When even higher resolution genetic mapping was applied to the people of Britain in 2015, families who had lived in Devon for multiple generations could be distinguished from the people of Cornwall, and when these precise differences were plotted on a map the boundary was the River Tamar, which for centuries has effectively been the county line. When the same technique was applied to the Iberian Peninsula in 2019, it showed vertical stripes of similarity, revealing that Spain’s history meant that people are fractionally but measurably more similar in a north–south axis than they are east to
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