404 INTRODUCTION TO EVOLUTION 



of red resulting from the mixture of red and brown pigment.) If one of the 

 enzymes in this series is not produced or is changed in some way so that it 

 does not perform its usual function, the chain is "broken" and no brown 

 pigment is produced. Such a change in an enzyme may result from mutation 

 of the gene upon which it is dependent for its presence or its specificity. As 

 a result of mutation of one of the genes connected with the chain of enzyme- 

 controlled reactions the eye is left with only bright red pigment, the resultant 

 eye color being called vermilion. The mutated gene (whose effect is to sup- 

 press brown pigment formation) is called a gene "for" vermilion eye. But 

 note that its effect is through alteration of one step in a series of enzyme- 

 controlled chemical reactions occurring in the body of the fly during its 

 embryonic development. Similarly, mutation of another gene will cause 

 failure to produce both brown and red pigment, the eye being left white 

 in color. 



Important for evolution are those genes that control the rates of 

 metabolic processes and the times in the life history at which the processes 

 occur. This aspect of gene action is complementary to that discussed in 

 the preceding paragraph since the so-called "rate-genes" doubtless act by 

 means of control of enzymes. From the standpoint of evolution we are 

 particularly interested in rate-genes expressing their effect during the 

 course of embryonic development, since the phenotype of the adult is a 

 resultant of the forces (genetic and environmental) that have acted on it 

 during its embryonic life. Natural selection acts, not on the genes them- 

 selves, but upon what those genes produce, or as Waddington (1959) has 

 expressed it, "natural selective pressures impinge not on the hereditary fac- 

 tors themselves, but on the organisms as they develop from fertilized eggs 

 to reproductive adults." This is important. Too often we think of individ- 

 uals as adults only; the individual is an organism, and hence subject to 

 natural selection, from the time it is a fertilized egg onward. 



An easily visualized example of a rate-gene in action is afforded by the 

 research of Ford and Huxley (Huxley, 1932) on eye color in the crustacean 

 Gammarus (an amphipod or "scud"). Early in embryonic life these crea- 

 tures have bright red eyes. At about the end of the first week of develop- 

 ment deposition of melanin (dark brown pigment) begins in the eyes. This 

 continues at such a rapid rate that three or four days later the eyes ap- 

 pear black. The investigators discovered a mutation which causes the eyes 

 of adults to be red, with a faintly brownish cast. They showed that in this 

 case deposition of pigment did not begin until the young were 4 weeks old, 

 and that then it proceeded so slowly that by the time sexual maturity was 



