SEXUAL SELECTION AND NATURAL-SELECTION IN BIRD SPECIATION

The role of sexual selection in speciation is investigated, addressing two main issues. First, how do sexually selected traits become specie s recognition traits? Theory and empirical evidence suggest that femal e preferences often do not evolve as a correlated response to evolutio n of male traits. This implies that, contrary to runaway (Fisherian) m odels of sexual selection, premating isolation will not arise as an au tomatic side effect of divergence between populations in sexually sele cted traits. I evaluate premating isolating mechanisms in one group, t he birds. In this group premating isolation is often a consequence of sexual imprinting, whereby young birds learn features of their parents and use these features in mate choice. Song, morphology and plumage a re known recognition cues. I conclude that perhaps the main role for s exual selection in speciation is in generating differences between pop ulations in traits. Sexual imprinting then leads to these traits being used as species recognition mechanisms. The second issue addressed in this paper is the role of sexual selection in adaptive radiation, aga in concentrating on birds. Ecological differences between species incl ude large differences in size, which may in themselves be sufficient f or species recognition, and differences in habitat, which seem to evol ve frequently and at all stages of an adaptive radiation. Differences in habitat often cause song and plumage patterns to evolve as a result of sexual selection for efficient communication. Therefore sexual sel ection is likely to have an important role in generating premating iso lating mechanisms throughout an adaptive radiation. It is also possibl e that sexual selection, by creating more allopatric species, creates more opportunity for ecological divergence to occur. The limited avail able evidence does not support this idea. A role for sexual selection in accelerating ecological diversification has yet to be demonstrated.