This rapid dispersal creates a specific problem for models that propose a second migration, or a significant admixture event, as the explanation for Population Y.

If the Americas were peopled rapidly and if the founding population quickly differentiated into geographically distinct groups, then there was a limited window of time during which a second, distinct population could have entered South America and mixed with the existing population without leaving detectable traces in North American populations.

The rapid expansion model is not itself in question — the genetic and archaeological evidence for it is strong.

The puzzle is how to fit the Population Y signal into the framework that the rapid expansion model creates.