436 RADIATION BIOLOGY 



exceedingly minute fraction of the total, they must occur occasionally 

 and the ciuestion therefore arises as to what role they may play in the 

 production of effects on future generations, following an exposure to radi- 

 ation. Under suitable conditions of selective multiplication, their role 

 may be inordinately out of proportion to their relative numbers. So, 

 for instance, in Gustafsson's (1947) extensive work with irradiated barley 

 seeds only one mutation in about 800, on the average, was found to be of 

 some use in adapting barley better for the needs of man, yet by discard- 

 ing the other 799 and actively propagating the one advantageous mutant 

 it was possible to establish an improved variety, and this procedure was 

 in the end profitable. Of course the same sort of thing happens, in a 

 slower and less regular fashion but on a far grander scale, in the natural 

 evolution of organisms by spontaneous mutation, except that in this case 

 the changes selected for multiplication are those which are advantageous 

 for the species itself rather than for man. That is, despite the fact that 

 the spontaneous mutations, like those produced by radiation, are in over- 

 whelming majority detrimental, a kind of advance in adaptation never- 

 theless results by virtue of the selective multiplication of the very few 

 gene mutations (and far fewer structural changes in chromosomes) that 

 happen to be helpful. Thus it might be thought that the continual 

 application of radiation would merely speed the advance, if other natural 

 processes were allowed to take their course. 



As a matter of fact this can and does happen under appropriate cir- 

 cumstances. Of the necessary conditions, the first is that the spontane- 

 ous mutation rate should not be already so high that when irradiation is 

 applied mutations occur too frequently to allow an equilibrium elimi- 

 nation rate and/or a genetic load low enough to be tolerated by the 

 population. A second condition is that the advantageous mutants 

 should multiply fast enough to replace the original type at a rate com- 

 mensurate with their increased rate of origination. A third requirement 

 is that the organism should not be at the limit of an evolutionary blind 

 end, i.e., that pathways of advantageous change still remain open to it. 

 Such opportunities will be present in greater abundance, allowing more 

 of the mutations that occur to be helpful in the given situation, if the 

 population has been placed in an environment, and subjected to condi- 

 tions of living, somewhat different from those previously natural to it; 

 for it must already have become so highly adapted to its natural condi- 

 tions as to make further progress difficult. Advance is also achieved 

 more readily if the population is one which has to some extent lost, 

 through genetic changes or recombinations, its original nicety of adap- 

 tation. This may have come about through the prior establishment of 

 some more or less harmful mutations, the effects of which can now be 

 overcome by reverse or counteracting mutations. Such prior retrogres- 

 sion is likely to have occurred if the given population has recently been 



