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veat is that the luxuries of confidence limits and certainty are ones that conservation biologists cannot now afford, given the rate of habitat de- struction documented in many of the chapters of this book. Constructive criticism is welcome, but to embrace the purist's motto of "insufficient data" is to abandon the bleeding patient on the operating table. SUMMARY 1. Conservation genetics can be applied to three distinct survival problems occurring on three time scales. In order of increasing temporal duration these problems are (1) immediate fitness; (2) adaptation within a phyletic line; and (3) speciation, the generation of evolutionary novelty by the splitting of phyletic lines. 2. Data from natural populations supports the inference that rela- tively heterozygous individuals have greater viability and, in some cases, fecundity, than relatively homozygous individuals. Within populations, relatively heterozygous individuals are observed to have high growth rates, high survivorship and greater morphological stability. In general, individuals from relatively polymorphic populations, compared to those from less polymorphic populations, seem to have similar advantages. 3. Inbreeding experiments provide another body of information rele- vant to conservation genetics. A review of the literature shows that even a small amount of inbreeding typically undermines fecundity and viabil- ity; a 10 percent increase in homozygosity may reduce total reproductive performance by as much as 25 percent. Most lines that are inbred become extinct after three to 20 generations, and the chance of surviving a bout of inbreeding may be virtually nil in some species fresh from nature. 4. Based on a rule of thumb employed by animal breeders, the basic rule of conservation genetics is that the maximum allowable rate of in- breeding is one percent, which corresponds to a genetically effective size of 50. In the world of real (in contrast to ideal) populations, several times this number may be needed. 5. The basic rule applies only to the maintenance of short-term fitness, because population sizes of this order of magnitude do not pre- vent the gradual erosion of genetic variation, and with it the raw material for future evolution. 6. Data from island populations of lizards support the thesis that nat- ural populations suffer attrition of genetic variation when their numbers drop below a few hundred. 7. Even the largest nature reserves are probably too small to guaran- tee the permanent survival of large herbivores and carnivores. 8. This century will see the end of significant evolution oflarge plants and terrestrial vertebrates in the tropics. Nature reserves are too small for the necessary isolation required by allopatric speciation. Further, it is 168