Download Chapter 9 Maintenance of Genetic Diversity

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