Final answer:
Gene flow is restricted when a physical barrier separates two populations, potentially leading to allopatric speciation as the populations evolve independently.
Step-by-step explanation:
Gene flow between two populations will be restricted if a physical barrier is present. A geographically continuous population typically has a relatively homogeneous gene pool, as individuals can move and mate with others, facilitating gene flow. When populations are separated by a physical barrier, such as the formation of a new lake, this movement of alleles is hindered. Over time, this can lead to allopatric speciation, as the separated populations evolve along different trajectories, with allele frequencies at numerous genetic loci diverging due to factors such as mutation, natural selection, and genetic drift.