Faster evolution of a premating reproductive barrier is not associated with faster speciation rates in New World passerine birds

<b>Abstract</b><br/><p>Why are speciation rates so variable across the tree of life? One hypothesis is that this variation is explained by how rapidly reproductive barriers evolve. We tested this hypothesis by conducting a comparative study of the evolution of bird song, a premating barrier to reproduction. Speciation in birds is typically initiated when geographically isolated (allopatric) populations evolve reproductive barriers. We measured the strength of song as a premating barrier between closely related allopatric populations by conducting 2,339 field experiments to measure song discrimination for 175 taxon pairs of allopatric or parapatric New World passerine birds, and estimated recent speciation rates from a global molecular phylogeny of birds. Taxon pairs with high song discrimination in allopatry failed to regularly interbreed in parapatry, evidence that song discrimination is indeed an important reproductive barrier. However, evolutionary rates of song discrimination were not associated with recent speciation rates, and song discrimination evolves faster in suboscine passerines than their more species-rich sister clade, the oscines. Our findings support the long-held idea that song is a key premating reproductive barrier in birds, but show that faster evolution of this reproductive barrier between populations does not result in faster diversification betweeen species.</p>