Data from: Unexpected positive and negative effects of continuing inbreeding in one of the world’s most inbred wild animals

Inbreeding depression, the reduced fitness of offspring of related
individuals, is a central theme in evolutionary biology. Inbreeding
effects are influenced by the genetic makeup of a population, which is
driven by any history of genetic bottlenecks and genetic drift. The
Chatham Island black robin represents a case of extreme inbreeding
following two severe population bottlenecks. We tested whether inbreeding
measured by a 20-year pedigree predicted variation in fitness among
individuals, despite the high mean level of inbreeding and low genetic
diversity in this species. We found that paternal and maternal inbreeding
reduced fledgling survival and individual inbreeding reduced juvenile
survival, indicating that inbreeding depression affects even this highly
inbred population. Close inbreeding also reduced survival for fledglings
with less-inbred mothers, but unexpectedly improved survival for
fledglings with highly inbred mothers. This counterintuitive interaction
could not be explained by various potentially confounding variables. We
propose a genetic mechanism, whereby a highly inbred chick with a highly
inbred parent inherits a “proven” genotype and thus experiences a fitness
advantage, which could explain the interaction. The positive and negative
effects we found emphasize that continuing inbreeding can have important
effects on individual fitness, even in populations that are already highly
inbred.