It may well be true that evolution has, through natural selection, ensured the survival of brown genes in the hot countries, while favouring pale skins in gloomier climes to maximise the ability to synthesise vitamin D
But genes are expressed in the environment in which they find themselves. Even brown skinned people in tropical countries turn noticeably paler when deprived of the sun during the rainy season
Genetic hardliners are having to come to terms with the fact that the way in which genes learn to express themselves, for example in babies conceived during famine or times of extreme shortages of food, may affect progeny
So the descendants of brown people who move to gloomy countries may well become pale, even without an admixture of ‘white’ genes
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The physical distinctions of race are produced by differences of climate & peculiarities of region, operating during many centuries. There are probably few natural divisions of the surface of the earth which have not a tendency to mould distinct types of men by the continual influences of sun & soil. However long a people may have lived in any particular country, however indelible may seem the constitutional bent which they may have received from its conditions, they cannot become dwellers in a foreign land without slowly & inevitably growing into the likeness in shape & colour of its original & indigenous inhabitants
WA O'Connor: History of the Irish People 1876
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