It has been my experience as both a college professor and a longtime observer of the creationism/evolution controversy that most people define evolution rather differently than do scientists. To the question, What does evolution mean? most people will answer, "Man evolved from monkeys," or invoke a slogan like "molecules to man." Setting aside the sex-specific language (surely no one believes that only males evolved; reproduction is challenging enough without trying to do it using only one sex), both definitions are much too narrow. Evolution involves far more than just human beings and, for that matter, far more than just living things.
The broad definition of evolution is a cumulative change through time. Not just any change counts as evolution, however. The Earth changes in position around the sun, but this is not evolution; an insect changes from egg to larva to adult during metamorphosis, but this is not evolution. An individual person (or a star) is born, matures, and dies but does not evolve. Evolution in this broad sense refers to the cumulative, or additive, changes that take place in phenomena like galaxies, planets, or species of animals and plants. It refers to changes that take place in groups rather than in individuals and to changes that accumulate over time.
Think of evolution as a statement about history. If we were able to go back in time, we would find different galaxies and planets, and different forms of life on Earth. Galaxies, planets, and living things have changed through time. There is astronomical evolution, geological evolution, and biological evolution. Evolution, far from the mere "man evolved from monkeys," is thus integral to astronomy, geology, and biology. As we will see, it is relevant to physics and chemistry as well.
Evolution needs to be defined more narrowly within each scientific discipline because both the phenomena studied and the processes and mechanisms of cosmological, geological, and biological evolution are different. Astronomical evolution deals with cosmology: the origin of elements, stars, galaxies, and planets. Geological evolution is concerned with the evolution of our own planet: its origin and its cumulative changes through time. Mechanisms of astronomical and geological evolution involve the laws and principles of physics and chemistry: thermodynamics, heat, cold, expansion, contraction, erosion, sedimentation, and the like. In biology, evolution is the inference that living things share common ancestors and have, in Darwin's words, "descended with modification" from these ancestors. The main—but not the only—mechanism of biological evolution is natural selection. Although biological evolution is the most contentious aspect of the teaching of evolution in public schools, some creationists raise objections to astronomical and geological evolution as well.
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