The variance in inbreeding depression and the recovery of fitness in bottlenecked populations

Theoretical analyses of inbreeding suggest that following an increased degr ee of inbreeding there may be a temporary recovery of fitness, because of s election either within or among inbred lineages. This is possible because s election can act more efficiently to remove deleterious alleles given the g reater homozygosity of such populations. If common, recovery of fitness fol lowing inbreeding may be important for understanding some evolutionary proc esses and for management strategies of remnant populations, yet empirical e vidence for such recovery in animals is scant. Here we describe the effects of single-pair population bottlenecks on a measure of fitness in Drosophil a melanogaster. We compared a large number of families from each of 52 inbr ed lines with many families from the outbred population from which the inbr ed lineages were derived. Measures were made at the third and the 20th gene rations after the bottleneck. In both generations there was, on average, su bstantial inbreeding depression together with a highly significant variance among the inbred lines in the amount of fitness reduction. The average fit ness of inbred lines was correlated across generations. Our data provide ev idence for the possibility of recovery of fitness at two levels, because (i ) the average fitness reduction in the F-20 generation was significantly le ss than in the Fg generation, which implies that selection within lines has occurred, and (ii) the large variance in inbreeding depression among inbre d lines implies that selection among them is possible. The high variance in inbreeding depression among replicate lines implies that modes of evolutio n which require a low level of inbreeding depression can function at le:ast in a fraction of inbred populations within a species and that results from studies with low levels, of replication should be treated with caution.