I'm assuming that as populations become larger (more successful?) that the selection process becomes more diffuse. It's clear that sub-populations have evolved

I'm assuming that as populations become larger (more successful?) that the selection process becomes more diffuse. It's clear that sub-populations have evolved or acquired useful traits (particularly dietary adaptations) in relatively short timescales. From TFA it appears that the original human population in Africa was pushed out of the interior by climate changes, it's not inconceivable that those conditions also provided evolutionary pressure