evolution

1. The process by which species of organisms arise from earlier life forms and undergo change over time through natural selection. The modern understanding of the origins of species is based on the theories of Charles Darwin combined with a modern knowledge of genetics based on the work of Gregor Mendel. Darwin observed there is a certain amount of variation of traits or characteristics among the different individuals belonging to a population. Some of these traits confer fitness—they allow the individual organism that possesses them to survive in their environment better than other individuals who do not possess them and to leave more offspring. The offspring then inherit the beneficial traits, and over time the adaptive trait spreads through the population. In twentieth century, the development of the the science of genetics helped explain the origin of the variation of the traits between individual organisms and the way in which they are passed from generation to generation. This basic model of evolution has since been further refined, and the role of genetic drift and sexual selection in the evolution of populations has been recognized. See also natural selection, sexual selection. See Notes at adaptation, Darwin.

2. A process of development and change from one state to another, as of the universe in its development through time.

Our Living Language  : Darwin's theory of evolution by natural selection assumed that tiny adaptations occur in organisms constantly over millions of years. Gradually, a new species develops that is distinct from its ancestors. In the 1970s, however, biologists Niles Eldridge and Stephen Jay Gould proposed that evolution by natural selection may not have been such a smooth and consistent process. Based on fossils from around the world that showed the abrupt appearance of new species, Eldridge and Gould suggested that evolution is better described through punctuated equilibrium. That is, for long periods of time species remain virtually unchanged, not even gradually adapting. They are in equilibrium, in balance with the environment. But when confronted with environmental challenges—sudden climate change, for example—organisms adapt quite quickly, perhaps in only a few thousand years. These active periods are punctuations, after which a new equilibrium exists and species remain stable until the next punctuation.