144 I The Process of Evolution 



that in adult males heterozygotes are significantly more common 

 than one would expect in a population in Hardy-Weinberg equilib- 

 rium. 



An extreme case of selective advantage of structural heterozygotes 

 occurs in a population of Drosophila fropicalis where there are two 

 common chromosome sequences. Both types of structural homo- 

 zygotes die early in development, and only the heterozygotes sur- 

 vive to breed. Although only half of the zygotes formed are viable, 

 this population flourishes. 



Long-term changes in inversion frequencies have been reported. 

 In California before 1941 only four Pikes Peak (PP) chromosomes 

 were found among 20,000 chromosomes studied. In 1957 PP chromo- 

 somes were found in all 10 localities sampled in the state, the mean 

 frequency being about .08 (a 400-fold increase). This increase 

 occurred at the expense of CH chromosomes. The agent behind 

 the change is not known, but it is almost certainly selection. Since 

 1941 the PP chromosomes have continuously predominated in popu- 

 lations found on the eastern slope of the Rocky Mountains and in 

 Texas. The California increase could not be due to migration from 

 this reservoir of PP chromosomes, as geographically intermediate 

 populations in Arizona and Utah did not change significantly be- 

 tween 1941 and 1957. The source of selection pressure is not known, 

 but widespread drought and increasing smog are two of the more 

 obvious factors which might be considered. 



The above is only a brief outline of some of the highly interesting 

 work done on the genetics of natural populations of Drosophila. 

 There can be little doubt that the different gene constellations or 

 supergenes in the inversion chromosomes (remaining together as 

 rather stable blocks because of the effect of the inversions in sup- 

 pressing the results of crossing-over) have different adaptive values 

 under different conditions. Many examples of chromosome fre- 

 quency changes correlated with environmental changes have been 

 elucidated. Less success has attended attempts to determine when 

 advantages and disadvantages occur in the life cycle. Describing 

 the exact nature of these adaptive changes also has proved diflBcult. 

 It is impossible to reproduce natural conditions in the laboratory, 

 and very little is known about the life history and ecology of the 

 various Drosophila species. Studies of D. pseudoohscura and D. 

 persimilis breeding in "slime fluxes" (yeast- and bacteria-infected 

 sap exudations on trees) in the Sierra Nevada, and laboratory 

 studies of many species, have yielded valuable information. Much 

 more work is needed. The complexity of the problem may be ap- 

 preciated by considering the factors determining pupation site in 



