NATURAL SELECTION: I 461 



possessed by individuals in any one portion of the range covered by the 

 species. There are, however, definite geographic trends in the distributions 

 of the arrangements, some arrangements being common in one locaUty, 

 rare in another. 



In one locahty, Piiion Flats, on Mount San Jacinto in southern Cali- 

 fornia, four such arrangements are found. They are identified by the 

 symbols ST, CH, AR, and TL, the meanings of which need not concern 

 us. The point of particular interest at present is that the relative frequen- 

 cies of these arrangements fluctuate with the seasons. As shown in Fig. 

 20.4, in March of each year about 

 52 percent of the chromosomes 

 found in these flies are ST chromo- 

 somes, about 23 percent are CH 

 ones, about 18 percent AR, and 

 about 7 percent TL. As the spring 

 progresses these frequencies 

 change for the first three men- 

 tioned, while the frequency of the 

 TL chromosome fluctuates but lit- 

 tle. The ST chromosome rapidly 

 diminishes in frequency, while the 

 CH and AR chromosomes corre- 

 spondingly increase. Fig. 20.4 

 shows the maximum frequency of 

 AR as reached in May, that of CH 

 in June. In the latter month the ST 

 chromosome is at its lowest fre- 

 quency. Following its peak, the 

 AR chromosome declines some- 

 what in frequency, while the CH 

 chromosome declines much more rapidly and to much greater extent. Con- 

 trariwise, the ST chromosome increases rapidly in frequency until by No- 

 vember it has practically the same frequency it had in March. 



How can we account for seasonal fluctuation of this kind in sene ar- 

 rangements of chromosomes? Parenthetically we should note that these 

 flies, of whatever gene arrangement, ah look alike as far as external visible 

 characteristics are concerned. Only microscopic examination of the 

 chromosomes in salivary glands of the larvae reveals any differences. The 

 differences obviously are of a most subtle nature. But subtle or not, they 

 evidently are important in the lives of the flies, important enough so that 



55 

 50 

 45 

 40 

 35 

 30 

 25 

 20 

 15 

 10 



5 







TL' 



TL 



I I I I I \ I 1 1 1 L 



55 

 50 

 45 

 40 

 35 

 30 

 25 

 20 

 15 

 10 



5 

 



JAN F M A M J J A S N DEC 



FIG. 20.4. Changes in frequencies of four 

 different gene arrangements in third chro- 

 mosomes of the fruit-fly population of Pition 

 Flats, California. Vertical axis: frequencies, 

 in percentage. Horizontal axis: successive 

 months. Combined data for six years of ob- 

 servation. (Redrawn from Dobzhansky, 

 "Adaptive changes induced by natural se- 

 lection in wild populations of Drosophila," 

 Evolution, Wo\. 1, 1947.) 



