CHAPTER 



20 



NATURAL SELECTION: I 



In the preceding chapter we emphasized the point that 

 populations have a tendency to remain in genetic equihbrium but that var- 

 ious forces tend to upset this equihbrium. Of these forces we discussed 

 mutation pressure and genetic drift, saving for later consideration the most 

 potent force of all: natural selection. In Chapters 2 and 15 we discussed 

 the general principles of natural selection, and in Chapter 16 we saw ex- 

 amples of it at work in a state of nature. Now we turn our attention to 

 some of the factors involved in natural selection and some of the ways in 

 which it operates to produce evolutionary change. 



MUTATIONS AS RAW MATERIALS 

 FOR NATURAL SELECTION 



Let us look more critically at mutations and ask 

 whether they really possess the qualifications for the important role as- 

 signed them in the modern theory of natural selection. 



Critics of the idea have emphasized the point that most of the mutations 

 we study in our laboratories are harmful, not beneficial. The multitudi- 

 nous mutations to which students of genetics in Drosophila devote them- 

 selves are almost all of the nature of abnormalities and malformations. 

 Even when no marked structural abnormality is involved, mutations fre- 

 quently reduce the viability, or fertility, of their possessors. Since, as we 

 have noted (p. 355), the leaving of disproportionately large numbers of 

 ofl'spring is the principal hallmark of success, reduced viability and fertil- 

 ity are at least as detrimental as are malformations of structure. 



Nevertheless the fact that not all mutations are harmful is emphasized 

 by the increasing use man makes of mutations to improve cultivated 



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