46 MUTATION AND PLANT BREEDING 



variability, occur in nature, a population containing a small amount 

 of mutants must be defined as the "normal" state of affairs. 



II. The balance maintained by selection scheme alone. Muta- 

 tion plays a very little role in determining an equilibrium value of 

 this type, because selection intensity is so much greater than mutation 

 rate. The gene frequencies involved in this type of equilibrium are 

 usually intermediate in value and thus result in genetic polymorph- 

 ism. Although greater fitness of heterozygotes is the simplest and the 

 most familiar mechanism by which selectional equilibrium may be 

 attained, it is not a necessary condition in more complicated 

 situations. A vast number of combinations of various factors may 

 lead to stable selectional balance. Probably most of the common 

 genetic characteristics, especially quantitative traits, are controlled 

 by selection rather than by mutation. 



Only very simple examples have been chosen to illustrate the 

 various points. The difficulty of comparing fitness of two different 

 populations has been discussed. Populations with the same average 

 fitness may have quite different selection schemes and different 

 genetic compositions. More detailed discussions may be found else- 

 where (3, 4, 12, 13, and 17). 



References 



1. Crow, J. F. 1948. Alternative hypotheses of hybrid vigor. Genetics, 



33: 477-487. 



2. Dempster, E. R. 1955. Maintenance of genetic heterogeneity. Cold 



Spr. Harb. Symp. Quant. Bio!., 20: 25-32. 



3. Dobzhansky, Th. 1957. Genetic loads in natural populations. Science, 



126: 191-194. 



4. , Krimbas, C, and Krimbas, M. G. 1960. Genetics 



of natural populations, XXIX: Is the genetic load in Drosophila 

 pseudoobscura a mutational or a balanced load? Genetics, 45: 741- 

 7 l >3. 



5. Haldane, J. B. S. 1937. The effect of variation on fitness. Amer. Nat., 



71: 337-349. 



6. Kojima, K. I. 1959 A. Role of epistasis and overdominance in sta- 



bility of equilibria with selection. Proc. Nat. Acad. Sci., 45: 984— 

 989.. 



7. . 1959. B. Stable equilibria for the optimum model. 



Proc. Nat. Acad. Sci., 45: 989-993. 



