l.i: MUTATION, SELECTION, AND POPULATION FITNESS 39 



Table 3. — Selectional Balance when the Heterozvgote has a Greater Fitness 



Value than Homozygotes. * 



*F — 0.20; t — 0.20; s = 0.30; p = 0.65; q = 0.35. 



E. Variable Fitness 



In all previous sections the value of the w's is assumed to be con- 

 stant. This may be true for certain traits affecting reproduction and 

 may not be so for others. It is quite possible that the fitness depends 

 not only on the genotype itself, but also, possibly to a large extent, 

 upon the genetic composition of the population as a whole. Some of 

 the simplest examples of this nature are given in Table 4, in which 

 the fitness value of a phenotype varies with the phenotypic frequency 

 in the population. In part A it is assumed that selective advantage or 

 disadvantage of one phenotype is proportional to the frequency of the 

 other phenotype. Thus, in the first example, when recessives are rare 

 in the population (R small and D + H = 1 — R large), the recessive 

 fitness becomes large, being 1 + s(\—R), and hence the recessive pro- 

 portion will increase. The same is true with the dominants. This type 

 of selection will lead to a stable equilibrium. Conversely, if as R 

 becomes small, the recessive fitness also becomes lower, 1 — s(D + H), 

 the equilibrium is unstable and one of the alleles will be reduced to 

 near extinction. The fitness of a phenotype in part B varies with fre- 

 quency of its own phenotype. 



The equilibrium condition, when fitness varies with gene fre- 



