Since the seminal papers of Fraenkel (1959) and Ehrlich and Raven (1964), much research has demonstrated the role of plant secondary metabolites (PSMs) as defence mechanisms against invertebrate and vertebrate herbivory. These metabolites can act directly on the herbivore as toxins (Theis & Lerdau, 2003; Gershenzon & Dudareva, 2007), digestibility reducers (Ayres et al., 1997; De Gabriel et al., 2009) and deterrents (Pass & Foley, 2000), and they can also act indirectly by, for example, attracting natural enemies of the herbivore (Dicke, 2009). The idea that the herbivores themselves are acting as selective agents on these PSMs has existed since it was first noted that these compounds may serve as anti-herbivore traits, and in some systems it is clear that herbivores may act as agents of natural selection on some specific PSMs (Simms & Rausher, 1989; Mauricio & Rausher, 1997; Stinchcombe & Rausher, 2001; Agrawal, 2005). However, in most systems there is still a dearth of evidence addressing this question, particularly in light of the vast number of herbivores that attack a single plant species across its entire life and the array of PSMs that are expressed in a plant species. Are all of these herbivores agents of selection and have all PSMs evolved because of the selective pressures by the herbivores, or are PSMs driven by selection from other pressures such as abiotic factors (Close & McArthur, 2002)? Knowing the answer to these questions is important when attempting to understand what is driving population divergence within species and the evolution and change in PSMs.
For selection to occur there must be additive genetic-based variability in herbivory within plant populations. This herbivory must correlate with additive genetic-based variation in plant defensive traits, and herbivory must affect plant fitness (see Box 2.1). Key papers in the late 1980s through to the late 1990s clearly demonstrated the evolutionary impact that invertebrate herbivores were having on plant chemical defences in some systems (Rausher & Simms, 1989; Simms & Rausher, 1992; Rausher, 1993; Mauricio & Rausher, 1997; Juenger & Bergelson, 1998).