Mutations are the ultimate source of genetic variation. A mutation is a random change in a DNA sequence, usually occurring because of errors in replication or repair processes. Most often, mutation gives rise to variations that are neither good nor bad, they are just different and are (at least for the time being) neutral. For instance, for most organisms, large parts of the genome constitutes of non-coding DNA, where single mutations have no affect what so ever. Just occasionally mutations generate new variations of alleles that can give an individual a fitness advantage; they are adaptive.