NATURAL SELECTION: 



463 



70 

 60 

 50 

 40 

 30 

 20 

 10 

 



• 



MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC 



TO 

 60 

 50 

 40 

 30 

 20 



10 

 



continued to rise rapidly in succeeding months (it is to be emphasized that 

 the temperature throughout this time remained constant, at 25° C, i.e., the 

 passing months did not bring temperature changes for the flies). An 

 equihbrium was reached by about the end of December (the graph, Fig. 

 20.5, does not include the concluding months of the experiment). In 

 experiments involving ST and CH chromosomes equilibrium was reached 

 when about 70 percent of the chromosomes were ST, about 30 percent 

 of them CH. The rapid increase in relative number of ST chromosomes 

 in a population maintained at a high temperature affords evidence that 

 the conclusion concerning the rela- 

 tive advantage of the ST arrange- 

 ment in the heat of summer (see 

 above) is valid. 



Of equal interest with this con- 

 clusion is the fact that an equilib- 

 rium is eventually established. If 

 the CH chromosome confers a 

 disadvantage at high temperatures 

 why does it not disappear entirely? 

 Following mathematical analyses 

 by Sewall Wright the experimenter 

 concluded that the CH chromo- 

 somes do not disappear entirely 

 because flies heterozygous for 

 them (i.e., having one CH third 

 chromosome and one ST third 

 chromosome — written : ST/CH ) 

 are more successful than are either 



flies homozygous for CH (i.e., CH/CH) or flies homozygous for ST (i.e., 

 ST/ST). It will be recalled that this is the explanation reached for the 

 experimental results obtained by L'Heritier and Teissier and by Reed and 

 Reed. Evidently ST/CH heterozygotes are characterized by that hybrid 

 vigor previously referred to (p. 459) and hence have an advantage over 

 both kinds of homozygotes (ST/ST and CH/CH). As Dobzhansky ex- 

 pressed it, "The populations at equilibrium contain the greatest possible 

 proportions of the well adapted heterozygotes compatible with the lowest 

 possible proportions of the relatively ill adapted homozygotes." 



It is of interest that in one experiment no equilibrium was reached. 

 This experiment involved the relatively rare TL chromosome (Fig. 

 20.4), placed in competition with the ST chromosome. The latter "won 



FIG. 20.5. Frequency of ST chromosomes 

 (in percentage) in on experiment in which 

 the ST gene arrangement was in competition 

 with the CH gene arrangement at the tem- 

 perature of 25 C. (Redrawn from Dobzhan- 

 sky, "Adaptive changes induced by natural 

 selection in wild populations of Drosophila," 

 Evolution, Wo\. 1, 1947.) 



