The Theory of Population Genetics 103 



nonrandom set of genotypes leaves more oflFspring than others, selec- 

 tion has occurred. In the broadest view, selection reduces the diver- 

 sity of living organisms; organisms containing certain types or com- 

 binations of genetic information are inviable or do not persist. This 

 process "creates" what we recognize as a certain order in nature, in 

 the same way that the order we see in a team with seven heavy, 

 muscular linemen is "created" by the game of football. It is only in 

 this restrictive sense of combating a trend toward increasing entropy 

 that selection is "creative." 



Critics of the theory of natural selection have claimed that selec- 

 tion can in no way be creative since it functions merely to eliminate 

 certain types. They would point out that, in the common analogy of 

 natural selection with a sieve, if large and small rocks are screened, 

 the end result is a pile of the smaller rocks from the original pile. 

 Nothing new has been created. What is forgotten is that the analogy 

 is too simple, for it ignores the potential mutational and recombina- 

 tional variability of biological entities. It is as if two small rocks 

 could mate and their offspring included rocks smaller than either 

 parent. If a sufficiently fine sieve were used, screening would create 

 a heap of rocks smaller than any in the original pile. 



There are two types of selection: natural selection, in which the 

 environment determines which genotypes are the most "fit," and 

 artificial selection, in which man determines which genotypes are the 

 most "fit." It is important to note that, while natural selection must 

 always operate on the genotype through the phenotijpe, it is some- 

 times possible for artificial selection to act directly on the genotype. 

 For example, let us suppose that a moth has a melanic form pro- 

 duced by a dominant gene M. If the dominance is complete, only 

 two distinguishable phenotypes will be presented to the environ- 

 ment, melanic and nonmelanic {M— and vim). As far as natural se- 

 lection is concerned, the homozygous dominant {MM) and hetero- 



Fig. 6.3 I Equilibrium with mutation and reverse mutation. See text 

 for explanation. {From Wright, 1940, The New Systematics, Oxford 

 University Press. ) 



