LECTURES IN BIOLOGICAL SCIENCES 



This fact explains in part the low opinion which many 

 biologists of a generation ago held in regard to Darwin's theory 

 of natural selection. In addition, the rediscovery of Mendel's 

 laws of inheritance focused attention upon unit characters, or 

 conspicuous differences between organisms which are inherited 

 as single units. When the origin of such unit characters by 

 genetic mutation was demonstrated in the evening primrose 

 (Oenothera) by De Vries and in the fly Drosophila by Morgan, 

 this seemed to many biologists to spell the doom of the natural 

 selection theory, based as it was on the accumulation through 

 selection of small differences between individuals. 



The past thirty years have seen a great revival of interest in 

 Darwinism, and an added confidence not only in the correct- 

 ness of the evolutionary theory itself, but also in Darwin's ex- 

 planation of natural selection. This revival has been brought 

 about by scientists who have applied modern quantitative and 

 experimental methods to the sphere of action in which the ev- 

 olutionary processes are actually at work. This is the genetics 

 of natural populations. Population genetics has erected three 

 great pillars which can support the edifice of evolutionary 

 theory. First, it has shown that multiple factor or polygenic 

 inheritance is the way in which most differences between nat- 

 ural populations are inherited. This leads to the corollary that, 

 as Fisher (1930) and many others have maintained, mutations 

 with relatively small effects, which contribute to the almost 

 imperceptible differences between individuals upon which Dar- 

 win relied, play a larger role in evolution than the conspicuous 

 mutations observed by De Vries and Morgan. Second, popula- 

 tion genetics has accumulated a great body of evidence show- 

 ing that the selective value of any single gene depends partly 

 upon the other genes with which it is associated in any indi- 

 vidual. This fact disposes simultaneously of any theories which 

 would make either the individual organism or the gene the basic 

 unit of evolutionary change. Mendelian segregation tells us 

 that the individual is merely the temporary home of a collec- 

 tion of genes which is partly scattered and assembled anew with 

 each successive generation of individuals. The rarity of muta- 

 tions makes each gene a relatively permanent entity, but as an 

 evolutionary unit its significance is reduced by its changing 

 adaptive values. Because of this fact, Dobzhansky (1955) and 



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