TABLE 18.2 Effects of different evolutionary forces on allelic frequencies within populations

Force	Short-term effect	Long-term effect
Mutation	Change in allelic frequencies	Equilibrium reached between forward and reverse mutations
Migration	Change in allelic frequencies	Equilibrium reached when allelic frequencies of source and recipient population are equal
Genetic drift	Change in allelic frequencies	Fixation of one allele
Natural selection	Change in allelic frequencies	Directional selection: fixation of one allele Overdominant selection: equilibrium reached Underdominant selection: unstable equilibrium