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Inbreeding avoidance among interacting females and males is not always
observed despite inbreeding depression in offspring fitness, creating an
apparent ‘inbreeding paradox’. This paradox could be resolved if selection
against inbreeding was in fact weak, despite inbreeding depression.
However, the net magnitude and direction of selection on the degree to
which females and males inbreed by pairing with relatives has not been
explicitly estimated. We used long-term pedigree data to estimate
phenotypic selection gradients on the degree of inbreeding that female and
male song sparrows (Melospiza melodia) expressed by forming
socially-persistent breeding pairs with relatives. Fitness was measured as
the total numbers of offspring and grand-offspring contributed to the
population, and as corresponding expected numbers of identical-by-descent
allele copies, thereby accounting for variation in offspring survival,
reproduction and relatedness associated with variation in parental
inbreeding. Estimated selection gradients on the degree to which
individuals paired with relatives were weakly positive in females, but
negative in males that formed at least one socially-persistent pairing.
However, males that paired had higher mean fitness than males that
remained socially unpaired. These analyses suggest that net selection
against inbreeding may be weak in both sexes despite strong inbreeding
depression, thereby resolving the ‘inbreeding paradox’.
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