462 INTRODUCTION TO EVOLUTION 



natural selection acts upon them. It seems reasonable to conclude that the 

 differences have something to do with either viability or fertility, or both. 

 It would seem, for example, that under conditions prevailing in the spring 

 months carriers of CH chromosomes have some advantage over carriers 

 of ST chromosomes, leaving more surviving progeny on the average and 

 thus leading to the observed increase in frequency of CH chromosomes, 

 with concomitant decrease in frequency of ST chromosomes. Following 

 the same line of thought, we may conclude that conditions during the 

 summer favor carriers of ST chromosomes; hence their numbers increase. 

 The same summer conditions are evidently unfavorable for carriers of 

 CH chromosomes, with resulting decline in numbers. Perhaps differences 

 in temperature are concerned in the matter. Specifically we might interpret 

 the graph (Fig. 20.4) as indicating that during the heat of summer carriers 

 of ST chromosomes are at a relative advantage (and hence increase in 

 numbers rapidly), while carriers of CH chromosomes are at relative 

 disadvantage (and hence decline in numbers). 



To test interpretations of the kind suggested, Dobzhansky set up experi- 

 ments in natural selection, using population cages much like those of 

 L'Heritier and Teissier (see p. 457). In one experiment several hundred 

 flies having two different gene arrangements in the desired proportions 

 were placed in a cage. Within a single generation the population increased 

 to the maximum compatible with the amount of food available — usually 

 to between 2000 and 4000 flies. The experimenter recorded that the 

 numbers of eggs deposited were tens to hundreds of times greater than 

 the numbers of adult flies that hatched. "The competition for survival is 

 intense." Once a month samples of eggs were taken and the salivary gland 

 chromosomes of larvae hatching from them were studied. In this way 

 changes in the relative frequencies of the different gene arrangements 

 were traced. 



It was found that when population cages were kept in the cold (16%° C.) 

 no changes in frequencies of gene arrangements occurred. The original 

 relative proportions continued generation after generation. At this 

 temperature there is evidently little if any difference in advantageousness 

 among the various gene arrangements. 



When the cages were kept at room temperature or higher (25° C), how- 

 ever, progressive changes occurred until a definite equilibrium was estab- 

 lished. Fig. 20.5 shows the results of one such experiment. The population 

 was established in March. In this population 10.7 percent of the third chro- 

 mosomes were ST, 89.3 percent were CH. As the graph shows, the 

 frequency of ST chromosomes nearly doubled in the first month and 



