Natural Selection 



107 



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Fig. 44. Females of the stonefly Allocapnia vivipara showing differences in 

 wing lengths found in nature. The stippled area on the abdomen is mem- 

 branous. 



sclerotized abdomens, a third have unusually short wings and a 

 third have the longest wings. In spite of this lack of adaptive cor- 

 relation, both traits are being carried in many populations and are 

 available for selection pressures to influence. 



At any time and through a variety of causes new viable genetic 

 components or combinations may occur and become a part of the 

 gene pool of the population. In other words, the genetic composition 

 of a population is dynamic and constantly adding an element of 

 variability into the relationship between gene frequency and natural 

 selection. Such changes might modify old selection pressures or 

 establish new ones. 



