Duplication of Molecules in Living Organisms 223 



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

 zygotes the 25% of type AA would die of malaria, the 25% of type S S would 

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

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

 cumstances 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-cell 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 

 wüd 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. 



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

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



