LECTURES IN BIOLOGICAL SCIENCES 



of phenomenon with which population genetics is concerned. 

 The argument of the last few paragraphs shows that, even if 

 we reject Lamarck's theory when it is interpreted in terms of 

 simple genetics, there are good grounds for accepting it when 

 it is interpreted in terms of evolutionary-population genetics. 



The view that has been put forward above gives no grounds 

 for suggesting that the environment directly stimulates the ap- 

 pearance of new hereditary variation. The individual differ- 

 ences in the capacity to respond to stress, on which selection op- 

 erates, may well have been present in the population before 

 the stress was applied. However, there is one important con- 

 clusion concerning natural variability which must be drawn. 

 Selection for capacity to respond to stress will produce indi- 

 viduals whose developmental systems are easily modifiable in 

 certain appropriate directions and are resistant to modifications 

 which would produce harmful alterations. The types of stability 

 and instability which are thus built into the developmental 

 systems of the organisms must influence the manner in which 

 new hereditary variability is expressed. A random change in 

 the molecular configuration of the hereditary material will af- 

 fect a system of developmental reactions which are easily mod- 

 ified in certain directions and more stable in other respects. 

 Random mutations of the genes will therefore tend to produce 

 phenotypie effects which are not completely random, but which 

 are to some extent determined by the earlier selection which 

 has determined the modificability of the developmental system. 



In this short summary of modern ideas about evolution, we 

 see that Darwin's theory of natural selection has provided the 

 basis for the great advances in our understanding which have 

 followed the development of modern genetics. The theoretical 

 and experimental studies of population genetics, both in the 

 laboratory and in nature, have given us a much deeper insight 

 into the mechanisms by which natural selection operates. They 

 culminate at the present time in the notion of a co-adapted 

 gene pool, but we can still see many vistas for future develop- 

 ment opening in front of us. When one regards the twin 

 problems of the manner in which the environment influences 

 evolution and of the origin of new variation, again the theory 

 of natural selection provides the essential clue. It remains very 



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