44 MUTATION AND PLANT BREEDING 



gamblers A and B. The former likes to play big stakes and wins or 

 loses a large sum in each bet but comes out even at the end of the 

 game; the latter plays cautiously and wins or loses a small sum in 

 each bet but also comes out exactly even at the end of the game. Who, 

 then, is a better gambler? When they do lose, A loses a total of $50 

 in two hands and B loses the same amount in ten hands, but both 

 of them recover their losses before the end of the game so that there 

 is no change in wealth distribution. Who shall we say is a better 

 gambler? 



Comparing two populations in selectional balance involves more 

 or less the same difficulty. For the sake of simplicity, let us assume 

 F = 0, so that the equilibrium condition is, according to section 4, 



s t 

 p — and q = 



s+f s+t 



and the average fitness is 



st 

 w = 1 - tp- - sq 2 = 1 



s + t 



In Table 6 are listed some examples of equilibrium populations 

 of this type. It is seen that populations 1 and 2 have the same genetic 

 composition but different average fitness, while populations 3 and 4 

 have different genetic composition but the same average fitness. Since 

 in the case of selectional balance neither allele A nor allele a can be 

 strictly considered deleterious, it may be easier to use w as an index 

 which, as before, measures the amount of selection going on in the 

 population, although the selection produces no change on the genetic 

 composition of the population at equilibrium. 



Finally, it should be pointed out that although w, based on rela- 

 tive fitness of genotypes, measures the amount of selectional turnover 

 per generation, it is not useful as an index for the absolute surviving 

 ability of a population. If two populations, one with w = 0.94 and one 

 with W = 0.64, are grown side by side in the same environmental con- 

 ditions, there is no way to foretell which population will win out. 

 The genetics of intrapopulation selection deal only with the genetic 

 composition of a population and give no information as to inter- 

 population competing abilities. The latter more properly belongs to 

 the realm of ecology, to be studied by field naturalists and geneticists. 

 In plants there are instances in which only heterozygotes survive, 



