Inbreeding load and purging: implications for the short-term survival and the conservation management of small populations

Using computer simulations, we evaluate the effects of genetic purging of inbreeding load in small populations, assuming genetic models of deleterious mutations which account for the typical amount of load empirically observed. Our results show that genetic purging efficiently removes the inbreeding load of both lethal and non-lethal mutations, reducing the amount of inbreeding depression relative to that expected without selection. We find that the minimum effective population size to avoid severe inbreeding depression in the short term is of the order of N-e approximate to 70 for a wide range of species' reproductive rates. We also carried out simulations of captive breeding populations where two contrasting management methods are performed, one avoiding inbreeding (equalisation of parental contributions (EC)) and the other forcing it (circular sib mating (CM)). We show that, for the inbreeding loads considered, CM leads to unacceptably high extinction risks and, as a result, to lower genetic diversity than EC. Thus we conclude that methods aimed at enhancing purging by intentional inbreeding should not be generally advised in captive breeding conservation programmes.