The Source of Variability 79 



observation, the presence of these "lethals" can be detected by the 

 faikire of eggs to hatch, seeds to germinate, or individuals to reach 

 full development. 



VIABLE MUTATIONS 



A large proportion of the viable mutations in both plants and 

 animals are of a deleterious nature, often expressed as reduced 

 fertility or vigor. Even if the homozygotes of these mutant alleles 

 are lethal or nearly so, the heterozygotes combining a mutant with 

 a normal allele may be nearly as productive and vigorous as normal 

 homozygotes. In this heterozygous fashion sublethal alleles may be 

 carried in populations almost indefinitely. 



Viable mutants have been observed for practically every gene 

 studied and therefore affecting every character studied. Many 

 exhaustive studies and compilations provide long lists of these 

 mutant types ( Goldschmidt, 1940; Lerner, 1958; Stebbins, 1950; 

 and many others). An enumeration of this tremendous array of 

 spontaneously arising mutants would convey the impression of 

 a fountain of new variability sufficient to account for the changes 

 inherent in the evolutionary mechanism. 



One point concerning these mutants is of special interest. In any 

 one line, the same mutant will appear independently many times. 

 Also homologous mutations have been observed in entirely different 

 species (Spencer, 1949). This fact provides a possible explanation 

 for the appearance of the same character in different phylogenetic 

 lines. It also supports the plausibility of Blum's (1951) contention 

 that the basis for mutation is a relatively simple chemical change 

 in the genie material. 



A number of investigators have speculated on the nature of this 

 change in the genie material which produces a new mutant. Several 

 authors have assumed that DNA is the genie material and have 

 postulated various mechanisms by which changes in the arrange- 

 ment of nucleotides might occur (Watson and Crick, 1953; Beadle, 

 1955) and how this might control the composition of proteins 

 synthesized through DNA activity (Gamow, 1955). When, how- 

 ever, one considers the possibility that DNA may be only one link 

 in a sort of ultimate molecular machine involving protamines, 

 DNA, and likely other compounds, it seems feasible that changes 

 in any member of the machine might be responsible for the origin 

 of mutations. 



In extremely detailed studies of several genes, however, Stadler 

 (1954) pointed out that the mutants he observed were due to 



