. Genetics in relation to agriculture. Livestock; Heredity; Variation (Biology); Plant breeding. 260 GENETICS IN RELATION TO AGRICULTURE biotypes can be differentiated from one anotlier so tliat tliey "breed ; How these distinct biotypes originate will be considered in the following chapter, the fact that they exist is the chief consideration here. The effect of "mass" selection in causing temporary changes in heteroge- neous varieties of plants and races of animals is easily understood by the aid of the diagram shown in Fig. 108. The area within the large curve repres
. Genetics in relation to agriculture. Livestock; Heredity; Variation (Biology); Plant breeding. 260 GENETICS IN RELATION TO AGRICULTURE biotypes can be differentiated from one anotlier so tliat tliey "breed ; How these distinct biotypes originate will be considered in the following chapter, the fact that they exist is the chief consideration here. The effect of "mass" selection in causing temporary changes in heteroge- neous varieties of plants and races of animals is easily understood by the aid of the diagram shown in Fig. 108. The area within the large curve represents a mixed population or phenotypically similar group containing a number of distinct genotypes indicated by the small curves A-Z. Every genotype has its own variation curve and is distinct from each of the others, but they intergrade with each other so completely that the population appears as an entity. Now if one should select individuals from either extreme of the population, say at 90 or 70, it is clear that such individuals might to any one of four or five geno-. tl06ir)2G:UMtai3rttt7li^GU70Tl7273 74 Fig. 108.—Schematic diagiam showing the relation of a population to the biotypes composing it, or of a phcnotype, to the genotypes or pure lines within it. (After Lang frojn Goldschmidt.) types. If selection in the same direction were continued a strain would be established with a mode distinct from the mode of the original popu- lation. These strains could be maintained b}^ continual selection and in time a single genotype might be isolated when selection would be said to have changed the type permanent]}'. But selection changed nothing—it only isolated a certain genotj'pe or genotypes from the origi- nal mixture. Tower's results in selecting for the purpose of creating albinic and melanic strains of beetles as illustrated in Fig. 109 may be explained in this way. The original population shown at A consisted of a number of distinct biotypes. By the isolation o
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