Diethard Tautz , Michael Doebeli , and Johan

When Terry Erwin from the Smithsonian National Museum of Natural History examined the diversity of beetles that lived on a single species of tropical trees, he found 682 different beetle species, 163 of which he classified as specialist species that lived exclusively on the particular tree species used in his study. Since there are around 50 000 tropical trees species, Erwin extrapolated that there must be on the order of 7 million specialist beetle species (Erwin 1982). Using similar extrapolations, Erwin (1982) also estimated the total number of tropical arthropod species as about 30 000 000. While these estimates may be too high (Schilthuizen 2000; Ødegaard 2000; Novotny et al. 2002), they are mind-boggling nevertheless and serve as an illustration of the incredible amount of species diversity that exists on our planet: estimates for the total number of extant species of plants and animals range from 10 million to 100 million (May 1990; Schilthuizen 2000). It is also estimated that the number of extant species represents only about 1% of the total number of species that ever existed during the history of life on earth. Together with the common phylogenetic ancestry usually inferred for the tree of life for higher organisms, this implies that speciation must have been truly rampant during the creation and evolution of our biosphere. Indeed, there was ample space and time for the evolutionary generation of diversity. After all, the past 3.5 billion years, during which life presumably evolved from some self-replicating molecules, has seen large-scale geographic changes, including the rise, shift, and disappearance of continents and oceans. For example, the fauna and flora of Australia is in many ways very different from that of any other place in the world, presumably because of the long-lasting geographic isolation of this continent from other landmasses.