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evidence that increased body size and metabolic requirements result in higher extinction rates in vertebrates and Terborgh and Winter in Chap- ter 7 argue that population size or density is the best predictor of extinc- tion in birds; thus on small islands, the probability of extinction for large organisms is very high. Assuming that their generalizations hold up, it means that the apparent thresholds for autochthonous speciation could, at least in part, be attributable to thresholds for speciation-extinction equilibria for numbers of species greater than one per genus.) In practice it makes little difference whether the area effect on specia- tion in Figure 4 is a reflection of taxon-specific thresholds for autochtho- nous speciation, or is a reflection of thresholds for speciation-extinction equilibria. Even if it is the latter, which I think is unlikely, it means that existing nature reserves are still too small to generate more species than they now contain. In fact, they will contain fewer and fewer species with the passage of time because most reserves are now supersaturated and will lose species rapidly over the next few centuries (Chapter 6). It there- fore appears inescapable that, for the first time in hundreds of millions of years, significant evolutionary change in most higher organisms is coming to a screeching halt in the tropics. One might object that this entire argument is misleading on the grounds that reserves can support speciational processes because of the isolation between them (that is, on the model of archipelagos). The mini- mum requirements for this kind of speciation is that a species occur in two or more reserves, and that the reserves be isolated long enough for the evolution of intrinsic isolating mechanisms such as differences in habitat preferences or post-mating reproductive barriers. Superficially, this model seems reasonable, assuming that reserves are continuously via- ble for thousands of years. The flaw in this line of thought is that it ignores the high rate of extinction of large organisms on islands. Figure 7 in Chapter 6 shows that we cannot count on the species surviving long enough to speciate, even in the largest tropical reserves. In fact, the extinction rate is so high that half of the original large mammals species in such a reserve are expected to go extinet in from 500 to 2000 years unless, that is, man intercedes. The irony is that such intercession itself will prevent speciation of this type from occurring. The prevention of extinction of such animals will require the exchange of individuals between the populations in the differ- ent reserves, this being the only available means of preventing genetic deterioration of the largest or least dense specieS. Large doses of gene flow, of course, are inimical to incipient speciation and the development of locally adapted races because it turns the subdivided species into a genetic omelette. But even if genetic deterioration were not a factor and gene flow was not a necessary preventative therapy, artificial migration would still be necessary in most instances in order to prevent stochastic 166