The phylogenetic study of adaptive zones: Has phytophagy promoted insect diversification?

Simpson's postulate that rapid diversification follows entrance into a new "adaptive zone" is frequently invoked a posteriori for groups of unusual diversity. The postulate can be tested more rigorously by defining an adaptive zone according to ecological criteria, independent of particular groups of organisms. The adaptive-zone hypothesis predicts that if multiple lineages have invaded a new adaptive zone, they should be consistently more diverse than their (equally old) sister groups, when the latter retain the more primitive way of life. Higher-plant feeding among insects is an independently defined, repeatedly invaded adaptive zone, to which a profound acceleration of diversification rate has been attributed. We have quantified the evidence for this hypothesis by comparing sister groups and species diversity of as many phytophagous insect groups as current taxonomic evidence allows. A sign test showed significant association of diversification rate with the adoption of phytophagy. The possible artifactual bases for this trend are discussed and provisionally rejected. We discuss several possible biological explanations for the association, including models of phylogenesis either dependent on or independent of ecological role.