This conflict can occur before or after fertilization. Before fertilization, the mother has an interest in picking the best suited father from a larger pool, while all fathers have an interest in being picked. After fertilization, fathers have an interest in maximizing maternal investment in their progeny, while mothers will have an interest in carefully partitioning resources among progeny to maximize their combined success.
A new study in the September issue of The American Naturalist argues that with increased self-fertilization, parental conflict decreases. Consequently, parents from frequently selfing groups should be competitively inferior with respect to this parental conflict. Yaniv Brandvain and David Haig examine crosses between selfing and outcrossing pairs and find that, in most cases, there are pre- and post-zygotic symptoms of outcrossers being "stronger" than selfers with regard to parental conflict. They contend that this competitive imbalance can explain a common pattern of unilateral incompatibility, in which pollen from self-incompatible populations can successfully fertilize ovules of self-compatible individuals, but the reciprocal cross fails. Since both pre- and post-zygotic consequences of this imbalanced conflict can perturb successful fertilization and development, they provide barriers to hybridization and may facilitate speciation.