108 I The Process of Evolution 



Table 6.7 



individuals to halve the gene frequency and reduce the number of 

 defective individuals to 1 in 40,000. The problem of carrying out 

 such a program without mistakes for such a protracted period makes 

 it highly unlikely that such meager results would justify the effort 

 involved. On the other hand, selection against dominants is rela- 

 tively highly effective. If a dominant gene became lethal, for in- 

 stance, it would be removed from a population in one generation. 



Homozygous Recessives Relatively Unsuccessful 



The situation where the dominants are favored over the recessives 

 but the homozygous recessive individuals make some contribution 

 to the gene pool of the succeeding generation is probably more 

 common than that of complete homozygous recessive lethality. 



If fitness values of 1 are assigned to the two dominant genotypes, 

 and 1 — 5 to the homozygous recessives, after one generation of 

 selection there would be p- AA individuals, 2pq Aa individuals, 

 and q^ — sq~ aa individuals out of a total of p~ + 2pq -\- q- — sq~ = 

 1 — sq- individuals. Using the same procedure as in the previous 

 example, the change in gene frequency per generation is 



Pq + qHl-s) -sqHl-q) 



' 1 — sq^ " 1 — sq"^ 



Thus the change in gene frequency under these conditions is 

 small when q is very large or very small and is relatively large when 

 the value of q is intermediate. When q is large, progress is slow be- 

 cause of the relatively large reproductive contribution of the reces- 

 sive homozygotes (in contrast with the dominants). As q becomes 

 small, the sheltering effect of the heterozygotes slows progress, as it 

 does when the homozygous recessives are lethal. A few sample 

 values are given in Table 6.8, where the selection coefficient oper- 

 ating against the homozygous recessives is 5 = .5. 



