Secondary Sympatry as a Sorting Process

A much remarked-upon pattern in nature is elevated trait disparity in sympatric relative to allopatric populations or species. Early explanations focused on secondary contact between allopatrically speciating taxa and emphasised adaptive divergence driven by costly interactions in sympatry (i.e., ‘character displacement’). Here we consider a related hypothesis, ‘species sorting’, which describes a bias in the outcome of secondary contact wherein lineages are unlikely to establish sympatry unless and until they evolve sufficient trait differences in allopatry. Sorting-like processes are prevalent in community assembly theory but are more seldom discussed in the context of speciation and secondary sympatry. We first define ecological and reproductive species sorting as analogous to ecological and reproductive character displacement, and we synthesise ‘differential fusion’ and the ‘Templeton effect’ within this framework. Through the logic of coexistence and assembly theories, we distinguish the types of allopatry-derived trait differences that will likely promote sympatry from those that likely will not, and we discuss biogeographic consequences of the latter. We then highlight new empirical approaches to distinguish sorting from displacement and survey the mixed evidence to-date. We finally suggest key priorities for future research into the hypothesized role of species sorting as a generator of major biodiversity patterns.