Genomic imprinting, sibling solidairity and the logic of collective action.

Genomic imprinting has been proposed to evolve when a gene's expression has fitness consequences for individuals with different coefficients of matrilineal and patrilineal relatedness, especially in the context of competition between offspring for maternal resources. Previous models have focused on pre-emptive hierarchies, where conflict arises with respect to resource allocation between present and future offspring. Here we present a model in which imprinting arises from scramble competition within litters. The model predicts paternal-specific expression of a gene that increases an offspring's fractional share of resources but reduces the size of the resource pool, and maternal-specific expression of a gene with opposite effects. These predictions parallel the observation in economic models that individuals tend to underprovide public goods, and that the magnitude of this shortfall increases with the number of individuals in the group. Maternally derived alleles are more willing than their paternally derived counterparts to contribute to public goods because they have a smaller effective group size.