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mately preserve many large species on purely genetic and evolutionary grounds. This conclusion buttresses the empirically based conclusion of biogeographers that extinction rates are. inversely correlated with area. THE END OF VERTEBRATE EVOLUTION IN THE TROPICS We now turn from the issues of fitness and evolutionary adaptation to the third level of biological survival-speciation, the capacity of evolu- tionary lines to generate new species. Speciation is the splitting of evolv- ing lineages. Many current evolutionists consider speciation to be the principal source of biological novelty, relegating the role of gradual phy- letic change to fine tuning of adaptations within already established spe- cies (Gould and Eldredge, 1977; Stanley, 1975). According to this thesis, speciation is the major, if not the only, process by which significant evo- lutionary changes come about. As a generator of diversity, therefore, spe- ciation emerges as preeminent. The question posed here is whether tropical nature reserves will be large enough to permit speciation in higher vertebrates and plants. I em- phasize the tropics because probably 90 percent of species are tropical, and also because the rate of destruction of tropical habitats (Chapter 17) means the imminent insularization of tropical floras and faunas. Fortunately, this question-the minimum area required for speciation -can be solved empirically. All that is necessary is to determine the size of the smallest island on which a particular taxon has speciated autoch- thonously (in situ)-that is, where there is sufficient room for a species to split into two or more species. For example, the occurrence of endemic genera and families of lemurS, chamaeleons, birds and plants on Mada- gascar is evidence that these taxa have speciated on that island. It must be understood, however, that most island radiations are not autochthonous in the sense meant here. Most insular radiations occur on archipelagos, water gaps between islands being the barrier to gene flow. The Darwin finches on the Galapagos, for example, did not speciate on a single island. Islands the size of these in the Galapagos archipelago are too small to have physical features large enough to isolate conspecific populations of birds within an island. Speciation by multiple invasion, like speciation in an archipelago, is not relevant to the problem at hand. This form of speciation requires that an island be invaded at least twice by the same taxon. For example, two endemic species of lizard in the roquet group of the genus AnoUs occur on the island of Grenada in the Caribbean, but this is the result of two sepa- rate colonizations (Yang et at, 1975). Figure 4 portrays the sizes of the smallest islands on which particular taxa are known to have speciated by processes occurring exclusively on the island. In simple terms, it appears that large or vagile organisms re~ quire much more space to produce two or more contemporaneous species 164