Yes, it is true that evolution is a theory, which is a well substantiated explanation for observable facts.
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction.
Different characteristics tend to exist within any given population as a result of mutation, genetic recombination and other sources of genetic variation.
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.
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. 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.
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