Plants, and the organisms that eat them, constitute the majority of terrestrial multicellular diversity (Speight et al., 2008). Indeed, co-evolutionary interactions between herbivores and plants are thought by some to be 'the major zone of interaction responsible for generating terrestrial organic diversity' with plant secondary metabolites (PSMs) playing a central role in co-evolutionary processes (Ehrlich & Raven, 1964). As typically described, plants gain fitness advantages and the potential for evolutionary radiation from mutation or recombination events that generate novel PSMs that deter herbivores (or other attackers and competitors, e.g. pathogens). In turn, counter-adaptations, or offences (Karban & Agrawal, 2002; Sorensen & Dearing, 2006), by herbivore populations favour cladogenesis in the consumers and exert further selection pressure for novel PSMs (Janzen, 1980). Antagonistic interactions between plants and herbivores are, therefore, seen as a driving force behind the great diversity of PSMs that occur in plant populations (Rosenthal & Berenbaum, 1992; Gershenzon et al., Chapter 4).
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