Inbreeding and interbreeding in Darwin's finches.

Studies of inbreeding and interspecific hybridization are generally pursued separately with different metrics. There is a need to integrate them because they have the common goal of seeking an understanding of the genetic and ecological basis of fitness variation in populations. We use mean expected heterozygosity as an axis of variation on which to compare the fitness of inbreeding and hybridizing Darwin's finches (Geospiza scandens and G. fortis) relative to the fitness of matched outbred controls. We find that relative fitness of inbred finches is less than one in the 1991 cohorts of both species. Inbreeding depression is stronger in the species (G. scandens) with the lower genetically effective population size. Relative fitness of hybrids (backcrosses) in the same cohort of G. scandens is greater than one. Evidence of heterosis in G. fortis is mixed. Thus the two interbreeding species displayed somewhat different fitness patterns under the same set of environmental conditions. Hybridization may enhance fitness to different degrees by counteracting the effects of inbreeding depression, by other additive and nonadditive genetic effects, and by producing phenotypes well suited to exploit particular ecological conditions.