As it was originally proposed, the extended phenotype comprised ‘all effects of a gene upon the world’ (Dawkins, 1989) and portrayed how the effects of a gene borne by an organism influenced its biotic and abiotic environments. The consideration of indirect genetic effects, in which an organism’s phenotype becomes part of the selective environment of conspecifics (Wolf et al., 1998), was developed rigorously in the population genetics context and the concept subsequently extended to include effects on heterospecifics (Whitham et al., 2003). The extended phenotype concept has been adopted as a framework by some evolutionary biologists and ecologists to study the roles of plant secondary metabolites (PSMs) since Whitham et al. (2003) used heritable variation in tissue tannin concentrations among Populus species and hybrids to develop the concept of community and ecosystem genetics (Antonovics, 1992).
Many studies of how genetically determined variation in plant traits, including PSMs, drive associated community phenotypes and processes, have been based on differences between hybrids (Dungey et al., 2000; Hochwender & Fritz, 2004; Bailey et al., Chapter 14). Fewer studies have investigated the effects on extended phenotypes of continuously varying PSMs or between known genotypes within a species (Whitham et al., 2006; Schweitzer et al., 2008; Barbour et al., 2009; O’Reilly-Wapstra et al., Chapter 2). A convenient approach to identification and utilisation of genotypic variation for the study of multiple effects of PSMs is provided by the use of genetic polymorphisms. A polymorphism can be defined as occurring when a trait such as a morphological or biochemical character exists in two or more distinct forms in a randomly mating population within a species (Ford, 1975). The approach is particularly useful in species that cannot be readily cloned. Here, we review examples of how intra-specific variation in a particular group of PSMs, the monoterpenes, has informed our understanding of how PSMs can play multiple ecological roles and mediate the extended phenotype of plants. The monoterpenes are a group of low-molecular-weight, volatile terpenoids which form a very diverse group in terms of number of compounds, structure and function (Gershenzon & Dudareva, 2007). We use variation within species which are polymorphic for concentrations or presence of monoterpenes to provide an insight into their ecological ramifications and larger-scale consequences, against the background of intra-specific variation in other traits.