454 



INTRODUCTION TO EVOLUTION 



FIG. 20.1. Approximate distribution of three "temperature races" of Drosophila funebris. 

 Solid line ( + 20°) is the isotherm of July; dashed line ( — 5°) is the isotherm of January; 

 dotted line (±25°) is the isoline of a difference of 25°C. between the mean temperatures 

 of July and January. (After TimofeefF-Ressovsky; from Allee, Emerson, Park, Park, and 

 Schmidt, Principles of Animal Ecology, W. B. Saunders Company, 1 949, p. 1 1 7.) 



it might eventually be found that mountaintops in the region were inhab- 

 ited by a black-eyed race of the insect. Such a race would have arisen, not 

 because there was any advantage in being black eyed, but because there 

 was advantage for a mountain dweller in having increased viability at low 

 temperatures. 



The second line of investigation likely to yield evidence of favorable 

 mutations involves study of animals not already at their "adaptive peaks." 

 We shall seldom find such relatively poorly adapted animals living in a 

 state of nature. Natural selection will have seen to that. But we can pro- 

 duce such animals experimentally, and then observe what natural selec- 

 tion does to them. Dobzhansky and Spassky (1947) produced popula- 

 tions of this kind in the form of stocks of Drosophila homozygous for one 

 or another of certain chromosomes known to contain recessive genes or 

 gene complexes which reduced viability and produced other deleterious 

 effects. Homozygotes for one such chromosome (designated PA748) had 

 very low viability, were slow in developing to the adult stage, and had 



