HUMAN ORGANIC EVOLUTION: FACT OR FANCY? 



there, they must also apply to man. From the genetic view- 

 point, evolution is essentially a change in gene frequencies. 

 To understand this it is important to recognize that although 

 genes do not change, and hence constitute the conservative 

 force mentioned above, they do combine in different ways in 

 different populations to produce different genotypes which 

 will interact with the environment to characterize certain 

 populations. The force for change is mutation, and this force 

 alters the gene so that changes are possible. If the altered 

 gene has adaptive value, it will become established in a popu- 

 lation. The third factor in change comes about through the loss 

 of genes. As far as is known, these processes have been 

 responsible for changes in the past. The changes brought 

 about by these processes may be discussed as four genetic 

 mechanisms which are primarily responsible for the alteration 

 of gene frequencies in a population — they are: Mixture, 

 mutation, natural selection, and genetic or random drift. 



Mixture is a very frequent phenomenon today and un- 

 doubtedly it has been important in the past. When two 

 populations with differing gene frequencies come together 

 and interbreed, the resulting population will be altered, 

 according to the proportions of new genes introduced by the 

 mixing populations. Consider a pair of hypothetical genes D 

 and d, for shortness and tallness respectively, and assuming 

 that tall stature is recessive to short stature, and the hetro- 

 zygote Dd produces intermediate stature, and that equal 

 numbers of the two parent stocks were involved, then the 

 result of the crossing of a pure short race DD with a pure 

 tall race dd would be the production of a new race in which 

 the population would be composed of 25% tall, 25% short, 

 and 50% intermediate individuals. Note, however, that there 

 are still tall and short people,- and that for no one individual 

 can it be predicted what he will be. This also illustrates, that 

 no genes have been lost, but rather that they have been, so 

 to speak, reshuffled in the population. The assumption here 

 is of course that no changes have taken place in the genes 

 themselves. 



Thus the effects of mixture will depend on gene frequencies 

 in the component populations, and the relative numbers of 

 individuals in each population. Though this can be calculated 



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