As stated above, there are three major racial stocks in the world. Besides, there are some races which do not fall clearly within a specific race category. These races constitute doubtful classification.
But, when we strictly examine a particular race, there arises a problem. It would be wrong to say that there is any pure race. There has been so much of inter-mixture that we do not find any pure type.
There also exist some hybrid races. In the case of caste, in India, we have a similar concept of varnashankar, according to which if a Brahmin marries a Sudra woman, the offspring of the two falls in the category of varnashankar, that is, a mixture of two castes. Thus, there are some difficulties in the racial classification. These are described as below:
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(i) Gene Mutation:
The contemporary advances in medical science are characterized by genetic engineering. According to this, if the genes are changed, the human body, that is, its whole physiology, would undergo changes.
Racial composition or characters undergo change by gene mutation. This is a change in the gene structure of the reproductive cell that influences bodily development.
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(ii) Natural Selection:
Darwin has explained evolution natural selection. In order to survive in a particular environment, the human body adapts itself to the circumstances. This brings out a change in racial composition.
Precisely, genes that produce variations and have survival value in a particular environment, will rapidly establish themselves. Thus, albinism offers no advantage and is rare, but heavy skin pigmentation is advantageous in the tropics and spreads quickly.
(iii) Genetic Drift:
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If a migrant group by chance bears characters unlike those of the larger group from which it separates, then in the course of time, specially, if further splitting occurs, a population different from the ancestral population would emerge. The American Indians have almost certainly lost some of the traits of their Mongoloid ancestors in this way.
(iv) Population Mixture:
Today, it is common to find racially distinct populations migrate from one place to another. This results in population mixture. It may also be called inter-breeding. Most existing racial groups today are of this kind.