Mutations result from mistakes in DNA replication or from unrepaired damage to DNA. The damage may be caused by reactive molecules produced in the normal course of metabolism, by chemicals in the environment, or by radiation of various types, including X-rays and ultraviolet light. Most genomes also contain DNA sequences that can “jump” from one position to another in the genome, and their insertion into or near genes is a source of mutation. Yet another source of mutation is incorrectly repaired chromosome breaks caused by reactive chemicals or radiation.

Most mutations are spontaneous, occurring by chance in the absence of any assignable cause. They occur randomly, unconnected to an organism’s needs—it makes no difference whether or not a given mutation would benefit the organism. Whether a favorable mutation does or does not occur is purely a matter of chance. This key principle, that mutations are spontaneous and random, is the focus of this section.