Evolution by natural selection was first demonstrated by the observation that more offspring are often produced than can possibly survive. The theory of evolution by natural selection was conceived independently by Charles Darwin and Alfred Russel Wallace in the mid-19th century and was set out in detail in Darwin's book On the Origin of Species. It is this process of evolution that has given rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms and molecules. The evolutionary pressures that determine whether a characteristic would be common or rare within a population constantly change, resulting in a change in heritable characteristics arising over successive generations. Evolution occurs when evolutionary processes such as natural selection (including sexual selection) and genetic drift act on this variation, resulting in certain characteristics becoming more common or rare within a population. Different characteristics tend to exist within any given population as a result of mutation, genetic recombination and other sources of genetic variation. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction.
Evolution is change in the heritable characteristics of biological populations over successive generations.
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