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evolution ev·o·lu·tion (ěv'ə-lōō'shən, ē'və-)
A continuing process of change from one state or condition to another or from one form to another.
The theory that groups of organisms change with passage of time, mainly as a result of natural selection, so that descendants differ morphologically and physiologically from their ancestors.
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.
A theory first proposed in the nineteenth century by Charles Darwin, according to which the Earth's species have changed and diversified through time under the influence of natural selection. Life on Earth is thought to have evolved in three stages. First came chemical evolution, in which organic molecules were formed. This was followed by the development of single cells capable of reproducing themselves. This stage led to the development of complex organisms capable of sexual reproduction. Evolution is generally accepted as fact by scientists today, although debates continue over the precise mechanisms involved in the process. (See mutation, punctuated equilibrium, and creation science.)
Note: The first cell is thought to have been formed when the Earth was less than a billion years old.