Book Viewer

Short Contents | Full Contents | Other books @ NCBI

Modern Genetic Analysis Chapter 17. Population and Evolutionary Genetics
Artificial Selection

In contrast with the difficulties of finding simple, well-behaved cases in nature that exemplify the simple formulas of natural selection, there is a vast record of the effectiveness of artificial selection in changing populations phenotypically. These changes have been produced by laboratory selection experiments and by selection of animals and plants in agriculture (as examples, for increased milk production in cows and for rust resistance in wheat). No analysis of these experiments in terms of allelic frequencies is possible because individual loci have not been identified and followed. Nevertheless, it is clear that genetic changes have occurred in the populations and that some analysis of selected populations has been carried out according to the methods described in Chapter 18. Figure 17-14 shows, as an example, the large changes in average bristle number achieved in a selection experiment with D. melanogaster. Figure 17-15 shows the changes in the number of eggs laid per chicken as a consequence of 30 years of selection.

The usual method of selection is truncation selection. The individuals in a given generation are pooled (irrespective of their families), a sample is measured, and only those individuals above (or below) a given phenotypic value (the truncation point) are chosen as parents for the next generation. This phenotypic value may be a fixed value over successive generations; then selection is by constant truncation. More commonly, a fixed percentage of the population representing the highest (or lowest) value of the selected character is chosen; then selection is by proportional truncation. With constant truncation, the intensity of selection decreases with time, as more and more of the population exceeds the fixed truncation point. With proportional truncation, the intensity of selection is constant, but the truncation point moves upward as the population distribution moves. Figure 17-16 illustrates these two types of truncation.

A common experience in artificial selection programs is that, as the population becomes more and more extreme, its viability and fertility decrease. As a result, eventually no further progress under selection is possible, despite the presence of genetic variance for the character, 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 to linked sterility genes that are carried along with the selected loci. In such cases, a number of generations without selection allow recombinants to be formed, and selection can then be continued, as in the upwardly selected line in Figure 17-14.

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 occurring 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.

© 1999 by W. H. Freeman and Company. All rights reserved.