In general, each species of fruit fly in the Hawaiian archipelago is restricted to a single island. One hypothesis to explain this pattern is that new species formed after a small number of flies colonized each new island. This mechanism is called:

Geographical isolation

Explanation:

The formation of new species by a small number of fruit flies colonized each new island. This mechanism is called geographical speciation or allopatric speciation.

The geographical speciation takes place when some members (biological populations) of the same species become isolated due to geographical changes such as mountain building or migration. Due to isolation, the organisms prevents two or more groups from mating with each other on regular basis. The gene flow between them is greatly reduced which eventually causing that formation of new species.