NATURE OF GENETIC HAZARD OF RADIATION 429 



human values. Even if we postpone a consideration of these by changing 

 the question to "What are the effects of a given increase in mutation 

 rate?", the problem is still a tremendous one. Muller has, in his paper 

 (p. 322), already pointed out some of the many unknowns: the relative 

 numbers of the various types of genes, their degrees of dominance, and 

 so forth. Other aspects, particularly effects at the population level, 

 present a major challenge to statistical geneticists. The problem is not 

 entirely novel to them. The same complex of variables — mutation and 

 selection pressures, population size, coefficients of inbreeding, etc. — has 

 been considered in dealing Avith the problems of evolution and improve- 

 ment in domestic plants and animals. In the present state of knowl- 

 edge of these variables, the diversity of theoretical possibilities that can 

 be derived from a consideration of them is almost overwhelming. An 

 excellent example of this diversity is provided by Wright (23) in his 

 speculations on the effects which an increased mutation rate might have 

 on just one single character of a population, namely, its reproductive 

 rate. 



Wright calculated that, with regard to completely recessive mutations 

 that affect reproduction in the population solely by depressing the re- 

 productive value of the mutants themselves, a somewhat optimistic 

 view can be taken on the chances of survival of the human population. 

 He points out, however, that this is not the only way in which the re- 

 productive value of the population could be affected. If the mutants 

 were of inferior quality (for example, with lowered intelligence or char- 

 acter), they could constitute a drag on the population as a whole in its 

 ability to utilize its resources and thus bring about a further reduction 

 in the reproductive value which, with high mutation rates, could con- 

 ceivably lead to collapse rather than to a new equilibrium. If the mu- 

 tants were inferior and not less fertile than the non-mutants, then the 

 population would tend towards collapse with even the slightest muta- 

 tion rate. A systematic increase in mutation rate would speed up the 

 process. Finally, if the mutants were inferior but more fertile than the 

 non-mutants, such a runaway process would be enormously enhanced. 



Wright suggests still another line of possibilities. For example, it 

 could be imagined that with social and economic changes resulting from 

 the increased number of mutants, the relative reproductive values of 

 mutants and non-mutants might change too, and even reverse their 

 order, thus, in the last case given above, leading to a new equilibrium 

 instead of collapse. 



It is apparent that the question of how great an increase in mutation 

 rate can be tolerated in the human population is exceedingly difficult 

 to answer even when consideration is limited to a single, quantitatively 



