MINI REVIEW Character displacement: in situ evolution of novel phenotypes or sorting of pre-existing variation?

Character displacement (Brown & Wilson, 1956), or what Darwin (1859) called divergence of character, is a commonly observed pattern in plants and animals (reviewed in Howard, 1993; Schluter, 2000; Dayan & Simberloff, 2005). Populations of two closely related species are often different phenotypically where the species occur together (‘sympatry’) but are indistinguishable where each species occurs alone (‘allopatry’; Fig. 1a). Character displacement may take two distinct forms. First, when species compete for resources, selection may lead to ‘ecological character displacement’ (Slatkin, 1980; Schluter, 2001). Ecological character displacement arises when competition between similar heterospecific individuals imposes directional selection on each species’ resource use and associated phenotypic characters, leading to divergence between species in these traits and a concomitant reduction in competition (reviewed in Robinson & Wilson, 1994; Schluter, 2000; Day & Young, 2004; Dayan & Simberloff, 2005). Secondly, when species interfere with each other’s ability to identify conspecific mates, or when they risk costly mismatings with one another, selection may lead to ‘reproductive character displacement’ (Blair, 1955; Crozier, 1974). Reproductive character displacement arises when Correspondence: Amber M. Rice, Department of Biology, University of North Carolina, Chapel Hill, NC 27599, USA. Tel.: 919-962-3595; fax: 919-962-1625; e-mail: [email protected]