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188 Biologists have long sought an answer to the question, “What determines species diversity?” Indeed, this question is one of only 25 key questions featured in the 125th anniversary issue of Science that were intended to expose critical gaps in scientific knowledge (Pennisi 2005). A Nature article by Emerson and Kolm (2005a; but see also Cadena et al. 2005; Emerson and Kolm 2005b) suggests that species diversity itself might help to promote speciation. Put another way, these authors argue that species cause species. Emerson and Kolm (2005a) used species lists of the plants and arthropods from the Canary and Hawaiian islands to address whether there is a positive relationship between species diversity and the rate of diversification (measured as the proportion of endemic species). Species richness was a strong predictor of the rate of diversification even after controlling for important biogeographical features such as island age, area, altitude, and proximity to the nearest neighboring island. Given this finding, one crucial question is what factors could drive the positive feedback between species diversity and speciation rates. A number of authors (e.g., Dawkins 1982; Jones et al. 1997; Odling-Smee et al. 2003; Erwin 2005) have suggested processes that could enhance the number of species in an area, including that new species may continue to subdivide resources via specialization as part of an adaptive radiation, “niche construction,” in which an organism modifies its own niche and thus occupies formerly unoccupied niche space, and “ecosystem engineering,” in which one species modifies the environment and in so doing facilitates the production of niches for other species. As a consequence of niche construction and ecosystem engineering, many organisms provide habitat and thus new niche opportunities for other species (Erwin 2005). Species Cause Species: Sympatric Speciation through Host-Race Formation Sympatric speciation through host-race formation may present a situation where specialization and resultant diversi fication in one species cause diversification in another. Speciation is typically envisioned as a multiple-step process in which populations of a species first become geographically isolated, then undergo divergence, and finally reproductive isolation occurs. This classical view holds that isolation and divergence come about over time and arise only when populations are located in separate geographic areas. Without geographic separation, gene flow between populations is expected to swamp any differences that may arise between populations. However, studies involving a number of taxa including fish (e.g., cichlids), birds (e.g., crossbills), crustaceans (e.g., parasitic copepods), mollusks (e.g., pulmonate snails), mites (e.g., hummingbird flower mites), and herbivorous insects (e.g., aphids, apple maggots, goldenrod gall flies, treehoppers) indicate that reproductive isolation and divergence can take place sympatrically (Wood 1980, 1993; Strong et al. 1984; Tauber and Tauber 1989; Bush 1994; Abrahamson and Weis 1997; Berlocher and Feder 2002). Most of the insect examples of sympatric speciation involve host-race formation. Under a scenario of sympatric speciation, an herbivorous insect initially shifts to a closely related or chemically similar host-plant species and changes its preferences for mating and/or oviposition. As a consequence , at least partial reproductive isolation of the hostassociated populations occurs. Isolation enables selection, drift, and mutation to produce additional differences between host-associated populations and facilitates differentiation between populations (Abrahamson et al. 2003). Host-race formation may be one of the primary reasons that FO U RTE E N Sequential Radiation through Host-Race Formation: Herbivore Diversity Leads to Diversity in Natural Enemies WARREN G. ABRAHAMSON AND CATHERINE P. BLAIR SEQUENTIAL RADIATION THROUGH HOST-RACE FORMATION 189 clades of herbivores undergo speciation more rapidly than their nonherbivore counterparts (Mitter et al. 1988). The bottom line is that herbivore biology strongly promotes evolutionary diversification (Funk et al. 2002). Sequential Radiation What has been little explored is whether diversification of herbivores in response to their host plants causes differentiation of their natural enemies. Does the genetic differentiation of an herbivorous insect create a new resource that when exploited by a natural enemy causes that natural enemy to undergo genetic and/or behavioral differentiation itself? In this chapter, we explore five examples in which the genetic diversification of herbivores has created new resource opportunities that have been exploited by natural enemies, which subsequently have undergone differentiation via host-race formation themselves. We have previously termed the process of herbivore differentiation causing natural-enemy divergence “sequential radiation ” (Abrahamson et al. 2003). In this sense, sequential radiation is characterized by a diversification of taxa (e.g., natural enemies) farther up...

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