THE DYNAMICS OF EVOLUTIONARY CHANGE 



others have correctly recognized the "Mendelian population" 

 in sexually reproducing organisms as the basic unit of evolu- 

 tionary change. The third pillar erected by population ge- 

 netics is the quantitative, experimental approach to the study 

 of selection itself. In the last century, evolutionists tried to 

 establish the validity of natural selection chiefly by showing how 

 a particular characteristic, such as body color, could have a 

 selective advantage in a given environment. They failed to 

 realize that the interactions between an organism and its en- 

 vironment are extremely complex, so that the way in which a 

 characteristic is advantageous is often indirect and very hard 

 to demonstrate conclusively. Population genetics has taught 

 us that the best way to show the action of natural selection is 

 to demonstrate statistically that a given gene, or gene com- 

 bination, has an adaptive advantage in a known, controlled en- 

 vironment. This demonstration is relatively easy in organisms 

 which are well known genetically, and can sometimes lead to a 

 further study of how selection acts. 



Supported by these three pillars, evolutionists have erected 

 a theory of how evolution works which has often been called 

 the "synthetic theory," and sometimes the "Neo-Darwinian 

 theory." Natural selection still serves as its cornerstone, but 

 the mutation theory, Mendelian genetics, and the statistics of 

 population dynamics are almost equally important components 

 of its foundations. The symposia which were held in various 

 parts of the world during the year of the Darwinian centennial, 

 1959, impressed most of their participants with the degree to 

 which biologists actively working in the field of evolution are 

 agreed on the soundness of this theory. A generation ago, 

 many leading biologists felt that none of the processes which 

 had been invoked to explain evolution was satisfactory and 

 that the true explanation would come with the discovery of 

 some hidden "cause" not then known. Now most biologists 

 agree in believing that the major causes of evolution are known, 

 but that no one of these causes is sufficient in itself. Muta- 

 tion, genetic recombination, and natural selection are comple- 

 mentary to each other. The task of the future lies chiefly in 

 showing how these processes act in relation to each other and 

 to the changing environments through which the earth has 

 passed and is passing. 



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