Final answer:
The absence of gene flow between two populations can lead to allopatric speciation, where populations evolve independently and may develop significant differences, possibly leading to the formation of different species.
Step-by-step explanation:
The student's question alludes to the concept of gene flow, which is a critical factor in the multiregional hypothesis of human evolution. This hypothesis predicts constant gene exchange between populations. However, if no evidence for gene flow between two regions exists, then this does not support the multiregional argument. Instead, it fits more with the concept of allopatric speciation, where a barrier like a new lake divides two populations, thus preventing gene flow and leading to the emergence of different species due to independent evolutionary paths. These pathways could include mutations, natural selection, and genetic drift affecting allele frequencies in differing environments. Without gene flow, populations can develop abrupt phenotype differences, and over time, these differences can become significant enough to lead to speciation.