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 …
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Many (most?) sharp racial boundaries are not found along major geographic fault lines, but appear because of recent massive migrations which obliterated "intermediate" groups.
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
