 Our discussion in this section has focused on deleterious mutations, i.e., ones which reduce the fitness of their host organism. This may seem odd, given that beneficial mutations play a key role in the evolutionary process. The reason is that in population genetics, a major concern is to understand the causes of the genetic variability found in biological populations. If a gene is beneficial, natural selection is likely to be the major determinant of its equilibrium frequency; the rate of sporadic mutation to that gene will play at most a minor role. It is only where a gene is deleterious that mutation plays a major role in maintaining it in a population.