DARWINISM AND MODERN GENETICS 



of populations, it seems probable that most evolutionary changes 

 by which species become adapted to the subtle variations of 

 their environment will involve many allelic differences rather 

 than few. The growing points of our present investigation of 

 natural selection are, therefore, the study of the changes which 

 can be brought about in co-adapted gene pools, rather than the 

 study of one or two genes of strong effect. Similarly, in the 

 study of artificial selection as it is involved in animal or plant 

 breeding, we again find that it is necessary to think in terms of 

 the statistical alterations in the frequencies of many genes. 



Population genetics deals with what may be called the tech- 

 nology of evolution— such matters as the range of genetic vari- 

 ability that will be present in a population, the rate of change 

 under natural selection or under the influence of statistical 

 fluctuations, and so on. The most fundamental problems con- 

 cerning evolution are, however, of a different kind. They arc 

 the two related questions of the manner in which new genetic 

 variation comes into being, and the nature of the influences 

 which the environment exerts over the direction in which evo- 

 lution will proceed. In pre-Darwinian evolutionary specula- 

 tions, such as those of Lamarck, these two questions were 

 thought to be intimately connected. With the rise of modern 

 genetics and its application to evolutionary theory, the two 

 problems were for some time thought to be quite unconnected 

 with one another. It is only in the last few years that we are 

 realizing that, in fact, they are closely connected, although the 

 connections are of a more complex nature than earlier biologists 

 had supposed. 



We have by now a very large, but still very unsatisfactory, 

 body of information concerning the processes by which new 

 hereditary variation arises. The unit hereditary determinants, 

 or genes, are found to undergo occasional alteration, a process 

 known as mutation. Mutations may occur spontaneously, that 

 is to say, in circumstances in which no particular causal ante- 

 cedents can be assigned to them. At one time it seemed ade- 

 quate simply to suppose that, since the gene must have a 

 highly complex molecular structure, mere thermal agitation or 

 similar chance processes might bring about mutational changes. 

 It was easy to comprehend in these terms the fact that physi- 



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