Biological evolution is the change in a population's inherited traits from generation to generation. These traits are encoded as 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 inheritable differences (genetic variation) between organisms. Evolution occurs when these differences become more common or rare in a population. This either happens through natural selection, which is caused by differences in the reproductive value of the traits, or randomly through genetic drift.