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In turn, that means reproductive isolation is not the best definition of a species, and speciation does not require allopatry (species to be reproductively separated).
Isolating mechanisms are things which prevent successful breeding between groups in a species.
In about 6,500 sq mi (17,000 km), the Hawaiian Islands have the most diverse collection of drosophilid flies in the world, living from rainforests to mountain meadows. Studies show a clear "flow" of species from older to newer islands.Each genus is split into a number of species, each of which is confined to a stretch of the north coast between rivers which descend from the Atlas Mountains to the Mediterranean.As soon as one crosses a river, a different but closely related species appears."...The ability to analyse the DNA sequence of living things has shown that there is often quite a lot of hybridisation between related species.That means genes have been transferred by these naturally occurring crosses.