Why is geographic isolation important? why must gene flow be stopped?

When an original population becomes seperated spatially because of geographic barriers, into two or more groups, these are termed as allopatric populations. The geographical barriers impose restrictions on the gene flow between the populations, so that the latter become reproductively isolated. These groups becomes more and more different and finally become different species. Ex : Darwin's finches of Galapagos islands are geographically isolated from related birds of South American mainland, and adaptive radiations in the Australian marsupials to form new species.