NATURAL SELECTION 



In fact, one northern race showed a shghtly higher survival rate than did 

 D. melanogaster. At the higher temperatures, however, the survival rate 

 of D. funebris was never as much as half of that of D. melanogaster. The 

 complete data are summarized in Table 2. 



Table 2. The Relative Viability of the Strains of Drosophila funebris of Differ- 

 ent Geographical Origin ( from Timofeeff-Ressovsky ) * 



• By permission from Dobzhansky, "Genetics and the Origin of Species," 3rd Ed., 

 Columbia University Press, 1951. 



Goldschmidt studied adaptation to climatic conditions in the gypsy 

 moth, Lymantria dispar. While the conditions of development were here 

 determined by climate, rather than by experiment, they nonetheless per- 

 mit an insight into the nature of the selective force. The moths lay their 

 eggs in the fall, but the eggs remain dormant until the following spring. 

 In the spring, when the sum of the daily temperatures has reached a cer- 

 tain minimum, development proceeds and the larvae emerge. Then they 

 feed on the green foliage of certain plants. Now the hatching time of 

 northern races is quite short, while that of Mediterranean races is much 

 longer. It is evident that a rapid developmental cycle is necessary for the 



235 



