Sympatry, syntopy, and species age: disentangling drivers of signal evolution in a large radiation of passerine birds (Meliphagides)

The evolution of signalling traits is crucial in species diversification, because they can become effective barriers to interspecific hybridization. Among various selection pressures acting on signal evolution, species interactions can reinforce signal divergence via reproductive character displacement, especially during secondary sympatry. Although previous evidence suggests that sympatry promotes signal divergence, methods using large-scale geographical range overlap fail to capture local co-occurrence (syntopy), an essential prerequisite of species interactions. In this study, we used 116 sympatric species pairs of Meliphagides, a large radiation of Australasian passerines, to test simultaneously the effects of node age, range sympatry, and, for the first time, local syntopy on colour and song divergence. Signal divergence increased with node age (although not consistently). However, we did not find conclusive evidence that sympatry was driving signal evolution, probably owing to a necessary exclusion of allopatric pairs from our analyses. Furthermore, we did not demonstrate any significant effect of syntopy on signal evolution. This could suggest that reinforcement of signal divergence in sympatry (and syntopy) is not as common as previously thought, with neutral trait drift potentially being a more dominant driver. Alternatively, constraints on signal evolution might prevent trait divergence.