. Genetics in relation to agriculture. Livestock; Heredity; Variation (Biology); Plant breeding. 258 GENETICS IN RELATION TO AGRICULTURE generations, the character under observation being degree of mealiness of the kernel (Schartigkeit), Johannsen concluded that the selection had produced no effect. Moreover the Swedish plant-breeding station at Svalof has been guided for years by the knowledge that their pedigree cultures, , pure lines, were not changed by selection. A similar con- clusion was reached by Tower after four to ten generations of rigorous selection of albinic individuals in t
. Genetics in relation to agriculture. Livestock; Heredity; Variation (Biology); Plant breeding. 258 GENETICS IN RELATION TO AGRICULTURE generations, the character under observation being degree of mealiness of the kernel (Schartigkeit), Johannsen concluded that the selection had produced no effect. Moreover the Swedish plant-breeding station at Svalof has been guided for years by the knowledge that their pedigree cultures, , pure lines, were not changed by selection. A similar con- clusion was reached by Tower after four to ten generations of rigorous selection of albinic individuals in three different attempts to establish an albinic race from a stable race (pedigree material) of the Colorado potato beetle {Leptinotarsa decemlineata). The history of these three. Fig. 106.—Four pure lines of wheat which have been grown by Vilmorin for 50 years. The original specimen in the seed museum is shown on the left in each case. The close similarity of the pairs of heads indicates that pure lines remain constant indefinitely. (After Haoedoom.) experiments are shown at A, B and C in Fig. 107. The small black polygons show for each generation the individuals selected to become the parents of the next generation. It will be noted that neither the range nor the mode of the population is permanently shifted in the direction of the selection. Thus we find that in races or varieties which are constant (homozygous) selection has no effect unless mutations occur. Various evidence has been brought forward to show that the principle does not hold for all organisms. But in all such cases among sexually propagated species we may assume that the material used was hetero- zygous for certain factors. Such has been shown already to be a satisfac- tory explanation of Castle's results in selecting for plus and minus strains in the hooded rats which is one of the cases originally advanced as evidence against the pure line theory. Digitized by Microsoft®. Please note that these images are
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