Races of man / by Sonia Cole.

  • Cole, Sonia, 1918-1982.
Date:
1965
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Races of man / by Sonia Cole. Source: Wellcome Collection.

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    is 2 23 , or more than 16 million. In practice, an individual is believed to be the product of about 44,000 pairs of genes. Of these, some 90% are held in common by everyone. Of the remaining 5,000 or so, at least half must be allowed for sexual differ ences and racial variations and only about 2,500 genes—3-5% of the whole— remain to account for individual variations. Genetics and Evolution Evolution is essentially a change in gene frequencies resulting from the produc tion of new genes, the loss of genes, or an accumulation of changes in gene fre quencies. Frequencies may be altered by mixture of populations, mutations, natural selection , and genetic drift or random variation. Races converge by mixture and diverge through natural selection, isolation and genetic drift. The most important agency in bringing about changes in gene frequencies of human populations is mixture ; at the present time, this is going on to such an unprecedented extent that it is bound to lead to a merging of the races we know. Perhaps the strongest isolating mechanism holding this process in check is 'assorta- tive mating' caused by class, social and other artificial barriers5 but as a result of better education, the general trend nowadays is towards a breakdown of these barriers. A mutation is the appearance of a new hereditary character that breeds true. Mutations arise from changes in the chemical composition of the gene or of the chromosome which alter its inherent potentialities. Most organisms are in such good equilibrium with their environment as a result of natural selection that any change is likely to be for the worse. Mutations, however, provide the raw material for evolution j from this material, natural selection determines which of the many potential characters shall survive. It is this interaction of mutations and natural selection which gives rise to evolutionary change. It has been shown mathematically that a very small selective advantage can lead to the rapid spread of new genes in a population. Natural selection operates most strongly under rigorous environmental conditions, or where there is much competition5 under such conditions, only the most efficient forms will survive. Where there is little 'selective pressure', on the other hand, there is scope for many different varieties. In the history of man over the past few thousand years, disease has undoubtedly had a very important selective effect. During the