The clinal concept works well some places on land, but ignores how severe a barrier oceans were pre-1492. There were areas of complete genealogical disjunction for many thousands of years until, say, 1492, such as the 1,600 mile wide gap between Africa and South America.https://twitter.com/HobbesianM/status/941052954618064896 …
Many (most?) sharp racial boundaries are not found along major geographic fault lines, but appear because of recent massive migrations which obliterated "intermediate" groups.
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For example, 4,000 years ago there was a smooth cline of Bushmen ancestry stretching from S. Africa to Ethiopia. Bantu and Middle Eastern migrants moved in and today's East Africans are 0% Bushman.pic.twitter.com/XygubetPPT
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