Natural selection is the process by which evolution produces adaptation. Other processes (such as genetic drift) cause evolution to occur, but in a random direction. Only natural selection guides the evolutionary process in a direction that produces adaptation to environmental and social conditions. Natural selection is the process that was first clearly explained by Charles Darwin, although some earlier writers such as W. C. Wells, Edward Blyth, and Patrick Mathew had presented fragments of the idea. Natural selection was independently discovered by Alfred Russel Wallace.

Darwin not only demonstrated that evolution had occurred but also explained how it occurred. Evolution became believable for the first time. Darwin's contemporaries largely accepted his demonstration that evolution had occurred, but the scientific community did not embrace natural selection for another 80 years.

Sexual selection, which was also first elucidated by Charles Darwin, operates in a manner similar to natural selection. Many scientists consider it a subset of natural selection. However, sexual selection usually produces arbitrary and sometimes outlandish characteristics that are not usually considered adaptation.

Both Darwin and Wallace alluded to artificial selection, the process by which humans breed crops and livestock in desired directions, in crafting the phrase natural selection. In both cases, some individuals are selected to propagate, and others are not selected to propagate, only in natural selection it is nature rather than humans that is doing the selecting. Both artificial and natural selection can produce genetic changes within populations and can cause divergence of populations into different varieties. Either could eventually cause varieties to be different enough that they would constitute different species. Artificial selection, however, has not gone on long enough for this to happen.

Natural selection occurs within populations. Individual organisms cannot evolve because they cannot undergo whole-body genetic changes. If mutations occur in somatic cells, which make up most of the body of an animal, the cells that contain the mutations will die when the organism dies. Only those mutations that occur in germ line cells, either eggs or sperm or the cells that produce them, can be passed on to the next generation. Therefore the genetic effects of evolution can only be observed in the next generation of the population. . .

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