Pervasive reinforcement and the role of sexual selection in biological speciation

Sexual selection has been widely implicated as a driver of speciation. However, allopatric forms are often defined as species based on divergence in sexually selected traits and it is unclear how much such trait differences affect reproductive isolation upon secondary contact, the defining feature of biological species. We show that in birds, divergence in song and plumage in allopatry corresponds poorly with whether species mate assortatively in hybrid zones and argue that this is because many other factors besides trait divergence affect propensity to hybridize, including rarity of conspecific mates and choice based on territory rather than male traits. We then present a general model for the establishment of sympatry that assumes a period of differentiation in allopatry followed by secondary contact and often hybridization, with hybridization subsequently reduced by reinforcement of mate preferences. We suggest that reinforcement commonly operates by a narrowing of a "window of recognition" for traits that are different between the species, rather than evolution of the traits themselves. Our arguments imply that it is important to study postmating as well as premating reproductive isolation in limiting sympatry and suggest that studies of reinforcement should focus on evolution of female preferences for diagnostic traits, rather than evolution of traits per se.