The Population Genetics of Selection

This chapter examines models of one- and two-locus selection in the absence of drift and mutation. Expressions for the per-generation rate of allele-frequency change and the expected time for a specified amount of change are developed for single-locus models, and their equilibrium structure is examined for those settings where selection retains more than one allele. The presence of selection-generated linkage disequilibrium greatly complicates the extension of single-locus results to two loci, and the chapter examines some of the resulting complications. Finally, it examines the nature of selection on a locus that underlies a trait under selection, and then uses this to develop the breeder's equation for the single-generation response in a trait under selection. One important result is that the loci for a trait under stabilizing selection experience fitness underdominance, and thus trait selection removes, rather than retains, genetic variation.