184 



PHYSIOLOGIC GENETICS 



Morgan 896 stated this viewpoint as follows: "If we had the complete ancestry of any 

 one animal or plant living today, we should expect to find a series of forms differing 

 at each step by a single mutant change in one or another of the genes, and each a 

 better-adapted or a differently-adapted form from the preceding." Under this theory 



Fig. 3 1 . Mean selective values of populations homallelic with respect to a 



MAJOR PAIR OF ALLELES (M, M') AND SIX MODIFIERS. 



Initial 

 Selective Peak 



M M M M 



ABCDEF A'BCDEF A'B'CDEF A'B'C'DEF 



A'B'C'D'E'F 



Later 

 Selective Peak 

 IvV 

 a'b'c'd'e'f' 



etc. 



etc. 



etc. 



M M M 



a'bic'd'ef A^'c'rJE'F a'b'c'd'e'f' 



etc etc. 



there is no recombination, and interaction effects are significant in evolution only in a 

 restrictive sense. 



The other two theories to be considered here are based on the concept that the 

 population is something more than a lot of individuals of the same genotype. Under 

 them a population is characterized genetically by an array of genie frequencies. The 

 elementary evolutionary process becomes a change of genie frequency. 373, 516, 1422 

 The observations which led to this concept were indeed largely on such obviously 

 variable populations as those of man, cultivated plants, domestic animals, and laboratory 

 rodents rather than on wild species, although some support could be drawn from such 



