Hitherto we have been concerned with the tactics adopted by individuals and the performance of populations, and we have demonstrated a number of applications of evolutionary theory to these levels of organisation. However, one of the most active areas of both theoretical and empirical ecological research concerns the interspecific interactions among individuals and populations. Such interactions fall within the ambit of community ecology or synecology. Most community properties are the sum of the properties of individuals and consequently reflect the separate adaptations of species. This causes difficulty in the application of evolutionary theory to explain the apparent structure and resilience exhibited by some communities with regard to phenomena such as species diversity (for an alternative view see Wilson (1980)). One type of individual selection which can increase the structure and stability of communities is evolutionary change in a trait of the individuals in one population in response to a trait exhibited by the individuals in a second population, followed by an evolutionary response by the second population to the change in the first. This process is called coevolution (Janzen, 1980), and may arise in a number of circumstances, depending on the nature of the effects of the interacting species on each other (Table 7.1).

Where two species interact to the benefit of both, the interaction is termed mutualism. Mutualism and other two-species interactions may arise without coevolution, but coevolution may enhance the relationship to the extent that the species become obligatorily interdependent (symbiosis). A more immediately obvious interaction is one where one species enhances its fitness to the detriment of another through predation or parasitism.