Mutation

How this is marked

The paying word is 'random' (mutations are spontaneous and not directed toward a need). Examiners reward linking mutation to the origin of new alleles, which is the source of genetic variation acted on by natural selection. Mention that ionising radiation and some chemicals increase the rate of mutation. A common error is saying mutations are 'always harmful'; many are neutral and some are advantageous.

In an exam answer

On the source of new variation: 'A new allele for antibiotic resistance arose by random mutation, and bacteria with this allele survived and reproduced.'

Full topic guide: Variation & selection