LECTURES IN BIOLOGICAL SCIENCES 



SEVEN BASIC POSTULATES 



At the Cold Spring Harbor Symposium of 1959, the pres- 

 ent writer became so strongly impressed by the solidity and im- 

 plications of this widespread agreement about the major proc- 

 esses of evolution that he undertook to formulate a series of 

 seven basic postulates which seem to express the extent of the 

 agreement and may serve as solid foundations for future work- 

 ing hypotheses about how the processes of evolution operate. 

 The discussion presented there (Stebbins, 1959b) served as a 

 summary of the symposium, which it concluded, and was there- 

 fore illustrated solely with examples taken from that sym- 

 posium. In the present article, the same postulates will be dis- 

 cussed in a broader fashion, with examples selected from all of 

 the evolutionary literature as most clearly illustrative of the 

 postulate being discussed. One must remember, however, that, 

 in a discussion as brief as the present one must be, only a tiny 

 fraction of the available evidence can be reviewed. 



The first basic postulate can serve as a summary of the pres- 

 ent introduction, or of the entire discussion. Its validity de- 

 pends upon that of the other six, so that no particular evi- 

 dence needs to be cited in its favor. It is stated as follows: 



At least in higher animals and plants, evolution proceeds 

 principally as the result of the interaction between four in- 

 dispensable processes: mutation, gene recombination, natural 

 selection, and isolation. 



The Role of Mutation and Gene Recombination 



The next two postulates are stated as follows: 

 Second, mutation neither directs evolution, as the early ev- 

 olutionists believed, nor even serves as the immediate source 

 of variability upon which selection may act. It is, rather, a 

 reserve or potential source of variability which serves to re- 

 plenish the gene pool as it becomes depleted through the action 

 of selection. 



Third, the mutations which are most likely to be accepted by 

 selection and so to form the basis of new types of organisms are 

 those which individually have relatively slight effects on the 

 phenotype, and collectively form the basis of polygenic or mul- 

 tiple-factor inheritance. 



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