restricted access 15. The Oscillation Hypothesis of Host-Plant Range and Speciation
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From a humble beginning in the early Cretaceous, angiosperm plants have quickly conquered the earth so that they now make up one of the most ubiquitous and species-rich groups (Crane et al. 1995; Wikström et al. 2001; Stuessy 2004; Friis et al. 2005). Likewise, the pioneer insects that once colonized this novel resource have multiplied to such an extent that they have become an ecologically dominating group in all terrestrial ecosystems (Mitter et al. 1988; Farrell 1998). Together, the seed plants and the insects that feed on them make up a good half of all described species, and their diversification is among the most remarkable in the history of life, both in terms of magnitude and relative speed. Consequently, if we want to understand the processes that generate biodiversity on earth, this is a good place to start. Even though it is clear that both seed plants and plantfeeding insects have undergone rapid diversification to a much larger extent than their respective sister groups, we have a surprisingly poor understanding of why this has happened . The analogous diversification of these groups has led to the idea that they must have diversified together, reciprocally influencing each other’s cladogenesis (Ehrlich and Raven 1964). However, apparent disparity in the relative timing of the diversification of the two groups has led many to suggest that they may have undergone rapid speciation for reasons that are not necessarily connected (Jermy 1984; Janz and Nylin 1998). Furthermore, while explanations for how insects could have influenced the diversification of flowering plants have typically focused on pollination (Pellmyr 1992; Waser 1998; Dodd et al. 1999; Grimaldi 1999), explanations for the potential impact of plants on the diversification of insects have often revolved around herbivory (Mitter et al. 1988; Farrell 1998; Kelley et al. 2000). Here, we will primarily deal with the latter question: how the utilization of flowering plants as food resources could have promoted speciation rates in insects. Seed plants are different from many other food resources. As already mentioned , the resource itself has quickly diversified, and a typical consequence of this seems to the development of high resource specialization among the insects that feed on them. Relative host specialists dominate most groups of plant-feeding insects, and there are indications that many polyphagous species often show considerable geographic specialization (Thompson 2005). Flowering plants may be an abundant resource, but different plant species offer very different chemical challenges, and it appears difficult to be able to cope with more than a few plant species at a time. These difficulties can be both metabolic (Via 1991; Joshi and Thompson 1995; Mackenzie 1996; Agrawal 2000) and neurological (Bernays and Wcislo 1994; Janz and Nylin 1997; Bernays 2001). This widespread specialization is a natural candidate in the search for mechanisms behind the high speciation rates among these insects and has often figured in such discussions (Jaenike 1990; Futuyma 1991; Thompson 1994; Kelley et al. 2000; Hawthorne and Via 2001). However, understanding the reasons for host specialization is just the beginning . The proposed role of specialization is that it can aid population fragmentation, either by the formation of host races (Bush 1975; Feder et al. 1988; Carroll and Boyd 1992; Hawthorne and Via 2001) or by an increased likelihood of geographic fragmentation (Peterson and Denno 1998; Kelley et al. 2000). Under both these scenarios, specialization acts as a pruning process; it increases the likelihood of population subdivision and speciation by removing plants from the repertoire. Thus, by itself, this process would very soon run out of fuel. The next challenge then, is to understand what processes generate novel variation in host use, the processes that have driven the spread of plant-feeding insects across the phylogeny of flowering plants. Even though there is a considerable conservatism in host-plant use among groups of related 203 F I FTE E N The Oscillation Hypothesis of Host-Plant Range and Speciation NIKLAS JANZ AND SÖREN NYLIN herbivorous insects, plant-feeding insects have, to various extents, conquered all major groups of flowering plants. How and why does the interaction diversify? What are the relative roles of host-range expansions and host shifts? This is an important part of our hypothesis for how plant diversity generates diversity in the insects that feed on them, but a final, and fundamental, part remains: does this diversification of the interaction indeed lead to elevated speciation rates? It is the purpose of...


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