You searched for: “allopatric speciation
allopatric speciation (s) (noun), allopatric speciations (pl)
The differences of populations in geographical separations to the point where they are recognized as isolated species: "Allopatric speciation exists when two biological populations of the same species become separated as a result of geographical changes or population dispersal and whose areas of existence are entirely disunited to such a degree that they do not occur in any one place together."

"Allopatric speciations involve changes that take place with related organisms to the point where they are different enough to be considered separate species and this happens when populations of certain species are separated and adapt to their new environment or conditions (physiological, geographic, or behavioral)."