382 CELL HEREDITY 



directed by conditions which allow the multiplication of only certain 

 kinds of cells. Although selection of variant cells may not be the most 

 important factor in normal development, when variants do arise, selection 

 may follow. The quality and intensity of the change will be a function 

 of the nature of the variant and the nature of the environment. 



CELL POPULATION DYNAMICS 



Some information is available about the explosive changes which occur 

 when microorganisms are exposed to drastically inhospitable environ- 

 ments such as those containing toxic agents or lacking required growth 

 factors. The various cells making up the population may also interact 

 with one another as, for example, in the modification of the environment 

 to make it favorable or unfavorable for one or another type. The selec- 

 tive actions involve the length of the lag before division, the rate of 

 multiplication, the efficiency with which the limiting factor is utilized, 

 and the rates of death during or after growth. Were it not for selection 

 and certain features of the expression of the variant phenotype, for every 

 allele an equilibrium should eventually be established with all others to 

 and from which it can mutate. The equilibrium would be achieved 

 when the proportion equals: 



Numberof mutants _ G (]9 }) 



Number of parents b 



where a is the rate of change to the variant, and b to the parental condi- 

 tion. As a consequence, considering the large number of genes behaving 

 in this way, the population should gradually become so heterogeneous 

 that it would not consist of predominantly one cell type. In practice, 

 this kind of heterogeneity is not observed. Bacterial cultures of one 

 cell type do not degrade into such a mixture despite the fact that their 

 haploid nature exposes mutations. 



The reason for this contradiction is that while a particular mutant is 

 rising in frequency in a population, other kinds of mutations arc also 

 occurring. Even a constant environment presents a challenge to a grow- 

 ing population; types fitter than their relatives will be selected for and 

 overgrow all others, including the mutant whose rise was being observed. 

 The result is that these mutants will be eliminated, only to start their 

 march toward ecjuilibrium again among the newlv established fitter 

 types. As this process continues, a new kind of equilibrium results 

 (Figure 12.13). Its level is determined by the rates of mutation and 

 the degree of fitness achieved rather than onlv by the rates of mutation 



