Natural Selection 



Although spontaneous changes or mutations in the 

 genetic system are normally produced independently 

 of the environment, their fate is ultimately deter- 

 mined by the forces of the environment. The or- 

 ganism cannot be divorced from its environment 

 which contains the necessities for the growth and 

 reproduction of the organism and at the same time 

 imposes a rigorous test of the fitness of the organism. 

 The environment therefore is the testing ground 

 for genetic changes, and its action determines which 

 changes will persist and which will perish. This 

 sifting is the process of natural selection. 



The process of natural selection is difficult to 

 measure on a short-term basis. Few contempora- 

 neous cases have been observed or adequately tested 

 in nature. Experimental evidence has contributed 

 much knowledge concerning the selective forces 

 exerted by simple laboratory variables on a few 



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