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For much of the twentieth century, the study of speciation had two major emphases. One was evaluating the geographic circumstances under which speciation occurred, and specifically whether geographic isolation (allopatry) was required (Mayr 1942, 1947; Bush 1969; Futuyma and Mayer 1980; Coyne and Orr 2004). The other was deciphering the genetic architecture of speciation, that is, the roles played by chromosomal translocations, and the kinds, numbers, linkage and epistatic relationships, and phenotypic effects of the genes involved in reproductive isolation (Noor et al. 2001; Orr 2001; Rieseberg 2001; Ortiz-Barrientos et al. 2002). Only recently have the contributions of natural selection to speciation been added as a major empirical focus of speciation studies. A few notable early articles notwithstanding (e.g., Dodd 1989; reviewed by Rice and Hostert 1993), the contributions of adaptive ecological divergence to reproductive isolation have primarily been evaluated—with increasing frequency—over the last decade or two (see reviews by Schluter 2000, 2001; Berlocher and Feder 2002; Drès and Mallet 2002; Funk et al. 2002; Coyne and Orr 2004; Rundle and Nosil 2005). And only over the last several years have approaches been developed to rigorously isolate the contributions of natural selection from other potential causes of the evolution of reproductive isolation (Schluter and Nagel 1995; Funk 1996, 1998; Fitzpatrick 2002; Funk et al. 2002, 2006). These comparative approaches take advantage of the recent availability of molecular genetic data, phylogenetic frameworks, and meta-analytic approaches. Such comparative approaches offer opportunities to tease out generalities about the complex and heterogeneous process of speciation that cannot be otherwise obtained. This chapter demonstrates how particular comparative approaches can be used to identify and quantify ecological contributions to reproductive isolation and speciation. Because this volume is about herbivorous insects, our chapter illustrates these points with reference to insect systems. This chapter has six parts. First, we discuss the nature and study of ecological speciation. Second, we present some advantages of evaluating herbivorous insect systems for such investigations. Third, we describe recently developed comparative approaches that have provided insights on this topic. Fourth, we analyze available herbivore data sets to illustrate these methods and the evidence they provide on ecology’s role in herbivore speciation. Fifth, we highlight important issues pertaining to the conduct and interpretation of these comparative studies. Sixth, we suggest future directions for advancing the multidimensional study of ecological speciation through the creative application of comparative approaches. Ecological Speciation Although the contribution of natural selection to speciation was not an explicit empirical focus of most of the twentieth century, it was often assumed to be an important factor in the evolutionary diversification of lineages. Indeed, such ideas characterized elements of the evolutionary synthesis. At a macroevolutionary scale, an association between ecology and diversification seemed evident in the oft-observed union of ecological diversity/novelty with species richness in particular taxa (Lack 1947; Simpson 1953; Ehrlich and Raven 1964; Van Valen 1965; Stanley 1979; Mitter et al. 1988; Bernatchez and Wilson 1998; Schluter 2000). Simpson (1953) explained such patterns as “adaptive radiations” driven by the evolution of “key innovations.” Such key innovations were traits that provided a lineage with access to a previously underexploited ecological niche and associated resources, namely, a new “adaptive zone.” Subsequent partitioning of these new resources, it was argued, would promote the formation and coexistence of a large number of ecologically varied species (e.g., Ehrlich and Raven 1964; Mitter et al. 1988; Hodges and Arnold 1995). 117 N I N E Comparative Analyses of Ecological Speciation DANIEL J. FUNK AND PATRIK NOSIL At a microevolutionary scale, midcentury verbal models described how the individual speciation events that cumulatively constitute an adaptive radiation might be causally connected to ecology (Muller 1942; Mayr 1947, 1963). Consider two populations that occupy different habitats/environments . These populations are expected to adapt to their respective habitats and genetically diverge as a consequence . Thus, at loci involved in this local adaptation, alternative alleles will be expected to rise to high frequency or fixation in the two populations. This genetically based adaptive divergence might be expected to incidentally promote the evolution of reproductive isolation between these populations for two reasons. First, divergent selection may have incidental (pleiotropic) effects on reproductive isolation . For example, consider divergent selection on body size in populations occupying alternative habitats. If ecologically adaptive divergence in body size occurs between populations of a species that exhibit size-assortative mating preferences , sexual isolation may be a pleiotropic consequence (Nagel and Schluter 1998; McKinnon et al. 2004...


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