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Why are the Neotropical Australian and Antarctic regions sometimes treated as separate biogeographic realms?

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World map using color coding to show the distribution of major floral kingdoms, subkingdoms, and regions. Key floral kingdoms include Boreal (green), Neotropical (pink), Paleotropical (yellow), South African or Capensic (purple), Australian (orange), and Antarctic (blue). Boundaries for subkingdoms, regions, and divisions are marked with different colored lines, and major regions such as Indoafrican, North American, and Malesian are labeled across continents. — biogeographic region Floral kingdoms, subkingdoms, and major regions of the world.

Some zoogeographers classify the Neotropical, Australian, and Antarctic regions as separate realms because their fauna and flora are so distinct from those of other regions that they warrant a higher level of classification.1

This distinction is rooted in historical biogeography, which considers the long-term effects of continental drift and isolation. While these regions were once connected as part of the supercontinent Gondwana, they separated and drifted apart over millions of years.234

Isolation: The Australian and South American (Neotropical) regions are noted for their high degree of isolation, which has fostered the development of a large number of unique species.5
Antarctic Separation: Antarctica became isolated as it drifted poleward and was cut off from other southern continents by the opening of the Drake Passage, leading to a harsh climate that severely limited terrestrial biotic evolution.34

Because these regions evolved in relative isolation following the breakup of Gondwana, they possess unique biological characteristics that differentiate them from the more interconnected regions of the Northern Hemisphere.134