Magic cues versus magic preferences in speciation

Abstract

Question: How does divergent natural selection lead to divergence in mating traits and the evolution of reproductive isolation? Background: Ecological speciation of non-allopatric taxa usually requires the evolution of an association between selective mating and the traits underlying ecological adaptation. 'Magic traits' affect both ecological fitness and assortative mating and may therefore mediate rapid evolution of reproductive isolation. Problem: When assortative mating is mediated by separate preferences and cues, as opposed to being based on trait similarity (e.g. assortment by body size or habitat), pre-mating reproductive isolation between non-allopatric populations often requires divergence in both mating preferences and mating cues. However, most proposed cases of magic trait speciation rely on observation of divergent mating cues alone, leaving the consequences for reproductive isolation uncertain. Solution: We propose that a distinction should be made between mating cues and mating preferences when documenting divergent natural selection on mating traits. We argue that immediate effects of ecological adaptation on mating preferences, through direct selection or through pleiotropy, will drive divergence in both preferences and traits much more predictably than ecological selection on mating cues. The distinction between 'magic cues' and 'magic preferences' is critical for evaluating the evolutionary consequences of divergent selection on mating traits, and implies a need for increased research effort into documenting variation in mating preferences in diverging taxa.