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were ignorant of the lethal dose. That is, enough is known about inbreed- ing to justify concern that any increase in homozygosity reduces absolute vigor and fecundity, but so far we have no way of deciding how much is tolerable. We can not even pinpoint with any certainty the cause of in- breeding depression, although we suspect that this loss of fitness is mostly attributable to a load of deleterious recessive genes and to altered gene interactions rather than to a loss of single gene heterosis concomi- tant with decreasing heterozygosity (Eberhart, 1977; Comstock, 1977; Falconer, 1977; Singh and Zouros, 1978). Finally, we know that a partial antidote for the accumulation of such deleterious genes during inbreeding is the winnowing effect of natural selection. Our problem is that we lack data on the relative strengths of the poison (genetic load) and the anti- dote (natural selection). We cannot look to population genetic theory and models for help be- cause the models for finite populations require several assumptions (about mutation rates, gene frequencies, selection coefficients) about which we can only guess. In addition, one must decide the mode of selec- tion (dominance, overdominance, or even neutrality, a paradoxical choice). Clearly, some more down to earth approach is needed. Perhaps the best way to proceed is to side-step theory and resort to empiricism. By trial and error, animal breeders have discovered how much in- breeding can be tolerated by domestic animals before the lines begin to decline in performance and fertility. Their rule of thumb is that the per generation rate of inbreeding should not be higher than two or three per- cent (Dickerson et aI., 1954; Stephenson et aI., 1953). Higher rates of in- breeding fix deleterious genes too rapidly for selection to eliminate them from the line. I prefer a slightly more conservative value, an upper limit of one percent inbreeding per generation. The reasons are (1) domesti- cated stocks have been partially purged of deleterious genes over the mil- lenia, so they can tolerate higher rates of inbreeding than can outbreeding species fresh from the wild, and (2) animal breeders can safely ignore some classes of phenotypic change resulting from inbreeding and genetic drift, whereas conservationists wish to preserve the "wild- type". That is, conservationists may have to practice selection on more traits than do animal breeders in order to preserve the original pheno- type, and the rate of inbreeding must be decreased if the number of traits being selected increases. How does this basic rule (the one percent rule) translate into popula- tion size? The rate ofloss of heterozygosity (increase in homozygosity) or f per generation is equal to 1/2~, where ~ is the effective population size. Thus, Ne must equal 50 or more if the inbreeding rate is to be kept below the one percent threshold. Even at the rate of one percent, however, the loss of genetic variation is appreciable after a few generations. That is, even though natural selec- tion may be able to nullify the most hannful inbreeding effects at this 160