Li: MUTATION, SELECTION, AND POPULATION FITNESS 



[J 



Table 6. — Equilibrium Condition when Selection Favors Heterozygotes. * 



Population 1 



Population 2 



Parameters in nature 



Gene frequency 

 Genotype proportion 



wi = 0.90 



J t = 0.10 



w 2 = 1.00 



w 3 = 0.85 



0.15 



p = 0.60, q = 0.40 

 0.36, 0.48, 0.16 



wi = 0.40 



} t = 0.60 



w , = 1.00 



w 3 = 0.10 



0.90 



/> = 0.60, q = 0.40 

 0.36, 0.48, 0.16 



Average fitness 



0.015 0.54 



w = \ - - = 1 - 0.06 w = 1 - - = 1 - 0.36 



0.250 1.50 



Population 3 



Population 4 



Parameters in nature 



Gene frequency 

 Genotype proportion 



Average fitness 



Wi = 0.92 



[ t = 0.08 



Wi = 0.95 



I = 0.05 



w . 2 = 1.00 



w 3 = 0.90 



iv, = 1.00 



0.10 



p = 5/9, q = 4/9 

 25/81, 40/81, 16/81 



0.008 

 w = \ - = 1 - 0.044 w 



0.180 



} s = 0.40 

 w 3 = 0.60 



p = 8/9, q = 1/9 

 64/81, 16/81, 1/81 



0.02 

 1 - - - = 1 - 0.044 

 0.45 



*wi = 1 — t; iV2 = 1; wz = 1 — 5. 



e.g., Oenothera; that is, w^ = u/ 3 = 0. The average fitness in such a 

 case is very low, meaning that there is a great deal of selection going 

 on, but the plant population may nevertheless thrive. 



Summary and Conclusion 



Two major types of genetic equilibrium have been discussed. 



I. The balance between recurrent mutation and persistent 

 selection. In this case, the gene frequency is always kept at a very low 



level, ranging from the order \/u to u, where u is the mutation rate 

 to the gene under consideration. The small immediate harmful effect 

 on the population as a whole must be viewed as the price the popula- 

 tion has to pay for preserving the gene for possible further use in the 

 course of evolution. Since mutations, the ultimate source of genetic 



