118 I The Process of Evolution 



The majority of alleles are fixed or lost at random. An intermediate- 

 sized population under opposing selection and mutation pressures is 

 shown in Fig. 6.10. There is random variation around modal values 

 established by the opposing pressures. The case of a large popula- 

 tion with gene frequencies at equilibrium points determined by the 

 magnitudes of opposing selection and mutation pressures is covered 

 in Fig. 6.11. Finally, Fig. 6.12 gives the gene frequencies in sub- 

 divisions of a large population fluctuating around modal values 

 established by opposing forces of migration and selection. 



Figs. 6.5 to 6.8 | Distributions of gene frequencies under different 

 selection pressures and in populations of different sizes. For details 

 see text. (From Wright, 1937, Proc. Nat. Acad. Sci. 23.) 



6.5 



6.7 



6.6 



6.8 



* 



