Ashkenazic and Sephardic Jews were first identified as distinct less than a millenium ago. That is fast. It took tens of thousands of years for Europeans to diverge from Africans. If Ashkenazim look different from Sephardim, it's because people living in northern Europe are more likely to marry northern Europeans, and people living in the Near East are more likely to marry Arabs.
Askenazi Jews seem to be about ~40 to 50% Southern European, the second largest component is basically classical Near Eastern (think Druze or Syrians) and they (according to the current interpretation of genetic data) seem to have been this way for centuries.
Its rather surprising that they seem to have Southern European admixture while there is only a few extra % of Eastern and Northern Admixture, they are among European peoples closest to Italians. Perhaps the mixture stabilised in the Late Roman Empire after Europe became less cosmopolitan? Perhaps bot...
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