DARWINISM AND MODERN GENETICS 



which is often known as genetic drift, has been particularly 

 stressed by Sewall Wright (1931). On the other hand, others 

 of the theoreticians, such as Fisher (1930), have argued that it 

 is not likely to be of much practical importance in nature. Only 

 detailed investigation of actual natural populations can settle 

 this, and the matter is still under debate. 



The theory of population genetics was soon reinforced by 

 experimental studies of actual populations as they exist in na- 

 ture. The first major advances in this field were made by a 

 group of Russian geneticists. Tschetverikov (1926) seems to 

 have been the first to draw any practical conclusions from the 

 consideration that the phenomenon of dominance implies that 

 wild populations may contain very many more recessive genes 

 than a casual inspection would suggest. Following his lead, 

 several of his pupils in Russia, such as Dubinin (1934), and 

 others who later worked outside the Soviet Union, such as the 

 Timofeeff-Ressovskys (1940), and Dobzhansky (1951), as well as 

 geneticists from many other countries, have profoundly increased 

 our knowledge of the genetic endowment of populations evolv- 

 ing in the field. 



From these studies three major new points have emerged. 

 In the first place, the wealth of genetic variation within a nat- 

 ural population has turned out to be much greater even than 

 the early students expected. In the early days of population 

 genetics, it was thought that a population consists of individuals 

 most of whose genes are of the same set of standard wild-type 

 alleles, with occasional rare recessive alleles at some few loci. 

 It has been found, however, that, although this picture is true 

 enough if we consider only allelic differences which produce 

 very marked effects, the idea that there are standard wild-type 

 genes is not adequate if we examine the matter in detail. On 

 the contrary, at almost every locus a population is found to con- 

 tain a number of slightly different alleles, all of which produce 

 relatively normal individuals, but individuals with slight dif- 

 ferences between them. Nearly all individuals taken from a 

 wild population are, in fact, heterozygous for very many more 

 genetic factors than anyone had suspected. 



From this basic fact, two further consequences seem to be 

 emerging. On the one hand there is much evidence, particularly 



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