Final answer:
Geographical isolation for many generations commonly leads to divergent evolution and reproductive isolation between two populations, often resulting in them being unable to produce viable offspring if they come into contact again. This process is called allopatric speciation.
Step-by-step explanation:
When two populations have been geographically isolated from each other for many generations, they can evolve along different trajectories, resulting in divergent adaptations and reproductive isolation. The separation impedes gene flow, allowing for independent mutations and distinct natural selection pressures to change allele frequencies at numerous genetic loci. This process is known as allopatric speciation. Over time, these mechanisms can lead to each population becoming reproductively and genetically independent, potentially to the point where they are unable to produce viable offspring with each other.
Although two populations might initially be able to interbreed producing hybrids in a hybrid zone, if the hybrid offspring have reduced fitness, natural selection may favor reproductive barriers, enhancing separation. Conversely, if these hybrids are viable and successful, this could indicate the two populations are not yet fully reproductively isolated and could still be considered the same species.