@134318c2 Sounds like they're working with "effective population size". It's more like a value in their simplified models than reality. Something like the minimum number of people needed to contain the observed genetic variation, assuming homogeneous mixing in the whole population. The effective population size of today's Tuscans in Italy for instance, is about 4000, with the real population being around 4 million. The effective population size of Tokyo is just 3500.  https://www.cambridge.org/core/journals/genetics-research/article/effective-population-size-of-current-human-population/E767DDCB8E895844FA35C9C44FA8B62F