142 I The Process of Evolution 



graphic variation in frequencies of three different chromosome types 

 in a series of locaHties transecting the southwestern United States. 

 The chromosomal frequencies also change with altitude, as can be 

 seen in Fig. 7.8, which shows the proportions of three difiFerent types 

 at different elevations in the Sierra Nevada of California. Super- 

 imposed on this geographic variation is a seasonal variation in fre- 

 quency. For instance, in the Sierra Nevada, the Standard gene 

 arrangement is commonest at lower elevations, becoming pro- 

 gressively less common with increasing altitude. Arrowhead, on the 

 other hand, has a frequency that is positively correlated with alti- 

 tude, being greatest in the subalpine zone (about 10,000 feet) and 

 least at the base of the mountains (850 feet). However, in general. 

 Standard chromosomes tend to increase in frequency as the season 

 progresses, whereas Arrowhead chromosomes tend to decrease in 

 frequency. Thus the frequencies of these chromosomes in late sum- 

 mer populations at high elevations tend to approach the frequencies 

 of spring populations at lower elevations. 



These changes may best be explained as the result of rather 

 strong selection pressures. Dobzhansky has tested this hypothesis in 



Fig. 7.8 I Frequencies of different kinds of chromosomes in populations 

 of Drosophila pseudoobscura at different altitudes in the Sierra Nevada. 

 Black columns, Standard; white columns, Arrowhead; hatched columns, 

 Pikes Peak. The areas of the squares are proportional to the fre- 

 quencies of chromosome types; ordinate, elevation in feet; abscissa, 

 horizontal distance in miles. The elevation of a locality is indicated by 

 the position of the base of the lowermost square. ( From Dobzhansky, 

 1948, Genetics 33.) 



10,000 



5,000 



