THE DYNAMICS OF EVOLUTIONARY CHANGE 



actual rates of mutation for changes in oil or protein content. 

 But the data of Stadler (see Dobzhansky, 1951) on rates of mu- 

 tation for other characteristics in corn tell us that for a par- 

 ticular characteristic the occurrence of one mutation in 50,000 

 plants is a relatively rapid rate. Hence the occurrence of even 

 one mutation in the desired direction during the course of the 

 experiment is rather unlikely. The slow, steady way in which 

 the populations responded to selection shows that many genetic 

 differences were being sorted out. Since these differences could 

 not have arisen by mutation during the course of the experi- 

 ment, they must have existed in the gene pool of the original, 

 unselected, but cross-fertilizing population. 



The experiments of Mather and his associates (Mather and 

 Harrison, 1949; Mather, 1955; Breese and Mather, 1957) on 

 selection for high and low number of abdominal bristles in 

 Drosophila gave similar results. In these experiments the re- 

 sponse to selection was less regular than in corn, and the un- 

 desirable secondary effects, particularly the appearance of a 

 high degree of sterility, were more marked. The actual response 

 was in a series of short bursts, lasting five to twenty generations, 

 and separated by intervals of several generations during which 

 no response to selection occurred. Nevertheless, selection was 

 effective even after one hundred generations, and the calcula- 

 tions of Mather and Harrison, like those given above for the 

 corn experiment, show that the response to selection was based 

 upon genetic differences already present in the gene pool of the 

 initial population, rather than mutations occurring during the 

 course of the experiment. Furthermore, Breese and Mather 

 (1957) tested six different marked regions of one chromosome 

 for genes affecting the difference between the line selected for 

 high and that for low bristle number. They found such genes 

 in every region, which suggested that the gene pool contained 

 a very large supply of genes affecting bristle number, at many 

 different loci. Indirect evidence leading to the same conclusion 

 was obtained by Clayton and Robertson (1955), who found that 

 an inbred line of Drosophila, which would be expected to con- 

 tain a smaller gene pool than the crossbred population used by 

 Mather and Harrison, responded very slowly to selection for 

 increased bristle number. 



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