Answer:
D. Total separation of two populations by a physical barrier
Explanation:
Allopatric speciation is a result of reproductive isolation where the reproductive isolation is brought about by a physical barrier. This physical barrier can be a geographical change like building of a mountain between to populations or permanent emigration of some members of population to some distant places. In such cases the gene flow between the two populations does not occur and the two populations evolve as two new species. Thus total separation of the two populations by a physical barrier is a primary requirement for the allopatric speciation to occur.
