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CHAPTER 28 



uct(s) of the defective gene with that of the 

 normal, in order to normaHze all of the 

 pleiotropic effects of the mutant. However, 

 insofar as most, if not all, current medicines 

 act later than this in the pedigree of causes 

 (Chapter 10), they serve to alleviate some 

 detrimental effects, but not others, and in 

 this way cause an increase in the totality of 

 human suffering by increasing persistence. 

 This situation will persist until genetics and 

 medicine have advanced somewhat further 

 than they have at present. 



In view of the foregoing you will agree 

 that it is primarily the euploid or nearly 

 euploid mutants which persist in the gene 

 pool, and it must be these which are primarily 

 responsible for changes in its composition 

 during the course of evolution. By far the 

 most common, and hence most important, 

 class of mutants of this type, is the point 

 mutant. 



You may have noticed, in Chapter 27, 

 that it was only suggested that mutations 

 provide the raw materials for biological 

 evolution. Our hesitance in specifying that 

 evolution is the natural outcome of changes 

 in gene pools was based upon the observa- 

 tion presented earlier that the great majority 

 of mutants, including point mutants, are 

 harmful in homozygous or hemizygous condi- 

 tion. In the present Chapter and in Chap- 

 ter 24, we have indicated that most mutants 

 are also detrimental when heterozygous. 

 Under these circumstances how can muta- 

 tion provide the more adaptive genotypes so 

 necessary for evolution to take place as a 

 result of changes in the gene pool? This 

 dilemma is resolved in the light of two facts. 

 It is true, in a given genotype under a given 

 set of environmental conditions, that the 

 great majority of point mutants are detri- 

 mental, and that perhaps only one point 

 mutant in a thousand increases the repro- 

 ductive potential of its carrier ever so slightly. 

 Yet, provided the mutation rate is not too 

 large and provided there is sufficient genetic 



recombination, these rare beneficial mutants 

 offer the population the opportunity to be- 

 come better adapted. The second point is 

 that mutants which lower biological fitness 

 under one set of environmental conditions 

 may be more advantageous than the normal 

 genes under different environmental cir- 

 cumstances. So, for example, a mutant like 

 vestigial wings in Drosophila is clearly inferior 

 to its normal genetic alternative in an en- 

 vironment where flight ability is advanta- 

 geous, but this mutant might be advantageous 

 for Drosophila living on a small island where 

 flight is not only unnecessary but harmful, 

 where insects that fly can be lost by being 

 blown out to sea. As a second example of 

 this, we can mention the fact that several 

 decades ago the environment was DDT-free, 

 and mutants which conferred immunity to 

 DDT were doubtless less adaptive than the 

 normal genetic alternatives present. But 

 once DDT was introduced into the insect 

 environment, such mutants, even if they were 

 detrimental in other respects, provided a 

 tremendous over-all reproductive advantage 

 over their alternatives, so that they became 

 established in the population as the new wild- 

 type genes. Still other examples could be 

 cited involving microorganisms, where mu- 

 tants occur that confer resistance to anti- 

 biotics; in an antibiotic-free environment 

 these mutants would be less adaptive than 

 the genes normally present. 



It becomes clear, then, that mutation pro- 

 vides the opportunity for a population to 

 become better adapted to its present set of 

 environmental circumstances. It also pro- 

 vides the raw materials needed to extend the 

 population's range to different environments, 

 which already exist in some other territory 

 or which will arise through change in the 

 environment of its present territory. A pop- 

 ulation that is already very well adapted to 

 its environment is appreciably harmed by 

 the occurrence of mutation. But the en- 

 vironments in different territories differ, and 



