The Maintenance of Genetic Variation

Abstract

For evolution to proceed according to the rules outlined in the previous CHAPTERs, there must be additive genetic variation. As shown in CHAPTER 4, directional selection tends to erode genetic variation, although it is perhaps unreasonable to expect selection to be continually acting in one direction. If selection is not acting continuously in one direction, is there a most fit genotype? If so, why is it that selection does not act to eliminate all deviant genotypes? This question is the focus of the present CHAPTER: Specifically, what mechanisms have been proposed that would prevent the erosion of additive genetic variance, and what is the empirical evidence for their presence? The first section examines stabilizing selection, a type of selection that can clearly maintain a phenotype within narrow bounds and thought to frequently act in natural populations. Does stabilizing selection maintain genetic variation or lead to its erosion until only a single optimal genotype remains? Disruptive selection maintains phenotypic optima by an entirely opposite mode of action: Does this lead to the opposite result from stabilizing selection?