There are two components to what is often loosely referred to as ``genetic diversity:'' additive genetic variance (the portion of the genetic differences among individuals that can respond to selection) and allelic diversity (the number of different types of alleles present at any locus).
Even in the most extreme situation imaginable, a population reduced to one hermaphroditic individual for one generation, at most 50% of the additive genetic variance in quantiative traits will be lost if the population rebounds quickly to a large population size (Figure 3). A less extreme case, 2 males and 2 females would lead to only a 12% reduction in additive genetic variance. Moreover, recessive lethal and deleterious alleles will retard the loss of variance at other loci, especially those to which they are linked. Several generations of greatly reduced population size are necessary to significantly deplete quantitative genetic variation.
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The variety of alleles present, however, is greatly affected by population bottlenecks. Rare alleles are particularly susceptible to loss.
Population bottlenecks have little effect on variance, but may have a dramatic effect on diversity.