SELECTION AND VARIABILITY 



second quick change of nearly o-6 occurred in both, to be followed 

 by another period of near stability until the end of the experiment. 

 The responses to the selection shown by tlie males were smaller, 

 but conformed to the same pattern. 



Tests were made to determine the effects of the three major 

 chromosomes on bristle number at generations o, 13 and 21, 



5-5 



50 



4-5 



40^ 



RESPONSE DUE TO CHANCE IN 

 IT CHROMOSOME II,in 



10 15 



GENERATIONS OF SELECTION 



Fig. 73. — The effects of selection for increased number of scutellar bristles in 

 Drosophila mclatiogaster. The average number of scutellar chaetae is plotted against 

 generations of selection, all matings being of brothers and sisters. The two selection 

 lines came from the same parents in generation i, and gave parallel results except 

 that the first major response occurred four generations later in one than the other, 

 and the second major response perhaps one generation later. The responses were 

 nevertheless of equal or nearly equal size, and due to change in the same chromosomes, 

 in the two lines (based on Sismanidis, 1942). 



i.e. before selection commenced, after the first chief advance, and 

 after the second. They showed that the first advance in both lines 

 was due entirely to an increase in bristle-producing power of 

 chromosome II; while again in both lines the later advance also 

 involved chromosome III. The X chromosome appeared not to 

 have changed in either line during the experiment. Such regular 

 changes can hardly be attributed to new variability arising by 

 mutation. To what are they due ? 



Selection, as we have seen, is effective in changing the mean 



286 



