David, this was interesting.

"By far his most common example is that of a quantitative trait controlled by several loci where the selective optimum for the trait is at an intermediate value, i.e. neither the highest nor the lowest that can be produced by the various possible combinations of alleles. In this situation it is likely that the optimum intermediate value of the trait can be produced by different allele combinations. The effect of an allele on fitness (not necessarily on the quantitative trait itself) is epistatic, i.e. dependent on the combination of other genes in the genotype. Which of the relevant alleles are favoured by selection may then depend on the accident of which allele at a locus happens to be most frequent when selection begins, with all other alleles at the locus being driven to extinction."

The above example is particularly interesting. Consider gene expression controlled by trans-acting transcription elements. Being short, such regulatory elements should remain functional long after a duplication event. Thus many functional copies could be spread around the genome. The total number of active copies would be maintainded by selection but no individual copy would be preserved. This mechanism would tend to preserve diversity in gene expression and support fast adaptation to new environments.

If the population consisted of many small tribes with limited gene flow then drift would favor "robust combinations", i.e., trait values would be kept near the optimum even when the genetic background shifted due to drift. If so, long term evolutionary pressure should produce modularization of traits that needed to vary with the environment, e.g., changing tooth shape to match diet without harming other traits.