The answer is
geographic isolation.When a new species form by geographic isolation, it is called
geographical speciation or allopatric speciation. Geographical isolation is a physical separation which prevents gene
flow between distinct populations (on the island and on continent). The island populations will eventually diverge from a single species that migrated to the island. The members of the populations, which once belonged to the same species, will
no longer be able to mate and produce offspring. Eventually, the formation
of new species will occur, for example kangaroos, koala, and wombats, form that one species.
