Inbreeding and extinction: Effects of rate of inbreeding

Deleterious alleles may be removed (purged) by natural selection in populations undergoing inbreeding. However, there is controversy regarding the effectiveness of selection in reducing the risk of extinction due to inbreeding, especially in relation to the rate of inbreeding. We evaluated the effect of the rate of inbreeding on reducing extinction risk, in populations of Drosophila melanogaster maintained using full-sib mating (160 replicates), or at effective population sizes (N_e) of 10 (80) or 20 (80). Extinction rates in the populations maintained using full-sib mating occurred at lower levels of inbreeding than in the larger populations, whereas the two larger populations did not differ significantly from each other. Inbreeding coefficients at 50% extinction were 0.62, 0.79 and 0.77 for the full-sib (N_e = 2.6), N_e = 10 and N_e = 20 treatments, respectively. Populations of N_e = 20 that remained extant after 60 generations, showed inbreeding depression, with the mean fitness of these populations being only 45% of the outbred controls. There was considerable variation among the 31 inbred populations in fitness, but none of the N_e = 20 populations had fitness that was higher than the outbred control. We conclude that purging may slow the rate of extinction slightly, but it cannot be relied on to eliminate the deleterious effects of inbreeding.