PATTERNS OF DIVERSIFICATION IN PHYTOPHAGOUS INSECTS

secondary chemistry types (e.g., Rodman et al. 1998), phylogenetic studies directed
explicitly at the evolution of plant defense are still few (but see, e.g., Armbruster 1997,
Wink 2003, Rudgers et al. 2004). Agrawal and Fishbein (2006) mapped an array of
putative defense traits that included total cardenolides (though not the hypothesized ‘arms
race’ aspects thereof) onto a molecular phylogeny for 24 Asclepias species. Rather than
reflecting plant phylogeny, these traits appear to define three distinct, convergently
evolved defense syndromes, each possibly optimal in the right circumstances. This
implicit optimality/equilibrium view of plant defense is very different from the
historically contingent view inherent in the “arms race” hypothesis. Under the latter, we
expect some lineages to have acquired novel defenses that confer, at least temporarily, a
ubiquitous fitness advantage over relatives lacking those innovations. The relative
applicability of these two views of defense evolution across the diversity of plants and
their defensive traits has yet to be determined.
Reinforcing the view that ancient host associations may have left widespread, if
not numerically dominant traces on contemporary assemblages is the increasing evidence
for broad-scale correspondence between the ages of currently-associated insect and plant
groups, over time spans encompassing major evolutionary changes in the global flora.
The case for this long-standing postulate (see e.g., Zwölfer 1978) is best developed for
the beetle clade Phytophaga (Chrysomeloidea + Curculionoidea, ~ 135,000 species),
whose hosts span the chief lineages of seed plants (Farrell 1998, Marvaldi et al. 2002,
Farrell and Sequeira 2004). Recent phylogeny estimates show most of the basal
phytophagous lineages in both superfamilies to feed exclusively on conifers or cycads,
36