140 I The Process of Evolution 



N=212 



100 

 90 

 80 

 70 

 60 

 50 

 40 

 30 

 20 

 10 



N=2U 



Fig. 7.6 I Comparison of Natrix sipedon litters and adults from the 

 Bass complex of islands in Lake Erie. Ordinate, percent in class; 

 abscissa, banding type. A, least banded; D, completely banded. ( From 

 Camin and Ehrlich, 1958, Evolution 14.) 



investigation is possible in Drosophila because of the giant polytene 

 chromosomes found in the saHvary glands of the larvae of these fruit 

 flies. These chromosomes show the close pairing usually associated 

 with the zygotene stage of meiosis. Their size and this somatic pair- 

 ing make them very useful tools for research. Inversion heterozy- 

 gotes in Drosophila may be detected by examination of the salivary 

 chromosomes for the characteristic inversion loops (Fig. 3.2). 



Vernacular names have been given to the different sequences of 

 the banding in the very variable third chromosome of D. pseudo- 

 obscura. The three most widely discussed inversions in evolutionary 

 literature are Standard (ST), Arrowhead (AR), and Chiricahua 

 ( CH ) . When two different kinds of chromosomes occur in the same 

 population, three types of individuals will be present: two inversion 

 homozygotes and one inversion heterozygote. For example, where 



