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Unformatted text preview: the character, because the selected individuals do not re- produce. 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 popula- tion 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 the 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 ge- netically 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 vari-and 1970....
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.
- Spring '08