Wallace's line revisited and explained
2004
van Welzen, P.C.(National Herbarium Nederland Leiden Universiteit branch, P.O. Box 9514, 1300 RA Leiden (The Netherlands))E-mail:[email protected]
The Malay Archipelago, also known as Malesia, comprises the countries Malaysia, Singapore, Indonesia, Brunei, the Philippines, and Papua New Guinea. The flora analysis used are based on the c. 6,000 (out of an estimated 30,000) species described in the Flora Malesiana Project so far. About 3/4th of the Malesian flora is endemic, with the major concentrations of endemic species on New Guinea, Borneo and the Philippines. Two major blocks of species are present, in the west the Asian influence on the Sunda Shelf, in the East the New Guinean-Australian influence on the Sahul Shelf. The blocks are divided by the Wallace line (or derivatives of it). Four reasons are provided why the Wallace line is so apparent: 1. the east Malesian island fragments rifted quite late towards Southeast Asia to form stepping stones for dispersalists; 2. most fragments in the central part of Malesia rifted submerged, only to emerge after collisions and thus they did not act as rafts for biodiversity; 3. the central Malesian part has a different climate , not everwet but with a dry monsoon; and 4. the sea straits in central Malesia remained intact as barriers during glacial periods. The different glacial and interglacial periods caused contractions, expansions and speciation in most genera. Phylogenies of everwet species show repeating patterns of speciation on the Sunda Shelf and species with a preference for a dry monsoon show a disjunct distribution nowadays: Southeast Asia mainland and central Malesia.
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