Secondly, mutations must not occur or else mutational equilibrium must exist. Mutational equilibrium is the state in which the number of genes mutating, per unit time, from a more common allele to a less common allele matches exactly the number of genes mutating, per unit time, from a less common to a more common allele. Because mutations are always occurring and because mutational equilibrium rarely exists, mutation pressure on the gene pool always exists. More stable alleles increase in frequency while more mutable alleles decrease in frequency. Yet, although mutations increase variability and, thus, are the raw material of evolution, mutation pressure is slow, at random, and, thus, rarely a significant fact in directing the nature of evolutionary change.
Thirdly, there must not be any immigration or emigration of genes into or out of the population since the immigration or emigration of genes, a condition called gene flow, can result in the introduction or loss of alleles to the gene pool.
