A pretty common example of geographic isolation is a population migrating to an island and becoming separated from the mainland population. This stops the gene flow between the groups of organisms. Over time, the two populations become reproductively isolated and they evolve separately.
OR
When a population is separated because of a geographic isolation , like distance, a canyon, a river, or a mountain range, those two subgroups of the population are no longer able to reproduce together which resulted in speciation.
