•220 



( IIAI'TER 16 



when heterozygous— from the population 01 



gene pool standpoint — than the greater det- 

 rimental effect when homozygous. How- 

 ever, each rare mutant, in terms of its etleet 

 on reproductive potential, is equally harmful 

 to a constant-sized population in that each 

 eventually causes a genetic death. Thus, hy- 

 poploid} 1 which aets as a dominant lethal per- 

 sists onlv one generation before it causes 

 a genetic death; a rare point mutant whose 

 reproductive disadvantage is only U U) % will 

 persist, on the average, one thousand gen- 

 erations before causing genetic death. 



Consider, on one hand, the gross chromo- 

 somal abnormality which kills in utero, de- 

 stroying a life early. Neither the individual 

 involved nor its parents suffer very long, 

 since such deaths may occur as abortions 

 which pass unnoticed. On the other hand, 

 the heterozygous point mutants in individuals 

 who are past the reproductive age — and, 

 therefore, already have or have not suffered 

 genetic death — will continue to subject these 

 people to the previously and newly pro- 

 duced, small phenotypic detriment of hetero- 

 zygosity which adds to their aches, pains, 

 and disease susceptibility. In this respect, 

 then, the mutant with a small effect on re- 

 productive potential can cause more suffer- 

 ing than one with a large effect, for the 

 longer the persistence, the more the damage 

 in postreproductive life. In general, speak- 

 ing not in terms of biological fitness but in 

 terms of the total amount of suffering to 

 which a human population is subject, point 

 mutants with the smallest heterozygous 

 detriment are the most harmful type of 

 mutant. 



One might at first suppose that the 

 amount of gene-caused human suffering can 

 be reduced by medical science. This pos- 

 sibility exists, particularly for an individual 

 such as the diabetic who takes insulin; no 

 doubt he is better off than he would be 

 without medicine. But remember that this 

 medicine does not cure the genetic defect. 



Moreover, by increasing the diabetic's repro- 

 ductive potential, the medicine serves to 

 increase the persistence of the mutants in- 

 voked, and the genetic death which must 

 eventually occur is only postponed to a later 

 generation — each intervening generation re- 

 quiring the same medication. The total 

 amount of human sutlering would be re- 

 duced only if medicine could correct the 

 gene-produced defect. To correct all of the 

 multiple effects of the mutant, the medicine 

 would have to replace the primary product 

 of the defective gene with normal product. 

 Insofar as most, if not all, currently known 

 medicines act later than this earliest stage 

 in gene action (Chapter 6), they serve to 

 alleviate only some detrimental effects, thus 

 causing an increase in human suffering by 

 increasing persistence. Unfortunately, this 

 situation will continue until medical science 

 is much further advanced. 



In view of the preceding discussion, we 

 can assume that it is primarily the euploid 

 or nearly euploid mutants which persist in 

 the gene pool and are mainly responsible for 

 changes in its composition during the course 

 of evolution. By far the most common and 

 most important class of such mutants is the 

 point mutant. 



Mutation and Evolution 



In Chapter 1 5 we only suggested that muta- 

 tions provide the raw materials for biolog- 

 ical evolution. The reason for our hesitance 

 in specifying evolution as the natural out- 

 come of changes in gene pools was that the 

 great majority of mutants, including point 

 mutants, are harmful in homozygous or hemi- 

 zygous condition. In this chapter and in 

 Chapter 15, we indicated that most mutants 

 are also detrimental when heterozygous. 

 Under these circumstances, how can muta- 

 tion provide the more adaptive genotypes 

 postulated as necessary for evolution? It is 

 true that for a given genotype under a given 

 set of environmental conditions the great 



