Plant Hybrid Zones and Insect Host Range Expansion

The hybrid bridge hypothesis suggests that plant hybrids ‘‘bridge’’ the genetic gap between actual and potential host species, and that, for this reason, herbivorous insects are more likely to evolve an expanded host range in the presence of hybrids. While intuitively appealing, the hypothesis has two implicit assumptions: that phenotypic gaps between potential hosts limit host range, and that characters controlling host use are additively inherited in plant hybrids. Evaluation of these assumptions suggests that operation of the hybrid bridge hypothesis is relatively uncommon. In addition, the hypothesis has not been well integrated into existing theoretical and empirical work on the evolution of host range in herbivorous insects. Proper evaluation of the hypothesis will require information on the effect of plant hybridization on both insect preference and insect performance. Ecological and genetic factors affecting range expansion in both hybrids and novel parents also require evaluation.