HUMAN ORGANIC EVOLUTION: FACT OR FANCY? 



constantly modify the environment to suit himself, rather than 

 the other way around.^^ gut on the level of micro-evolution, 

 there are undoubtedly many changes in which natural selec- 

 tion works directly even on man.23 



The fourth and last evolutionary mechanism to be explored 

 is known as random genetic drift, or isolation, because it 

 depends for its operation on the isolation of one small popu- 

 lation from other populations with which it could interbreed. 

 Isolation must have been a factor of great importance in the 

 early history of man when geographic or other barriers 

 prevented people from mixing freely, especially during glacia- 

 tions in certain parts of the world. Reference to this process 

 was made earlier in connection with the Neanderthal fossils. 

 The basic mechanism is simple enough and may best be 

 described in terms of an example. 



Assuming a very small population of say only four men and 

 four women was placed on an island. If they came from a 

 normal European population, it might well happen that no 

 individual of blood group B (which accounts for only 10 to 

 15% in European populations) would be represented. But 

 even if a B individual were represented, by chance, the 

 individual most likely would be a heterozygote, that is of the 

 genotype BO, and any child of his has a 50-50 chance of not 

 belonging to bloodgroup B. Even if there were a fair number 

 of progeny the gene B might still not be represented in the 

 new generation, and as soon as this hypothetical original 

 group B individual passed the age of reproduction or died, 

 the gene would be irretrievably lost to the island population 

 unless it were re-introduced by mutation. Furthermore, the 

 possibilities of loosing the B gene are enhanced by the 

 individual possessing it getting killed before maturity, by 

 sterility, or for some other reason. 



Sewall Wright has shown mathematically that even in 

 larger populations the chance fluctuation in gene frequencies 

 with succeeding generations will often result in a gene being 



^ Cf. Charles G. Wilber, "Physiological Regulations and the Origin of 

 Human Types," Human Biology, vol. 29, no. 4, pp. 329^ — 336. 



^ Cf. S. L. Washburn, op cit.. pp. 298—304; Dobzhansky, op cif./,^ 

 W. W. Howells, "Universality and Variation in Human Nature," 

 Current Anthropology (W. L. Thomas, ed.), 1956, pp. 227—236. 



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