Geographic isolation

Spatial or geographical isolation is the kind of isolation that prevents interbreeding between allopatric people because they are physically separated. This isolation persists for a long time and could lead people/populations to differentiate morphologically in response to check for different environments. If the geographic barrier disappeared two situations may happen:
a) populations may return to interbreeding thus forming a single population;
b) whether the geographical isolation continues for a sufficiently long period, populations may differentiate such that the interbreeding between them is no longer possible, thus appearing reproductive isolation.