Post-insemination signalling systems and reinforcement

Theory indicates that the conditions necessary for the operation of reinforcement are very restrictive, yet two recent surveys of the literature report the widespread occurrence of patterns predicted by the reinforcement hypothesis. We begin a reconciliation of theory and data by focusing on one of the most troublesome restrictions placed on reinforcement: the need for strong selection against hybridization. Laboratory hybridization studies often do not find evidence of the required selection. We point out that the design of most laboratory hybridization studies precludes the detection of barriers to fertilization. Recent work on the ground crickets Allonemobius jasciatus and A. socius, which we summarize here, suggests that barriers to fertilization evolve quickly. If mating is a risky business, such barriers can promote the evolution of premating isolation. Post-insemination signalling systems are little understood by evolutionists and worthy of much greater attention.