Is Pairing with a Relative Heritable? Estimating Female and Male Genetic Contributions to the Degree of Biparental Inbreeding in Song Sparrows (Melospiza melodia)

The degree of inbreeding expressed within populations can profoundly shape evolutionary dynamics. The degree to which individuals inbreed is frequently assumed to evolve in response to selection, for example, resulting from inbreeding depression. Such evolutionary responses require additive genetic variance (V-A) in the degree to which individuals inbreed. However, the magnitude of V-A in the degree of biparental inbreeding has never been estimated. We devised a quantitative genetic model to estimate sex-specific V-A in the degree to which individuals inbreed while accounting for effects of individuals' own coefficients of inbreeding and genetic effects stemming from immigration. We applied this model to the degree of inbreeding expressed through social pairing in free-living song sparrows (Melospiza melodia). Estimates of V-A for both sexes appreciably exceeded 0 and the cross-sex genetic covariance was strongly positive, creating substantial total V-A in the degree of inbreeding. Our analyses also revealed inbreeding depression in the degree of inbreeding, such that more inbred individuals paired with closer relatives, and immigrant effects, such that individuals with greater genomic contributions from immigrants paired with more distant relatives. We thereby demonstrate that the degree of biparental inbreeding can show substantial V-A in nature and might consequently evolve in response to selection.