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Because the selected individuals do not reproduce. The loss of fitness may be a direct phenotypic effect of the genes for the selected character, in which case nothing much can be done to improve the population further. Often, however, the loss of fitness is tied not to the genes that are under selection but to linked sterility genes that are carried along with them. In such cases, a number of generations are allowed to breed without selection until recombinants form by chance, freeing the genes under selection from their association with sterility. Selection can then be continued, as in the upwardly selected line in Figure 20-13.

We must be very careful in our interpretation of long-term agricultural selection programs. In the real world of agriculture, changes in cultivation methods, machinery, fertilizers, insecticides, herbicides, and so forth, are taking place along with the production of genetically improved varieties. Increases in average yields are consequences of all of these changes. For example, the average yield of corn in the United States increased from 40 bushels to 80 bushels per acre between 1940 and 1970. But experiments comparing old and new varieties of corn in common environments show that only about half this increase is a direct result of new corn varieties (the other half being a result of improved farming techniques). Furthermore, the new varieties are superior to the old ones only at the high densities of modern planting for which they were selected.

The use of h 2 in breeding Even though h2 is a number that applies only to a particular population and a given set of environments, it is still of great practical importance to breeders. A poultry geneticist interested in increasing, say, the growth rate of chickens is not concerned with the genetic variance over all possible flocks and all environmental distributions. Given a particular flock (or a choice between a few particular flocks) under the environmental conditions approximating present husbandry practice, the question becomes, Can a selection scheme be devised to increase growth rate and, if so, how rapidly can it be increased? If one flock has a lot of genetic variance for growth rate and another only a little, the breeder will choose the former flock to carry out selection. If the heritability in the chosen flock is very high, then the mean of the population will respond quickly to the selection imposed, because most of the superiority of the selected parents will appear in the offspring. The higher the h2 is, the higher the parent-offspring correlation is.

If, on the other hand, h2 is low, then only a small fraction of the superiority of the selected parents will appear in the next generation. If h2 is very low, some alternative scheme of selection or husbandry may be needed. In this case, H2 together with h2 can be of use to the breeder. Suppose that h2 and H2 are both low, which means that there is a large proportion of environmental variance compared with genetic variance. Some scheme of reducing must be used. One method is to change the husbandry conditions so that environmental variance is lowered. Another is to use family selection. Rather than selecting the best individuals, the breeder allows pairs to produce several trial progeny, and parental pairs are selected to produce the next generation on the basis of the average performance of those progeny. Averaging over progeny allows uncontrolled environmental variation and developmental noise to be cancelled out, and a better estimate of the genotypic difference between pairs can be made so that the best pairs can be chosen as parents of the next generation.

If, on the other hand, h2 is low but H2 is high, then there is not much environmental variance. The low h2 is the result of a small proportion of additive genetic variance compared with dominance variance. Such a situation calls for special breeding schemes that make use of a nonadditive variance. One such scheme is the hybrid–inbred method, which is used almost universally for corn. A large number of inbred lines are created by selfing. These inbred lines are then crossed in many different combinations (all possible combinations, if it is economically feasible), and the cross that gives the best hybrid is chosen. 

Then new inbred lines are developed from this best hybrid, and again crosses are made to find the best hybrid cross. This process is continued cycle after cycle. This scheme selects not only additive effects but also dominance effects because it selects the best heterozygotes as parents for the next cycle; it has been the basis of major genetic advances in hybrid maize yield in North America since 1930. Yield in corn does not appear to have large amounts of non-additive genetic variance, however, and so it is debatable whether this technique ultimately produces higher-yielding varieties than those that would have resulted from years of simple selection techniques based on additive variance.