Pattern and process in the evolution of insect‐plant associations:Yponomeutaas an example

Phylogenetic studies are increasing our understanding of the evolution of associations between phytophagous insects and their host plants. Sequential evolution, i.e. the shift of insect herbivores onto pre‐existing plant species, appears to be much more common than coevolution, where reciprocal selection between interacting insects and plants is thought to induce chemical diversification and resistance in plants and food specialization in insects. Extreme host specificity is common in phytophagous insects and future studies are likely to reveal even more specialization. Hypotheses that assume that food specialists have selective advantages over generalists do not seem to provide a general explanation for the ubiquity of specialist insect herbivores. Specialists are probably committed to remain so, because they have little evolutionary opportunity to reverse the process due to genetically determined constraints on the evolution of their physiology or nervous system. The same constraints might result in phylogenetic conservatism, i.e. the frequent association of related insect herbivores with related plants. Current phylogenetic evidence, however, indicates that there is no intrinsic direction to the evolution of specialization. Historical aspects of insect‐host plant associations will be illustrated with the small ermine moth genusYponomeuta.Small ermine moths show an ancestral host association with the family Celastraceae. The genus seems to be committed to specialization per se rather than to a particular group of plants. Whatever host shift they have made in their evolutionary past (onto Rosaceae, Crassulaceae, and Salicaceae), they remain monophagous. The oligophagousY padellusis the only exception. This species might comprise a mosaic of genetically divergent host‐associated populations.