Natural selection<span> acts upon two major sources of genetic variation: </span>mutations<span> and</span>recombination<span> of </span>genes<span> through sexual </span>reproduction<span>. Most </span>mutations<span> do not affect the reproductive fitness of individuals -- some may be beneficial, some may be harmful, and </span>many<span> may be neutral. </span>Mutation rates<span> per gene are generally low</span>
