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Genetic variation has two sources: Mutation generates new variation, and recombination followed by segregation of homologous chromosomes during meiotic cell division shuffles mutations to create new combinations. In both cases, new alleles are formed, as shown in Fig. 21.2.

As we saw in Chapter 14, mutations can be somatic, occurring in the body’s tissues, or germ-line, occurring in the reproductive cells and therefore passed on to the next generation. From an evolutionary viewpoint, we are primarily interested in germ-line mutations. A somatic mutation affects only the cells descended from the one cell in which the mutation originally arose, and thus affects only that one individual. However, a germ-line mutation appears in every cell of an individual derived from the fertilization involving the mutation-bearing gamete, and thus appears in its descendants.

Mutations can also be classified by their effects on an organism (Chapter 15). Mutations occur randomly throughout the genome, and, because most of the genome consists of noncoding DNA, most mutations are neutral, having little or no effect on the organism. Most mutations that do occur in protein-coding regions of the genome, however, have a deleterious, or harmful, effect on an organism. Rarely, a mutation occurs that has a beneficial effect. Mutations like these are advantageous if they improve their carriers’ chances of survival or reproduction. Advantageous mutations, as we will see, can increase in frequency in a population until eventually they are carried by every member of a species. These mutations are the ones that result in a species that is adapted to its environment—better able to survive and reproduce in that environment.