11.1 Introduction

The existence of sympatric speciation has been a contentious issue because empirical support was scarce and the underlying theoretical mechanisms were not as fully understood as we might like (e.g. Futuyma & Mayer, 1980; Rundle & Nosil, 2005). The view on sympatric speciation is currently changing, however. Recent theories demonstrate how ecological adaptations can drive speciation (Dieckmann et al., 2004; Doebeli et al., 2005). In concert with theoretical development, empirical evidence corroborating this view is accumulating (Barluenga et al., 2006; Panova et al., 2006; Savolainen et al., 2006). An obstacle for sympatric speciation is the exchange of alleles between lineages and the homogenising effect of recombination in sexual reproduction (Felsenstein, 1981; Rice & Salt, 1988). The current view on sympatric speciation is therefore that disruptive selection for evolutionary divergence has to be correlated with assortative mating and reproductive isolation (Felsenstein, 1981; Rundle & Nosil, 2005). This can be through linkage between ecological genes and mating genes, or a pleiotropic effect of ecological genes on mating behaviour. Orr & Smith (1998) make the distinction between extrinsic and intrinsic barriers to gene flow. Extrinsic factors are physical barriers in the environment that prevent encounters between individuals. Intrinsic factors are genetic traits that increase pre- or post-zygotic reproductive isolation. They define sympatric speciation as ‘the evolution of intrinsic barriers to gene flow in the absence of extrinsic barriers’.

Recognition operates at many levels in biological organisms. At the suborganismal level, immune systems manufacture antibodies that are able to recognize and bind to foreign substances (antigens), thereby initiating a process that leads to antigen destruction. At the organismal level, individuals discriminate between objects in their environments as a function of the objects', say, nutritional value. At the population level, social structures are set up by individuals who are able to classify their conspecifics in terms of belonging to a particular group or class of individuals. If group structure is based on kinship between individuals then some type of kin recognition system is usually required to maintain the integrity of kin groups.

Recognition systems at all levels involve communication of information, whether the information is stored in the stereochemistry of molecules or the morphology of body features. In the simplest recognition systems, the messenger carrying the information is the object itself (e.g. an antigen) and the entity receiving the information executes the action (e.g. a lymphocyte). In more complex recognition processes, an object encodes a message in the form of a signal that is propagated by some physical (light, sound) or chemical transport (odour) process. Communication is completed when this signal is intercepted by a sensory system, decoded, and processed by the brain (an action may be initiated or the organism may decide not to respond). This definition is not limited to biological systems: it covers machines such as barcode readers.