G. W. BEADLE 



tion, it is possible to suggest a scheme of evolution that has some sup- 

 port at least in analogy. It has become increasingly evident that, with 

 respect to their need for and use of vitamins of the B group, purines, 

 pyrimidines, choline, amino acids, and other compounds, all cellular 

 organisms are fundamentally very similar (23,26,53). To consider 

 a specific example, carboxylase is presumably present in all protoplasm 

 and apparently always contains thiamin as thiamin pyrophosphate. 

 Many organisms, e. g., most plants, are able to synthesize the thiamin 

 they need, while others are dependent on an external supply of this 

 essential compound. From an evolutionary standpoint, this difference 

 is presumably determined by whether or not it is of advantage to a par- 

 ticular organism to synthesize thiamin. Evidently for Neurospora 

 there is selective advantage in being able to carry out this synthesis 

 for we find in wild strains that all essential genes concerned with it are 

 present in active form. In mammals, on the other hand, thiamin is 

 presumably so frequently present in the diet that the genes originally 

 concerned with its elaboration have been permitted by natural selection 

 to become inactive so far as thiamin synthesis goes. It may well be 

 that they have not disappeared entirely but have been modified so 

 as to enable the mammal to carry out chemical reactions of which the 

 bread mold is incapable. In a similar way, mammals have become 

 specialized through loss of ability to synthesize other vitamins, the 

 indispensable amino acids, and other compounds. With the develop- 

 ment of parasitism it would be expected that still further loss in syn- 

 thetic ability would be encountered. As Knight (23), Lwoff" (26), 

 Schopfer (38), and others have pointed out, this is indeed the case. 

 The work on Neurospora makes it most probable that the dropping out 

 of specific chemical reactions no longer of selective advantage is the 

 result of gene mutation. The limit of such parasitic specialization is 

 probably represented in the molecular viruses that have lost all power 

 of heterosynthesis and have retained only the one property essential 

 for their continued existence in an environment in which all necessary 

 compounds are available — the property of autosynthesis. 



One may quite properly raise the question as to the course of 

 positive evolution in terms of chemical reactions — how are new syntheses 

 developed in the course of organic evolution? Unfortunately, the 

 experimental evidence bearing on this is meager, which is not surpris- 

 ing, for obviously it should be much easier to destroy or inactivate a 



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