Resolving the Conundrum of Inbreeding Depression but no Inbreeding Avoidance: Estimating Sex-Specific Selection on Inbreeding by Song Sparrows (Melospiza melodia)

9 Inbreeding avoidance among interacting females and males is not always observed despite 50 inbreeding depression in offspring fitness, creating an apparent ‘inbreeding paradox’. This 51 paradox could be resolved if selection against inbreeding was in fact weak, despite 52 inbreeding depression. However, the net magnitude and direction of selection on the 53 degree to which females and males inbreed by pairing with relatives has not been explicitly 54 estimated. We used long-term pedigree data to estimate phenotypic selection gradients on
55 the degree of inbreeding that female and male song sparrows (Melospiza melodia) 56 expressed by forming socially-persistent breeding pairs with relatives. Fitness was measured 57 as the total numbers of offspring and grand-offspring contributed to the population, and as 58 corresponding expected numbers of identical-by-descent allele copies, thereby accounting 59 for variation in offspring survival, reproduction and relatedness associated with variation in 60 parental inbreeding. Estimated selection gradients on the degree to which individuals paired 61 with relatives were weakly positive in females, but negative in males that formed at least 62 one socially-persistent pairing. However, males that paired had higher mean fitness than 63 males that remained socially unpaired. These analyses suggest that net selection against
64 inbreeding may be weak in both sexes despite strong inbreeding depression, thereby 65 resolving the ‘inbreeding paradox’.