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216 We are faced with three seemingly conflicting observations regarding the diversification of plant-feeding insects. Insects can evolve at astoundingly rapid rates when confronted with new selection pressures, as shown in hundreds of studies in recent decades. Nevertheless, most insect lineages remain highly conservative in the range of species with which they interact. Occasionally, though, insects make great phylogenetic jumps, even jumping between eudicotyledonous and monocotyledonous plant taxa. It is the juxtaposition of these three observations that has historically created the conceptual tension between fields of study that focus on current selection, such as evolutionary ecology and population genetics, and fields that focus on higher-level patterns in the diversification of life, such as systematics and paleobiology. Studies of the geographic mosaic of coevolution, specialization , and population divergence have begun to resolve these apparently conflicting observations. In this chapter I discuss how our developing knowledge of the geographic mosaic of coevolution and cryptic speciation may help us better understand how plant-insect interactions persist for millions of years despite ongoing rapid evolution in everchanging environments. Throughout the chapter I focus on interactions between prodoxid moths in the genus Greya, their host plants, and parasitoids as exemplars of how our understanding of the dynamics of interactions involving phytophagous insects is advancing as researchers integrate results from multiple subdisciplines. A Blending of Perspectives: Populations, Species, and Species Interactions One of the most important results we have obtained from detailed study of insect species over the past several decades is that insect populations may often become adapted to their local host species (Craig et al. 2000; Boggs et al. 2003; Singer 2003; Ehrlich and Hanski 2004; Fischer and Foitzik 2004; Gotthard et al. 2004; Nielsen and de Jong 2005). Moreover, it is becoming evident that local adaptation may often occur at a finer geographic scale than is evident in phylogeographic analyses on the scale of population differentiation identified by using neutral molecular markers. For example, multiple Greya moth species, which are close relatives of yucca moths, occur in the northern U.S. Rocky Mountains (Thompson 1985; Davis et al. 1992; Pellmyr et al. 1996). Populations of some of these species differ geographically in their use of plant species, even within regions in which there is little evidence of neutral molecular differentiation among populations (Brown et al. 1997; Thompson et al. 1997; Janz and Thompson 2002; Thompson and Cunningham 2002). In turn, some wasps in the braconid genus Agathis, which attack larvae of Greya moths by searching within and among host plants, differ geographically in their search behaviors in ways that match the local availability of Greya larvae (Althoff and Thompson 2001). As with the moths, the geographic scale of differences in searching behavior is smaller than the scale of differentiation found in concomitant analyses of neutral molecular markers (Althoff and Thompson 1999, 2001). Hence, there is a complex mosaic of ecological differentiation in these plant-insectparasitoid interactions that is underestimated by analyses of neutral molecular differentiation among populations. Differences in the geographic scale of population differentiation obtained by these different kinds of analysis should not be surprising. These moths and parasitoids are living in post-Pleistocene environments that have been available for only about the past 10,000 years, allowing little time for molecular differentiation in genes that are diverging through mutation and random genetic drift alone. Genes under strong natural selection should be expected to evolve much faster than molecular regions that are diverging through mutation and random genetic drift alone, espeS I XTE E N Coevolution, Cryptic Speciation, and the Persistence of Interactions JOHN N. THOMPSON cially in fairly large populations. Hence, the scale of population differentiation is much more apparent in genes subject to selection than in molecular regions assessed using neutral markers. The important point is that insect populations probably are often more differentiated from one another than is evident in the burgeoning number of studies of molecular differentiation among insect populations. Moreover, we know that evolutionary changes in local adaptation can sometimes occur very quickly, reshaping insect populations within decades, as shown in the adaptation of some insect populations to the introduction of novel plant species. Examples include rapid biochemical adaptation of lepidopteran larvae to the novel defensive plant chemistry of invasive plants (Berenbaum and Zangerl 1998; Zangerl and Berenbaum 2003), modification of the length of the piercing mouthparts and other traits of hemipterans to gain access to seeds of introduced plants (Carroll et al. 1998, 2001), and changes in oviposition...

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