M-S Balance - Background
Typically, selection and mutation are opposing forces. The reason for this is that most mutations are disadvantageous. Thus, selection acts to rid the population of these alleles; The strength of this selection is usually denoted by s, the selection coefficient. Mutation occurs at a relatively constant rate, , and acts to increase the frequency of the mutant allele in the population. Mutation rates are typically between and .
Because were talking about deleterious alleles here, selection invokes a negative change in q = f(a), whereas mutation invokes a positive change. When these two forces come into balance with each other, q comes to an equilibrium value (no change in q).