Mutations are variations in the nucleotide sequence of a genome. They can occur because of errors during DNA replication or damage to DNA.

Mutations caused by DNA damage may be of two types: induced or spontaneous. Induced mutations result from exposure to mutagenic chemicals, UV rays, x-rays, or some other environmental agent. Spontaneous mutations occur without any exposure to environmental agents; they are a result of natural reactions taking place within the body.

Mutation is the ultimate source of new alleles, or new genetic variation in any population. Only mutations in gametes or gamete-producing cells (germ cells) can be passed on to offspring. However, there is also emerging evidence that some elements of the lifestyle of an organism can be passed on through epigenetics.

The genetic changes caused by mutation can have one of three outcomes on the phenotype:

  • The mutation reduces fitness — causes a lower likelihood of survival or fewer offspring.

  • The mutation increases fitness.

  • The mutation has no effect on the fitness. These are called neutral mutations.

The scale of such effects can vary. Impact on fitness also depends on the environment the organism lives in.

Mutations can also cause diseases in an organism, such as cancer.