Increasing Selection Response
A breeder can increase the expected genetic gain either by increasing the heritability or by increasing the selection differential. One approach to increasing heritability is reducing the environmental variance. This reduces the phenotypic variance, but the genetic variance is unchanged; thus, the heritability is increased. In a prior section of this lesson, the concept of blocking was introduced as a way of controlling environmental variance. Another approach to increasing heritability is to choose or create a population that is extremely variable (i.e., large genetic variance). However, if genetic variance of the population is increased at the expense of decreasing the average performance of the population, then nothing is gained.
A breeder can increase the selection differential by selecting fewer individuals. Consider the following grain weights (in grams) of individual plants from a population:
If the 10 best plants are selected, then the selection differential equals 38.2 grams; if only the top five are selected, then the selection differential increases to 50.4 grams. However, increasing the selection differential by selecting fewer individuals has a major drawback. This issue is explored in the next section.