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is taken up in Chapter 8. It will be mentioned here only briefly. The fol- lowing section is, in part, abstracted from a more comprehensive account of conselvation genetics (Frankel and Soule, in press, Chapter 3). SHORT-TERM FITNESS AND SURVIVAL Many conservation programs are holding actions, particularly those involving small numbers of individuals in captive or controlled environ- ments. Intensive projects of this type are often necessary until permanent survival programs can be established. Such programs, though, have a built-in hazard-genetic drift (random genetic change and the fixation of deleterious genes or gene combinations). Breeders of plants and animals have learned through experience that vigor, fecundity and other aspects of fitness decline at a rate proportional to the degree of random genetic change, and that this in turn is inversely related to population size. Therefore, the goals of such captive breeding programs must be to (1) minimize genetic and phenotypic deterioration and change and (2) to minimize the loss of genetic variation so that future adaptive options are retained. The cost of ignoring these objectives is almost certain failure ( extinction). The central problem of short-term conservation genetics is the rela- tionship between population size and fitness. Two biological disciplines converge on this problem and contribute to its solution; they are empiri- cal population genetics and quantitative genetics. Quantitative genetics, the oldest of the two, provides a wealth of data on the relationship be- tween inbreeding and fitness in domesticated or laboratory lines. These data permit us to generalize about the effect on fitness produced by dif- ferent rates and amounts of inbreeding. AI> discussed below, the key to our problem is found in this literature. To proceed on this basis alone, however, and extrapolate from the inbreeding effects produced in the lab- oratory or on the farm to the consequences of genetic drift and inbreeding in organisms fresh from nature, is to be nagged by certain doubts. A more satisfying approach, the one used here, is to demonstrate that the fitness of individuals in both captive and natural populations is related to their relative levels of heterozygosity (proportion of heterozygous loci). The following section explores the qualitative evidence for a relationship be- tween population size and fitness in nature. Heterozygosity and Fitness in Natural Populations Hundreds of electrophoretic studies and surveys have been performed, but only a fraction are useful in addressing whether a decrease in heter- ozygosity (or genetic variance) in a natural population will lead to a dimi- nution of fitness. Nevertheless, there now exist several such studies, and they provide us with a nearly unanimous answer (notwithstanding that 152