> How are the traits spreading across the entire population in that 1000 years enough to make the "large difference" instead of being averaged out again? Matin

> How are the traits spreading across the entire population in that 1000 years enough to make the "large difference" instead of being averaged out again? Mating isn’t random. People are more likely to mate with those who live near them. So Southern Chinese are more like each other than Northern Chinese. Geography matters. Distance matters. If there’s reproductive isolation difference can emerge and persist. If there isn’t it won’t. Geography isn’t the only way populations stay distinct. Religion, language, means of sustenance can do it too but interbreeding can reduce difference very, very quickly as you note.