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Chapter 9 Maintenance of Genetic Diversity Levels of genetic diversity result from the joint impacts of: Mutation & migration adding variation Chance & directional selection removing variation Balancing selection impeding its loss The balance between these factors depends strongly on population size and differs across characters. Conservation biologists need to understand how genetic diversity is maintained through natural processes if conservation programs are to be designed for its maintenance in managed populations. Maintenance of extensive genetic diversity in natural populations is one of the most important, largely unresolved, questions of evolutionary genetics. The balance of forces maintaining genetic diversity differs between large and small populations. Selection has a major impact in large populations. However, its impacts are greatly reduced in small populations where genetic drift has an increasingly important role. Five major points about genetic diversity in small populations: Genetic drift fixes alleles more rapidly in smaller populations. Loci subjected to weak selection in larger populations approach effectively neutral in small populations. Mutation-selection equilibria are lower in smaller than larger populations. The effects of balanced polymorphisms depends upon the equilibrium frequency; the frequency of fixation of intermediate frequency alleles is retarded, but balancing selection accelerates fixation of low frequency alleles. Balancing selection can retard loss of genetic diversity, but it does not prevent it in small populations. The consequence of these effects is that genetic diversity in small populations is lower for both neutral alleles and those subjected to balancing selection