The biogeographic isolation leads to the speciation. Speciation can be defined as the formation of the new species through the evolutionary processes. The biogeographic isolation, which is the separation of a species because of the physical and biological forces leads to the evolution of the species into new, distinct ones. These new species that evolved as a result of this isolation are no longer reproductively compatible with the parent species.
Hence, the blank can be filled with the 'speciation'.