Evolution is genetic change that takes place among members of a population over time. Several different forces are potentially capable of bringing about evolution, and we will explore those forces and the process of evolution more fully in Chapter 18. Here, we consider how one of those forces—natural selection—can bring about genetic change in a quantitative characteristic.

Charles Darwin proposed the idea of natural selection in his book On the Origin of Species in 1859. Natural selection arises through the differential reproduction of individuals with different genotypes. Because of the genes they possess, some individuals produce more offspring than others. The more successful reproducers give rise to more offspring, which inherit the genes that confer a reproductive advantage. Thus, the frequencies of the genes that confer a reproductive advantage increase with the passage of time, and the population evolves. Natural selection is among the most important of the forces that bring about evolutionary change. Through natural selection, organisms become genetically suited to their environments; as environments change, groups of organisms change in ways that make them better able to survive and reproduce.

For thousands of years, humans have practiced a form of selection by promoting the reproduction of organisms with traits perceived as desirable. This form of selection, called artificial selection, has produced the domestic plants and animals that make modern agriculture possible.