Abstract

Management of nature reserves, of multiple-use lands, and of captive breeding programs requires knowledge of the minimum population sizes below which the combined effects of random genetic changes and demographic variation would likely result in extinction. One prerequisite to estimating such minimum viable population sizes is the determination of the effects of inbreeding on fitness. Two hypotheses make distinct predictions about the relative tolerance of populations to inbreeding: If inbreeding depression results primarily from the expression of deleterious recessive alleles, then selection would have removed most such genes from populations with long histories of inbreeding, and those populations would be resistant to further inbreeding impacts. If inbreeding depression occurs because of a general selective advantage of heterozygosity throughout the genome, then previously inbred populations would have reduced fitness presently and would fare no better under future inbreeding than would large and heterogeneous populations. We tested the hypothesis that small, isolated populations of Peromyscus mice would show less depression in fitness when inbred than would large, central populations. Remnant, insular populations had one-quarter to one-third the genie diversity of large, central populations. Although the populations varied greatly in the rate of loss of fitness when experimentally inbred, the severity of inbreeding depression did not correlate with initial genie diversity of the stocks or, therefore, with the size and degree of insularity of the wild populations. Neither simple theory of inbreeding depression could account for the varied responses of the populations. It remains an important task for conservation biologists to discover phylogenetic, ecological, or genetic predictors of genetically minimum viable population sizes.