The founding population of the Americas was not a single band at a single moment.

It was a set of people — probably numbering in the thousands, possibly more — who had been living in Beringia and adjacent parts of northeastern Asia for an extended period, accumulating the specific genetic character that distinguishes Native Americans from their Asian relatives, and gradually differentiating into subgroups through the ordinary processes of population structure that operate in any large, geographically distributed population.

Within the founding population, some groups were in regular contact with others and therefore maintained gene flow — the exchange of genetic material through interbreeding that keeps populations genetically connected.

Other groups became more isolated, either through geographic barriers or through the specific patterns of movement across the broad Beringian landscape, and began to accumulate distinct genetic signatures through the process of genetic drift.