Mutations and reversions are events that change the expression of traits of an organism. They change the organism's phenotype. The change can result from a change in a base--a constituent of the deoxyribonucleic acid--or a change in one of the amino acid constituents of the ribonucleic acid that is used as a blueprint to make a protein.

Each time DNA replicates, mistakes can occur. If the mutation is deleterious, the organism does not survive. But, if the mutation changes the organism's phenotype in a nonlethal fashion, the mutation can persist. The accuracy of DNA replication, the occurrence of damage to the DNA, and the effectiveness of mechanisms for DNA repair determine the rate at which mutations occur. Bacteria and other organisms exhibit natural mutation and reversion rates, where mutations and reversions can occur spontaneously.

Salvador Luria and Max Delbrück provided...