For practical reasons, managers and policy-makers have to make rapid decisions with limited information on the status of endangered species. Because of its relative ease of acquisition, the most common surrogate used to derive inferences about the risk of extinction is population size. This is not to say that population size is the best indicator of risk; other considerations, such as the recent rate of population decline, are of clear importance (Mace & Lande, 1991). A popular rule of thumb for the critical population size necessary for the maintenance of adequate genetic variance for adaptive evolution in quantitative traits, originally espoused by Franklin (1980) and Soulé (1980), has been an effective population size (Ne) of 500 individuals. More recent assessments, based on both empirical and theoretical developments, suggest that this number should be revised upwards to Ne≃1000–5000 (Lande, 1995a; Lynch, 1995; National Research Council, 1995; Bürger & Lynch, 1997). Here we address the arguments of Franklin & Frankham (1998) that a critical Ne≃500–1000 is adequate for conservation purposes.