( Contribution from the Zoological Laboratory of Indiana University, 



No. 165.) 



An Experiment to Test the Nature of the Vari- 

 ations on which Selection Acts 



By Ferj^^vndus Payne, Ph.D., Associate Professor of Zoology, Indiana 



University 



Introduction 



In recent years there have developed two contrasted views 

 concerning the action of natural selection, or rather concern- 

 ing the nature of the variations on which selection acts. On 

 the one hand, we have the work of Johannsen and his fol- 

 lowers, who maintain that selection can do nothing more 

 than isolate pure lines. They further maintain that the only 

 method of change within the pure lines is by mutation. Since 

 by selection so-called unit characters in bisexual forms can 

 be modified, a further development of the pure line theory 

 has been necessary. This is the multiple factor hypothesis, 

 the basis of which was given by Nilsson-Ehle ('09) when 

 he showed that in certain crosses of oats and wheat two or 

 three factors may be involved. Selection acts in such cases 

 by eliminating or accumulating such multiple factors, and acts 

 for a few generations only, or until a homozygous strain is 

 produced. The second view concerning the variations on 

 which selection acts has been developed by Castle, who holds 

 that the individual variations of a given character are 

 expressions of the variations of the gene which stands for 

 that character. 



My own experiment was devised and carried out primarily 

 to find the nature of the variations on which selection acts. 

 The character chosen was bristle number on the scutellum 

 of , Drosophila ampelophila. By starting with a female with 

 one extra bristle (four is the normal number) the number 

 of bristles was gradually increased until a maximum of eleven 

 extra bristles was produced. The mean bristle number was 

 ijicreased until it reached 9.123. Tests were made and are 

 described in the text which show conclusively, I think, that 



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