Answer :
Speciation can be difficult to observe in nature, but the it is hypothesized that the following occurs in a allopatric speciation's intermediate stage:
(1) closely related species are differentiated through geographic variation (2) then there's reduced fertility in one a geographic race of the species (3) and in the geographic race of subspecies that are to become species.
(1) closely related species are differentiated through geographic variation (2) then there's reduced fertility in one a geographic race of the species (3) and in the geographic race of subspecies that are to become species.