SHORT COMMUNICATION Pattern, process and geographic modes of speciation

Sympatric speciation has been fascinating and controversial because the evolution of two distinct gene pools is hampered by sexual reproduction, which tends to homogenize genetic variation among individuals. However, there is not a simple mapping between the spatial pattern of sympatry (literally, being in the same place) and the processes of gene flow and recombination. As a consequence, investigators using a biogeographic definition of sympatric speciation might appear to come to different conclusions than those using population genetic processes to define sympatric speciation. In addition, it has become increasingly clear that the classification of speciation as allopatric, parapatric or sympatric is an artificial subdivision of a continuum and misses many potentially important nuances (Endler, 1977; Rice & Hostert, 1993; Schluter, 2001; Gavrilets, 2003; Coyne & Orr, 2004; Xie et al., 2007). In recognition of this, and to avoid further confusion arising from different usage of the terms, we and others have advocated research that emphasizes modelling and estimating important parameters for describing the process of divergence, rather than evaluating criteria for classifying geographic patterns (Kirkpatrick & Ravigné, 2002; Dieckmann et al., 2004; Bolnick & Fitzpatrick, 2007; Butlin et al., 2008; Fitzpatrick et al., 2008). Mallet et al. (2009) promote a different view. Although they agree that the traditional taxonomy depends on ‘artificially discrete categories’ and the most precise definitions often imply absurd empirical standards, they advocate retaining a geographic framework chosen such that ‘sympatric speciation is likely to be more common than generally assumed’. Coyne & Orr (2004) also favour the geographic classification (albeit with a different emphasis), claiming that the geographic mode of speciation can be identified and is useful because the relationship between geography and gene flow has clear implications for the nature and strength of the processes contributing to divergence. Here, we briefly review some major generalizations that can be drawn from decades of research on the geographic modes of speciation. Then we discuss arguments in favour of continuing to classify speciation as allopatric, parapatric, or sympatric. We end by arguing that the studies discussed by Mallet et al. might be better appreciated as detailed analyses of Correspondence: Benjamin M. Fitzpatrick, Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA. Tel.: +1 865 974 9734; Fax: +1 865 974 3067; e-mail: [email protected]