A mutation is a change in the base sequence of DNA. In a substitution, one base is replaced by another. In an insertion, an extra base is added. In a deletion, a base is lost. Because DNA is read in triplets (groups of three bases), insertions and deletions shift every triplet after them.

Mutations happen randomly when DNA is copied, but mutagens such as UV light, X-rays and some chemicals make them more likely. Most mutations are neutral, some are harmful and a few are beneficial.

The theory of evolution by natural selection: individuals in a population show variation. More offspring are produced than can survive, so they compete. Individuals with characteristics best suited to the environment are more likely to survive and reproduce, passing on their alleles. Over many generations, favourable characteristics become more common.

A species of moth has light and dark forms, controlled by a gene. Birds eat moths that they can see resting on tree trunks. In areas where soot from factories made tree bark dark, dark moths were better camouflaged.