HUMAN ORGANIC EVOLUTION: FACT OR FANCY? 



lost. A subdivision of a species into isolated populations, plus 

 time to allow a sufficient number of generations to elapse 

 (the number of generations being a function of the population 

 size) is all that is necessary for race formation. The isolation 

 may be brought about by many factors, such as geography, 

 ecological isolation, social isolation and so on. 



One of the important phenomena which makes this me- 

 chanism of genetic drift very important for evolutionary theory 

 is the factor of size of population. This has been studied 

 experimentally for non-human organisms, but probably also 

 operates in man. The smaller the population, the greater will 

 be the effect of drift. Reductions of populations have often 

 been observed through such phenomena as social fission, when 

 small groups of people have left the native habitat to establish 

 themselves in isolation elsewhere. Also diseases may play an 

 important role in this reduction of a population to a point 

 where genetic drift might become operative at a greater rate 

 than in larger populations. Furthermore, it should be pointed 

 out that the effective breeding population in man is generally 

 considerably smaller than the total population. Factors of age, 

 fertility, social taboos, etc., greatly limit an effective breeding 

 population. 



This mechanism of genetic drift has also other consequences, 

 some of which are not necessarily beneficial. In small popu- 

 lations alleles favored by natural selection may actually be 

 lost while disadvantageous alleles may become fixed. In such 

 cases, evolutionary changes by this same process of drift may 

 go against the direction of selection. This may perhaps explain 

 the aberrant forms of certain populations possibly contributing 

 to their extinction. 



It is probable that the four major mechanisms for 

 producing change rarely work singly, but in combina- 

 tion. The relative importance of each for the future of the 

 population under consideration seems to depend upon size of 

 the group or groups involved, and partly upon accident, which 

 is another way of saying that we have as yet little knowledge 

 of the causes of mutation though we know that radiation plays 

 an important role, and that chemical changes do also. We can 

 conclude however, that as populations increase, genetic drift 

 becomes much less important, and other factors play a much 



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