Wallace Line

The complex biogeography of the Indo-Australian Archipelago is a result of its location at the merging point of four major tectonic plates and other semi-isolated microplates in combination with ancient sea levels.

[1] He proposed a line to the east of Bali since "all the islands eastward of Borneo and Java formed part of an Australian or Pacific continent, from which they were separated.

"[3] The proposal of the line, however, was not the main objective of Wallace's endeavours: His primary purpose was in fact to understand the geological phenomena and the colonization events that caused the boundaries in faunal distribution in the region through the development of his theories of evolution and biogeography.

[4] On the other hand, the lack of knowledge of tectonic plates, and the uncertainty about biodiversity in the Philippines, left Wallace with some contradicting points he had to deal with concerning his theory on biogeography.

[1] Because of the complexity of the geographical landscape and the differences in diversity of organisms around the archipelago, continuous attempts to characterize faunal and botanical boundaries were carried out after Wallace.

[1] More recent work assessing biodiversity assemblages, phylogeny, and using computer-based geospatial tools to analyze previous boundaries have led to patterns of division similar to those proposed through the 19th century, although some special cases not explained before are reinforced by these modern analyses.

Consequently, for over 50 million years, deep water between those two large continental shelf areas created a barrier that kept the flora and fauna of Australia separated from those of Asia.

[10] Alternatively, "Weber's line" runs through this transitional area (to the east of center), at the tipping point between Asian species against those with Australian origins.

On the Australian side, many species of marsupials are present, and some monotremes, alongside native rodents—although the occurrence of rodents in this case is derived from more recent colonization events.