Rapid genetic deterioration in captive populations: causes and conservation implications

Many species require captive breeding to ensure their survival. The eventual aim of such programs is usually to reintroduce the species into the wild. Populations in captivity deteriorate due to inbreeding depression, loss of genetic diversity, accumulation of new deleterious mutations and genetic adaptations to captivity that are deleterious in the wild. However, there is little evidence on the magnitude of these problems. We evaluated changes in reproductive fitness in populations of Drosophila maintained under benign captive conditions for 50 generations with effective population sizes of 500 (2 replicates), 250 (3), 100 (4), 50 (6) and 25 (8). At generation 50, fitness in the benign captive conditions was reduced in small populations due to inbreeding depression and increased in some of the large populations due to modest genetic adaptation. When the populations were moved to 'wild' conditions, all 23 populations showed a marked decline (64-86%) in reproductive fitness compared to controls. Reproductive fitness showed a curvilinear relationship with population size, the largest and smallest population size treatments being the worst. Genetic analyses indicated that inbreeding depression and genetic adaptation were responsible for the genetic deterioration in 'wild' fitness. Consequently, genetic deterioration in captivity is likely to be a major problem when long-term captive bred populations of endangered species are returned to the wild. A regime involving fragmentation of captive populations of endangered species is suggested to minimize the problems.