At the molecular level, a mutation is a change in the nucleotide sequence of a genome. Such changes can be small, affecting one or a few bases, or large, affecting entire chromosomes. We begin by considering the origin and effects of small-scale changes to the DNA sequence. While mutation provides the raw material that allows evolution to take place, it can play this role only because of an important feature of living systems: Once a mutation has taken place in a gene, the mutant genome is replicated as faithfully as the nonmutant genome.