CHAPTER 18 



GENETIC CHANGE AND 



EVOLUTIONARY CHANGE 



In the preceding chapter we noted that genes act by 

 determining the presence or the nature of enzymes. Enzymes, in turn, 

 control the processes of metabohsm and of development. Hence, changes 

 in genotype result in changed enzymes and so in changed phenotype — and 

 changed phenotypes are the raw materials of evolution. Consequently 

 understanding of the nature of gene action is important to understanding 

 of evolution. 



Gene Action 



We have mentioned a strain of the bread mold, Neurospora, lacking the 

 ability to utihze lactose in its nutrition (p. 401). Normal Neurospora can 

 use this compound sugar; an enzyme called lactase splits lactose to the 

 simple sugars glucose and galactose. The strain unable to do this was 

 produced by irradiation (Bonner, 1948). Chemical analysis showed that 

 organisms in this strain did not possess lactase, or that if it was present it 

 was not in its active form. The lactase-less strain differed from normal ones 

 by mutation of a single gene. 



In Chapter 5 we discussed the fact that most metabolic processes in the 

 body involve a long chain of chemical reactions each one of which is cata- 

 lyzed by an enzyme. One such chain of reactions, involving a whole series 

 of intermediate chemical substances each synthesized under the influence 

 of the appropriate enzyme, results in the production of the brown pigment 

 in the eye of the fruit fly, Drosophila. (The wild-type eye color is a shade 



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