Flowers and Insect Evolution

Brian D. Farrell (1) provides a clear illustration of the utility of phylogenetic reasoning in evaluating the evolutionary history of ecological or behavioral traits, or both. With the use of morphological and molecular evidence, Farrell demonstrated that phytophagous beetle lineages feeding on angiosperms had considerably higher rates of diversification than their gymnosperm-feeding sister taxa (table 1 in the report). These results support Ehrlich and Raven's hypothesis that insect diversity is intimately tied to the rise and diversification of the angiosperms in the Cretaceous (2). Farrell argues that the pattern he observed in beetles is of broad significance and should be reflected in other lineages of insects showing close ecological associations with angiosperms.

Bees [Series Apiformes, Superfamily Apoidea (3)] provide support for this hypothesis, but have often been overlooked in this context (4), because, instead of feeding internally or externally on plant vascular tissues, they consume angiosperm pollen and provide a direct benefit to plants through their activities. Like many beetle lineages, bees form a speciose group, and, like beetles, many species of bees have intimate and often specialized (oligolectic) ecological associations with specific plant genera or species (5). Like angiosperm-feeding beetles, bees may have arisen in the early Cretaceous (6), and diversified along with the early angiosperms.

Recent phylogenetic evidence indicates that bees form a monophyletic lineage that has arisen from within the spheciform wasps [Series Spheciformes, Superfamily Apoidea (3), or “hunting wasps”], thus rendering the hunting wasps paraphyletic (7,8). However the precise position of the bees within the Spheciformes is unclear. Recent studies indicate that bees either arise basally within this lineage as sister to all Spheciformes except the families Sphecidae and Ampulicidae (including ∼7000 species) (7), or as sister to the family Philanthidae (including ∼1100 species) (8). The vast majority of hunting wasps are predatory on insects and other arthropods (9). Association with angiosperms in this case has led to enhanced rates of speciation: bees include ∼20,000 species, or 3 to 18 times the number of species included in their putative sister clade.

However, not all aculeate Hymenoptera support Farrell's hypothesis. The vespid subfamily Masarinae includes angiosperm-associated wasps [also called pollen wasps (10)], which are ecologically very similar to bees, despite their independent origin. Like bees, masarines arose in the Cretaceous (11) from within a lineage of predatory wasps (the family Vespidae). Masarine wasps include ∼300 species (10) and form the sister group to a clade of over 3500 species of predatory solitary and social wasps (12). In this case the angiosperm-associated lineage shows lower diversity than its non-herbivorous sister clade.

Phylogenetic reasoning of this kind can provide an extremely powerful tool for assessing the implications of angiosperm associations in insects. However, before fully accepting the hypothesis that angiosperms have had a direct impact on insect diversity, we should seek additional evidence from other speciose phytophagous insect orders, including Hemiptera, Thysanoptera, Orthoptera, Phasmida, Lepidoptera, and Hymenoptera (especially sawflies, bees, and pollen wasps). Finally, we should distinguish between herbivores that feed on plant vascular tissues and pollenivores, and ask whether these two groups exhibit similar patterns of coevolution with angiosperms.

By.......

1. Bryan N. Danforth and
2. J. Ascher

Department of Entomology,College of Agriculture and Life Sciences,Cornell University,Comstock Hall,Ithaca

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