SELECTION AGAINST INBRED SONG SPARROWS DURING A NATURAL-POPULATION BOTTLENECK

THE genetic and demographic consequences of population subdivision hav e received considerable attention from conservation biologists. In par ticular, losses of genetic variability and reduced viability and fecun dity due to inbreeding (inbreeding depression) are of concern(1-3). St udies of domestic, laboratory(4,5) and zoo populations(2,6,7) have sho wn inbreeding depression in a variety of traits related to fitness. Co nsequently, inbreeding depression is widely accepted as a fact. Recent ly, however, the relative impact of inbreeding on the viability of nat ural populations has been questioned(8-10). Work on the cheetah (Acino nyx jubatus), for example, has emphasized the overwhelming importance of environmental factors on mortality in the wild(9,10). Here we repor t that song sparrows (Melospiza melodia) that survived a severe popula tion bottleneck were a non-random subset of the pre-crash population w ith respect to inbreeding, and that natural selection favoured outbred individuals. Thus, inbreeding depression was expressed in the face of an environmental challenge. Such challenges are also likely to be fac ed by inbred populations of endangered species. We suggest that enviro nmental and genetic effects on survival may interact and, as a consequ ence, that their effects on individuals and populations should not be considered independently.