Conservation managers typically need to make prompt decisions based on limited information and
resources. Consequently, generalisations have essential roles in guiding interventions. Here, we (i) critique
information on some widely accepted generalisations and variables affecting them, (ii) assess
how adequately genetic factors are currently incorporated into population viability analysis (PVA) models
used to estimate minimum viable population sizes, and (iii) relate the above to population size thresholds
of the IUCN Red List criteria for threatened species that were derived from genetic considerations.
Evidence accumulated since 1980 shows that genetically effective population size (Ne) = 50 is inadequate
for preventing inbreeding depression over five generations in the wild, with NeP100 being required to
limit loss in total fitness to 610%. Further, even Ne = 500 is too low for retaining evolutionary potential for
fitness in perpetuity; a better approximation is NeP1000. Extrapolation from census population size (N)
to Ne depends on knowing the ratio of Ne/N, yet this information is unavailable for most wild populations.
Ratio averages (0.1–0.2) from meta-analyses are sufficient, provided adjustments are made for dissimilar
life histories. Most PVA-based risk assessments ignore or inadequately model genetic factors. PVA
should routinely include realistic inbreeding depression, and genetic impacts on evolutionary potential
should be incorporated where appropriate. Genetic generalisations used in conservation, the treatment
of genetics in PVAs, and sections of the IUCN Red List criteria derived from genetic considerations, all
require revision to be more effective conservation tools.