HEREDITY IN SOMATIC CELLS 



383 



200 



400 



Generations 



(a) 



600 



200 



400 



Generations 



ih) 



600 



FIGURE 12.13. Mutational equilibrium and orthoselection in populations of bacterial 

 cells. 



(a). Achievement of mutational equilibrium for antigenic mutants in Salmonella 

 where the rates of mutation for these characters are unusually high. (b). Opera- 

 tion of orthoselection in E. co/i in maintaining the level of methionine-independent 

 mutants in a methionine-requiring population at a level of ca. 10 *' instead of 10 

 which is the ratio of mutation rates (10~^:10~*') (from Stocker, 1949, J. Hygiene, 

 47:398; and Atwood, Schneider, and Ryan 1951, C. S. H. Symp. Quant. Biol., 16: 

 345). 



