Dynamics of inbreeding depression due to deleterious mutations in small populations: mutation parameters and inbreeding rate

A multilocus stochastic model is developed to simulate the dynamics of muta tional load in small populations of various sizes. Old mutations sampled fr om a large ancestral population at mutation-selection balance and new mutat ions arising each generation are considered jointly, using biologically pla usible lethal and deleterious mutation parameters. The results show that in breeding depression and the number of lethal equivalents due to partially r ecessive mutations can be partly purged from the population by inbreeding, and that this purging mainly involves lethals or detrimentals of large effe ct. However, fitness decreases continuously with inbreeding, due to increas ed fixation and homozygosity of mildly deleterious mutants, resulting in ex tinctions of very small populations with low reproductive rates. No optimum inbreeding rate or population size exists for purging with respect to fitn ess (viability) changes, but there is an optimum inbreeding rate at a given final level of inbreeding for reducing inbreeding depression or the number of lethal equivalents. The interaction between selection against partially recessive mutations and genetic drift in small populations also influences the rate of decay of neutral variation. Weak selection against mutants rel ative to genetic drift results in apparent overdominance and thus an increa se in effective size (N-e) at neutral loci, and strong selection relative t o drift leads to a decrease in N-e due to the increased variance in family size. The simulation results and their implications are discussed in the co ntext of biological conservation and tests for purging.