20 AN ANALYSIS OF THE EFFECT OF SELECTION. 



GENERAL RESULTS OF SELECTION EXPERIMENTS. 



In every case the selected lines showed means that differed from the 

 mean of unselected Dichsets in the direction in which selection had 

 been carried on. Owing to the apparently large environmental influ- 

 ence on bristle number, it is in most cases difficult to be sure how this 

 result was brought about, or, rather, at what stage in the process. 



In the case of the 1331 (speck) minus line, however, the change seems 

 to have been effected fairly rapidly at first, and slowly, if at all, later 

 on. In the case of the 1002 line there was probably no effect in the 

 later generations. Reversed selection was uniformly successful if 

 begun in the early generations, but not usually so at later stages. 

 These are the results that would be expected on the view that modify- 

 ing genes are involved. 



It is to be observed in the case of the plus lines that the means vary 

 inversely as the standard deviations — that is, that the two curves 

 are much like mirror images. In the minus lines the two quantities 

 usually vary together, giving curves that are nearly parallel. These 

 relations hold surprisingly closely for many of the curves, especially 

 those of the plus lines. They are due to the fact that a change in the 

 mean is almost always brought about by an elimination or great de- 

 crease in the number of individuals at one extreme of the population 

 rather than by a marked change in the position of the mode or of the 

 other extreme. This is strongly in favor of the view that selection 

 has been effective in eliminating ''unfavorable" combinations rather 

 than in producing entirely new types. 



The relation between the crossbred and inbred series is too much 

 obscured to repay detailed analysis. Evidently such experiments 

 with this character would have to be carried out under carefully con- 

 trolled environmental conditions before they could have any great 

 significance. 



CROSS OF TWO INBRED PLUS LINES. 



Since the two inbred plus lines, 864 and 1002, came from slightly 

 different sources (see above), and were kept separate while being plus 

 selected, it seemed possible that different plus modifiers had been 

 isolated in the two lines. If this were the case, crossing them should 

 result in increasing the variability in F2, and the parent-offspring 

 correlation when the F2 individuals were bred to produce F3. The 

 F2 population should contain genetically unlike individuals, and 

 should yield to selection in either direction. As a matter of fact, no 

 such result was obtained. 



Table 17 gives the result of the experiment. The 1941 set is per- 

 haps the clearest case, so we may consider it alone. The parents of 

 1941 came from 1763 (Fio of the 864 line) and 1788 (F7 of the 1002 

 line). As table 17 and figure 10 show, the standard deviation in F2 



