(Ethnogenesis, restricted gene flow and isolation by distance) The evolutionary model that posits humans evolved as an interconnected polytypic species from a single origin in Africa. The small population effects during initial colonization outside Africa, and adaptations to local conditions, helped establish regional differences, which were subsequently maintained through isolation-by-distance and adpative variation. Advantageous changes spread widely because of genic exchanges and the common background of the evolving cultural system whose elements could also spread.