All tropical forest regions contain related organisms, for reasons of biogeography. Hundreds of millions of years ago, almost all land was in the form of one large continent, Pangaea. Plants and animals both were widely distributed across this continent, with few geographical barriers to impede their dispersal. More than 200 million years ago, this land mass began to break up into two parts: Laurasia (which would eventually disintegrate into North America, Europe, Asia, Greenland and Iceland) and Gondwanaland (later to break up to become South America, Africa, Australia, Antarctica and India). Most current tropical areas arose from Gondwanaland, and, as the evolution of flowering plants had begun before its breakup, there are many similarities among the plants in tropical forest areas. Today there are more than 300 pantropical (i.e., appearing in all tropical areas) flowering plant genera and almost 60 pantropical families. Moreover, plants at high elevations, wherever they are, resemble each other more than they do plants in the lowlands of their own area. This is what Terborgh (1992a) calls “global parallelism” and indicates the conservative persistence of groups for millions of years.
However, there are also many regional differences within tropical areas because much evolution has occurred since Gondwanaland broke up. Southeast Asia has many conifers, while there are only two species in the New World tropics and one in Africa; dipterocarps are found only in Southeast Asia, and so on (Whitmore, 1995). An interesting case in point is that of Malesia. When one part of Gondwanaland moved north, it collided with a part of southern Laurasia, and these merged to create what is now called Malesia. Since Gondwanaland and Laurasia both had unique sets of flora and fauna, western and eastern Malesia, as their “descendants,” also have very different organisms. This abrupt demarcation between types of organisms in Southeast Asia is known as “Wallace’s line” (after Alfred Russel Wallace, who explored this region in the 19th century), and lies between the island of Lombok in Indonesia and islands farther east, such as Sumba, Timor, The Moluccas, and New Guinea. Dipterocarps, the huge hardwood trees so prominent in Southeast Asian forests, are found from the Malay Peninsula west through Sri Lanka, and fossils of such species have been found in East Africa, but they are not found east of the Wallace Line. Pitcher plants (Nepenthes) are also found from Madagascar to the Malayan peninsula but not farther east. But organisms in Madagascar have relatives in India and South America, so these areas must have been linked for a considerable period of time after the initial breakup of Gondwanaland. The plants of eastern and western Malesia are less sharply demarcated than are the animals, partly because flowering plants arose before the breakup of Pangaea (mentioned above) and partly because they can disperse over long distances, even across large bodies of water. In South America, the emergence of the Andes mountains separated east and west portions of the continent, and today the coastal forests and the Amazon basin have quite different species.