140 LINUS PAULING 



of heterozygotes AS in the population, and that of the progeny of two hetero- 

 zygotes the 25% of t}'pe AA would die of malaria, the 25% of type SS would 

 die of sickle-cell anaemia, and the 50% of type AS would be protected against 

 malaria and would not have the disease sickle-ceU anaemia. Under these cir- 

 cvunstances the heterozygotes would rapidly replace the original type AA. In 

 some highly malarial regions in Africa the incidence of heterozygotes AS is as 

 great as 50%. If the assumption is made that all of the sickle-ceU homozygotes 

 SS die without progeny, we may calculate from this incidence of the hetero- 

 zygotes that the advantage that the heterozygotes have over the normal individuals 

 AA is 50% — there will be an increase by the factor 1-5 in the ration of hetero- 

 zygotes to normal individuals in each generation. This is a very great increase, 

 such that within one thousand or two thousand years the mutant type of human 

 being, carrying a sickle-cell gene, would be found to have largely displaced the 

 wild type, without the mutant gene. So far as I know, this is the only quantitative 

 information that is available about the rate of evolution in man. The information 

 shows that the process of evolution can, under favourable circumstances, be a 

 very rapid one, and we may understand how it is that the process of selection of 

 a pool of favourable genes, such as is now represented in the germ plasm of the 

 human race, could have taken place so effectively during the period as short 

 as 10^ years since the origin of life on Earth. 



REFERENCES 



1. P. Jordan, Phys. Z., 39, 711, 1938; Z. Phys., 113, 431, 1939; Fundam. radioL, 5, 43, 



1939 j ^- ImmimForsch., 97, 330, 1940. 



2. L. Pauling & M. Delbruck, Science, 92, 77, 1940. 



3. F. Breinl & F. Haurowitz, Hoppe-Seyl. Z., 192, 45, 1930. 



4. J. Alexander, Protoplasma, 14, 296, 193 1. 



5. Stuart Mudd, J. Immunol., 23, 423, 1932. 



6. L. Pauling, J. Amer. chem. Soc, 62, 2643, 1940. 



7. L. Pauling, D. Pressman, D. H. Campbell, C. Ikeda & M. Ikawa, J. Amer. chem. 



Soc, 64, 2994, 1942, and later papers. 



8. J. D. Watson & F. H. C. Crick, Nature, Land., 171, 737, 1953; F. H C. Crick & 



J. D. Watson, Proc. Roy. Soc, 223A, 80, 1954. 



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10. J. D. Watson & F. H. C. Crick, Nature, Lond., 171, 964, 1953. 



11. L. Pauling, in Aspects of Synthesis and Order in Growth. Princeton University 



Press, p. 3, 1955. 



12. L. Pauling, D. Pressman & A. L. Grossberg, J. Amer. chem. Soc, 66, 784, 1944. 



13. L. Pauling, Festschrift Prof. Dr. Arthur Stall. Birkhaiiser, Basel, p. 597, 1957. 



14. L. Pauling, H. A. Itano, S. J. Singer & I. C. Wells, Science, no, 543, 1949. 



15. L. Pauling, Proc. nat. Acad. Sci., Wash., 18, 293, 1932. 



16. L. Pauling, Les Prix Nobel en 1954. Norstedt & Soner, Stockholm, p. 91, I955- 



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London, 1956. 



Contribution No. 2207 from the Gates and Crellin Laboratories of Chemistry, California 

 Institute of Technology, Pasadena, California, U.S.A. 



'^ 



