8.6: Mutation—a direct change in the DNA of an individual—is the ultimate source of all genetic variation.

In describing the first of four evolutionary mechanisms, it is helpful to keep in mind our precise definition of evolution: a change in the allele frequencies found in a population. Mutation is an alteration of the base-pair sequence of an individual’s DNA, and when this alteration occurs in the DNA that codes for a particular gene, the change in the DNA sequence may change that allele. (See Section 5-9 for a detailed discussion of the mutation process.) Say, for example, that a mutation changes one of a person’s two blue-eye alleles into a brown-eye allele. If this mutation occurs in the sperm-or egg-producing cells, it can be passed on to the next generation; the offspring may carry the brown-eye allele. When this happens, the proportion of blue-eye alleles in the population is slightly reduced, and the proportion of brown-eye alleles is slightly increased. Evolution has occurred.

When considering mutation as a mechanism of evolution, it is important to remember that the cells of a multicellular eukaryotic organism can be divided into two types: somatic cells and reproductive cells. Somatic cells—the cells forming the body of an organism—are not passed from parent to offspring. Only reproductive cells are. For that reason, only mutations that affect reproductive cells can be inherited, and so we consider only those mutations as altering the allele frequencies within a population. The causes of mutations, however, are the same for somatic and reproductive cells.

Mutations can occur spontaneously during the complex process of cell division and, as we saw in Chapter 5, can be induced by a variety of environmental factors—including radiation, such as X rays, and some chemicals. But although these factors can induce mutations and influence the rate at which mutations occur, they do not generally influence exactly which mutations occur. Thus, we tend to say that “mutations are random.” What this means, more precisely, is that (1) we cannot predict ahead of time which individuals will have which mutations, and (2) we cannot predict whether the consequences of a mutation will be benign, harmful, or useful. This doesn’t mean, however, that all mutations have an equal probability of occurring. They do not. Some mutations are far more likely than others.

Treatment of an agricultural pest with harmful chemicals, for example, might increase the number of mutations occurring in that pest population, but it does not increase the likelihood that a particular mutation is beneficial or detrimental.

In addition to being a mechanism of evolutionary change, mutation has another, more important, role relevant to all mechanisms of evolution, including natural selection: mutation is the ultimate source of genetic variation in a population. We saw that a mutation may lead to the conversion of one allele to another that is already found within the population, as in our blue eye and brown eye example. More importantly, though, a mutation may create a completely novel allele that codes for the production of a new protein (FIGURE 8-11). That is, a change in the base-pair sequence of a person’s DNA may cause the production of a gene product that has never existed before in the human population: instead of blue or brown eyes, the mutated gene might code for yellow or red eyes. If such a new allele occurs in the sperm- or egg-producing cells, and if it does not significantly reduce an individual’s “reproductive fitness” (which we’ll consider below), the new allele can be passed on to offspring and remain in the population. At some future time, the mutation might even confer higher fitness, in which case natural selection could lead to an increase in frequency of this allele in the population. For this reason, mutation is critical to natural selection: all variation—the raw material for natural selection—must initially come from mutation.

Despite this vital role in the generation of variation, however, nearly all mutations have either no impact on an organism’s fitness or a negative impact by causing early death or reducing the organism’s reproductive success. Mutations to a normally functioning allele that codes for a normally functioning protein typically result in either an allele that codes for the same protein or a new allele that codes for a non-functioning protein. The latter case almost inevitably reduces an organism’s fitness. For this reason, our bodies protect our sperm- or egg-producing DNA with a variety of built-in error-correction mechanisms. As a result, mutations are rare (in humans, on the order of 1 mutation in every 30 million base pairs in each generation).