THE HYBRID BRIDGE HYPOTHESIS - HOST SHIFTING VIA PLANT HYBRID SWARMS

Discusses mechanisms by which herbivorous insects acquire or shift to new host plants without involving preadaptation or mutation. The authors use their work on insect distributions on cottonwood trees to suggest the potential importance of hybrid plants that morphologically, genetically and spatially bridge gaps between parental species and which may allow herbivores to shift hosts in a series of gradual steps. Predictions are 1) monophages are more likely to benefit from a hybrid bridge than polyphages; 2) hybridising plants will share more herbivores than non-hybridising species; and 3) the pattern of hybridisation allows for specific predictions. The "hybrid bridge' hypothesis is tested using galling insects on Fremont cottonwood Populus fremontii × narrowleaf cottonwood Populus angustifolia hybrids in N Utah. -P.J.Jarvis