There is actually a ton of population discontinuity: Central Asian highlands, Sahara and Arabia, the oceans Even the political borders which marked the edges of the Caliphate (Arab, African admixture) and the Middle Kingdom (Han) show pretty sharp discontinuities https://twitter.com/DesertFoxMulder/status/941140289439305728 …
discontinuities: you don't need zero flow for a discontinuity, just a gradient which suddenly gives way to a cliff https://www.biorxiv.org/content/early/2017/12/13/233486 …pic.twitter.com/NhwDikHMjn