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The evolutionary ecology of polyphagy by phytophagous insects has been overshadowed by an intense focus on the evolutionary ecology of their host specificity. This bias reflects the preponderance of host specificity in phytophagous insects (reviewed by Weis and Berenbaum 1989 and Novotny and Basset 2005) and its fascinating consequences for community structure and evolutionary diversi- fication. Truly, the study of host-specific herbivores has provided many key insights and motivated the conceptual side of the study of plant-insect interactions (e.g., Brues 1924; Dethier 1954; Fraenkel 1959; Ehrlich and Raven 1964; Feeny 1976; Rhodes and Cates 1976) as well as the broader issues of ecological specialization (e.g., Futuyma and Moreno 1988; Novotny et al. 2002), coevolution (e.g., Thompson 1994, 1999; Becerra 1997; Berenbaum and Zangerl 1998), and speciation (e.g., Schluter 2000; Nosil et al. 2002; Stireman et al. 2005). However, I believe the development of evolutionary-ecology theory and the strength of its empirical tests require further attention to the full range of host-plant use observed in insect herbivores. After all, specialist and generalist herbivore species co-occur in the same communities (e.g., Novotny and Basset 2005), have overlapping host-plant ranges (e.g., Novotny et al. 2004), and have frequently sprung from the same phylogenetic lineages (Janz et al. 2001; Nosil 2002; Morse and Farrell 2005). Therefore, any complete theory of evolutionary ecology must ultimately aim to explain the existence of variation in diet breadth, in addition to the preponderance of host specificity seen in most phytophagous insect lineages. I argue here that the ultimate causes of host specificity may become clearer by greater consideration of the polyphagous exceptions to the rule. The study of dietary generalists, of course, also illuminates the causes of polyphagy, which has both conceptual and practical value of its own, as many invertebrate and vertebrate herbivores and many pestiferous insect herbivores are polyphagous. In my approach to the evolutionary ecology of polyphagy, I will assume that ultimate explanations for host-plant use by phytophagous insects are generally historical and functional (i.e., adaptive). That is, I contend that functional explanations of microevolutionary processes are the most important cause of macroevolutionary patterns. I raise this point because there is a widespread awareness in evolutionary biology that natural selection operates within various genetic-developmental constraints, and the role of such constraints in determining macroevolutionary patterns is unresolved (e.g., Pigliucci and Kaplan 2000; Arthur 2004). Accordingly, some authors have recently invoked such constraints as ultimate causes of the macroevolutionary patterns of host use either in particular instances or more generally. These genetic or developmental constraints are thought to limit the intrinsic evolutionary potential of particular phylogenetic lineages, creating phylogenetically conservative patterns of host-plant use (Futuyma et al. 1995; Janz et al. 2001; Price 2003). Although such constraints on host use may be important in certain cases and in ecological time (but see Radtkey and Singer [1995] and Singer et al. [this volume] for counterexamples), alone they offer limited explanatory power for macroevolutionary patterns of host-plant use when one considers polyphagy as well as stenophagy at the level of herbivore clades. Hostspeci fic herbivore species nested within lineages or clades that use a phylogenetically disparate set of host-plant species (case B in Table 3.1) indicate the widespread past occurrence of shifts between phylogenetically disparate hosts. Although the frequency of phylogenetic lability in host use has not been quantified among randomly selected lineages, several known examples indicate that this pattern is not uncommon in nature (Janz and Nylin 1998; Powell et al. 1998; Lopez-Vaamonde et al. 2003; Farrell and Sequeira 2004). Unlike explanations that focus on constraints as properties of lineages, functional explanations based on 29 TH R E E Evolutionary Ecology of Polyphagy MICHAEL S. SINGER ecological constraints can explain host specificity or polyphagy at the species (or lower) level, nested within a clade with either broad or restricted host-plant use (cases A–D in Table 3.1). The following discussion of host-plant use will focus on host range at the level of herbivore species or below (population, individual). General Explanations for Host Specificity Theoretical discussions of host-plant range by phytophagous insects reveal several plausible adaptive explanations with scattered empirical support, a complex state of affairs that is poorly resolved (e.g., Jaenike 1990; Mayhew 1997). For context, I will first focus on the familiar ground of host-plant specificity. The strongest general...


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