Changes in Populations 141 



only Standard and Chiricahua chromosomes are present, the three 

 different kinds of individuals are the two homozygotes ( ST/ST and 

 CH/CH) and the heterozygote (ST/CH). The homozygotes can be 

 distinguished only by progeny testing or careful study of the pattern 

 of banding, while the heterozygote is easily recognized by the in- 

 version loop. 



The different kinds of chromosomes in D. pseiicJoohsciira have 

 different geographic distributions, apparently because of their vary- 

 ing adaptive values in different habitats. Figure 7.7 shows the geo- 



Fig. 7.7 I Frequencies ( in percent ) of different kinds of chromosomes 

 in populations of Drosophila pseudoobscura in southwestern United 

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

 columns. Pikes Peak. {From Dobzhansky, 1947, Evolution 1.) 



