THE EFFECT OF SELECTION UPON THE SEX-RATIO 

 IN DROSOPHILA AMPELOPHILA. 



DON C. WARREN. 

 FROM THE ZOOLOGICAL LABORATORY OF INDIANA UNIVERSITY.! 



It is a well-known fact that sex-ratios in most animals approxi- 

 mate equality. The ratio is seldom one of exact equality, how- 

 ever, and the preponderance may be in favor either of the males 

 or the females. The variation from equality is fairly constant 

 for the species. For man the ratio has been found to be 100 

 females to 105 males; for the horse, 100 to 98; for the cow, 100 

 to 107; for the sheep, 100 to 97; for the pig, 100 to in; for the 

 dove, 100 to 105; and for the hen, 100 to 94. These variations 

 are so constant that they cannot be attributed to chance and 

 they are irregularities for which our present theories of sex deter- 

 mination offer no adequate explanation. 



The writer has attempted to determine the sex-ratio of the 

 fruit fly, Drosophila ampelophila. The determination was made 

 from three unrelated stocks in which the best possible environ- 

 mental conditions were provided. All matings were in single 

 pairs. Over 35,000 flies were examined and the ratio was found 

 to be 100 females to 95 males. 



Moenkhaus tested the effect of selection upon the sex-ratio 

 of Drosophila ampelophila and decided that the sex-ratio in this 

 species is "amenable to selection." If it be true that the relative 

 number of males and females in a strain can be varied by selection, 

 the present theories of sex determination must be somewhat 

 modified. Although this fact would not necessarily disprove 

 the theories of sex determination, it would necessitate the assump- 

 tion of an hypothesis of selective fertilization or of differential 

 mortality or viability of the determining elements. 



In view of the vital bearing of Moenkhaus's findings upon the 

 theories of sex determination, it has been deemed worth while 

 to repeat his work. 



1 Contribution No. 159. 



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