Directional Selection

Abstract

There are basically three types of selection: directional, stabilizing, and disruptive (Fig. 4.1). Of these, the first is the most important from the perspective of changes in the mean value of a quantitative trait (the other two types are discussed in CHAPTER 9). The term directional selection applies to selection in which the mean value of the parents which contribute to the next generation differs from that of the population from which the parents are drawn. The prediction of response to directional selection can be derived using the theoretical framework developed in CHAPTER 2. However, it is necessary to distinguish short-term response to selection from the long-term response, the former requiring fewer assumptions. In this chapter, I consider both types of responses with respect to a single trait. The consequences of the existence of genetic correlations between traits is explored in chapter 5.