Geographical and reproductive isolation of origins gradually leads to speciation
The geographical isolation is the physical separation of two populations by the geographical barriers. This occurs through adaptive radiation and allopatric speciation. The reproductive isolation is the separation of two populations of the same species, preventing interbreeding and production of a fertile offspring.
Scientists think that geographic isolation is a common way for the process of speciation to begin: rivers change course, mountains rise, continents drift, organisms migrate, and what was once a continuous population is divided into two or more smaller populations. ... This may or may not be sufficient to cause speciation.