• endangered species;
  • evolutionary potential;
  • genetic diversity;
  • heterozygosity;
  • inbreeding depression;
  • population management

Is this short review we explore the genetic threats facing declining populations, focusing in particular on empirical studies and the emerging questions they raise. At face value, the two primary threats are slow erosion of genetic variability by drift and short-term lowering of fitness owing to inbreeding depression, of which the latter appears the more potent force. However, the picture is not this simple. Populations that have passed through a severe bottleneck can show a markedly reduced ability to respond to change, particularly in the face of novel challenges. At the same time, several recent studies reveal subtle ways in which species are able to retain more useful genetic variability than they ‘should’, for example by enhanced reproductive success among the most outbred individuals in a population. Such findings call into question the validity of simple models based on random mating, and emphasize the need for more empirical data aimed at elucidating precisely what happens in natural populations.