The evolutionary pressures that determine whether a characteristic is 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 more rare within a population. Variation tends to exist within any given population as a result of genetic mutation and recombination. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Evolution is the change in heritable characteristics of biological populations over successive generations.
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