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In biology, evolution is the change in a population's inherited traits from generation to generation. These traits are the expression of genes that are copied and passed on to offspring during reproduction. Mutations and other random changes in these genes can produce new or altered traits, resulting in heritable differences (genetic variation) between organisms. New traits can also come from transfer of genes between populations, as in horizontal gene transfer or breeding between species. Evolution occurs when these differences become more common or rare in a population, either randomly through genetic drift or nonrandomly through natural selection.