Character displacement is one of the most studied phenomena in evolutionary biology and community ecology, yet recurrent criticisms have called the inferential power of past research into question. We propose a new experimental approach, adopted from the contemporary coexistence literature, that tests the intensity of competitive interactions among sympatric and allopatric populations of species using the 'invasibility criterion', rather than traditional characters. The invasibility criterion tests the population growth rates of competing species when each is rare and its competitor is abundant -- it can be used to assess the strength of character displacement, as well as its ecological consequences for coexistence outcomes and relative abundances.
Results/Conclusions
Using tests of the invasibility criterion, we demonstrate an increased ability to (i) identify instances of character displacement among organisms that are amenable to experiments, (ii) understand the evolutionary pathways through which it is achieved, (iii) quantify its effect on competition and coexistence, and (iv) perform comparative tests across taxa. The invisibility framework can also reveal less intuitive forms of character displacement, such as character convergence or parallel character evolution. Though these forms are predicted by theory, empirically identifying their existence has been hindered by a lack of appropriate tests. Overall, our approach expands the scope of research in character displacement to forms that are not readily identified by specific characters and to explore its community impacts.