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.
Premating isolation in animals involves decision-making processes that affect whether individuals accept or reject heterospecific mates. An integrative understanding of the behavioural processes underlying heterospecific acceptance can clarify the conditions under which premating isolation evolves. As an illustration, we review how Reeve's (Reeve 1989
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, 407–435. (
doi:10.1086/284926
)) acceptance threshold model can help make sense of patterns of premating isolation in nature. This model derives a threshold trait value for acceptance for rejection of recipients of an action (e.g. mating) based on the fitness consequences of these decisions. We show that the maintenance of partial reproductive isolation can be an outcome of optimal acceptance thresholds, even in the face of reinforcement. We also use this model to clarify how the composition of multispecies communities can shape premating isolation. The acceptance threshold model can also be viewed as the behavioural underpinning of reproductive character displacement and cascading reinforcement. Finally, we highlight potential limitations of the acceptance threshold model with respect to investigating the role of sexual selection in speciation, and we propose that integration of behavioural models in speciation research will help us gain a full picture of the mechanisms underlying premating isolation.
This article is part of the theme issue ‘Signal detection theory in recognition systems: from evolving models to experimental tests’.
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