restricted access 18. The Phylogenetic Dimension of Insect-Plant Interactions: A Review of Recent Evidence
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240 The dramatic expansion of research on insect-plant interactions prompted by Ehrlich and Raven’s (1964) essay on coevolution focused at first mainly on the proximate mechanisms of those interactions, especially the role of plant secondary chemistry, and their ecological consequences. Subsequently , in parallel with the resurgence of phylogenetics beginning in the 1970s and 1980s, there arose increasing interest in the long-term evolutionary process envisioned by Ehrlich and Raven (e.g., Benson et al. 1975; Zwölfer 1978; Berenbaum 1983; Mitter and Brooks 1983; Miller 1987). Since the early 1990s, spurred in part by the increasing accessibility of molecular systematics, there has been a happy profusion of phylogenetic studies of interacting insect and plant lineages. The results so far have reinforced skepticism about the ubiquity of the particular macroevolutionary scenario envisioned by Ehrlich and Raven, now commonly termed “escape and radiation” coevolution (Thompson 1988). However, this model continues to inspire and organize research on the evolution of insectplant assemblages because it embodies several themes of neo-Darwinism, each of interest in its own right, which have been taken up anew in the modern reembrace of evolutionary history. In this chapter we attempt to catalog some of the postulates about phylogenetic history derivable from Ehrlich and Raven’s essay and evaluate their utility for explaining the structure of contemporary insect-plant interactions . The escape and radiation model (reviewed in Berenbaum 1983) tacitly assumes, first, that the traits governing species’ interactions, such as insect host-plant preference, are phylogenetically conserved due to constraints such as limited availability of genetic variation. Such constraints create time lags between successive insect and plant counteradaptations, allowing the lineage bearing the most recent innovation to increase its rate of diversification. Second, a related general implication is that, because of genetic or other constraints on evolutionary response to new biotic surroundings, the structure of present-day insect-plant interactions (e.g., who eats whom) will be governed more by long-term evolutionary history than by recent local adaptation. This postulate parallels a broader recent shift in thinking about community assembly, from a focus on equilibrium processes to a greater appreciation of the role of historical contingency (Webb et al. 2002; Cattin et al. 2004; DiMichele et al. 2004). Third, the radiation component of escape and radiation perfectly encapsulates the “new synthesis” view, lately enjoying a revival (Schluter 2000), that diversification is driven primarily by ecological interactions. Insect-plant interactions have figured prominently in the modern reexamination of all three of these broad postulates. This chapter surveys the recent evidence on the phylogeny of insect-plant interactions, focusing chiefly on among-species differences in larval host-plant use by herbivorous insect lineages (largely neglecting pollinators, which are treated by Adler in this volume), and organized around the themes sketched above. We draw mostly on literature of the past dozen years, that is, subsequent to early attempts at a similar survey (e.g., Mitter and Farrell 1991; Farrell and Mitter 1993). Given the great diversity of phytophage life histories and feeding modes, full characterization of host-use evolution will require, in addition to hypothesis tests in particular groups, the estimation of relative frequencies of alternative evolutionary patterns across a broad sampling of lineages . Our emphasis here is on the latter approach. A complete catalog is no longer feasible, but we have made a concerted and continuing effort to compile as many phylogenetic studies of phytophagous insect groups as possible. These are entered into a database that at this writing contained over 1000 entries, many of which were obtained from the Zoological Record database. Our analyses and conclusions are based chiefly on approximately 200 of these reports that contain both a phylogenetic tree and information on E I G HTE E N The Phylogenetic Dimension of Insect-Plant Interactions: A Review of Recent Evidence ISAAC S. WINKLER AND CHARLES MITTER host-plant use. Many of the phylogenies are based on DNA sequences, while for others the chief evidence is morphology . This database, intended as a community resource to promote further synthesis, is available at www.chemlife, as are the data compilations and other supplementary materials mentioned in the text. Our nomenclature follows APGII (2003) for angiosperm families and higher groups, and Smith et al. (2006) for ferns. Conservatism of Host-Plant Use Full understanding of the influence of evolutionary history on insect-plant associations will require a broad accounting of the degree to which the different dimensions of the feeding...