Sympatric speciation by sexual selection

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There is increasing evidence1,2,3,4,5,6 for the process of sympatric speciation7,8, in which reproductive isolation of species occurs without physical isolation. Theoretical models9,10,11,12,13,14 have focused on disruptive natural selection as the crucial pressure for splitting a species. Here we report the theoretical finding that sympatric speciation may be caused by sexual selection even without disruptive natural selection. Specifically, we show that variation in a male secondary sexual character with two conspicuous extremes and the corresponding variance in female mating preference around no preference may jointly evolve into bimodal distributions with increasing modal divergence of the male and female traits, pulling a population apart into two prezygotically isolated populations. This mode of speciation, driven by two runaway processes15,16,17 in different directions, is promoted by an increase in the efficiency of females in discriminating among males or a decrease in the cost of male conspicuousness, indicating that sympatric speciation may occur more readily if barrier-free or predator-free conditions arise. Although even a slight cost of female preference would cancel the runaway process of sexual selection18, it would not cancel the divergent runaway processes of sympatric speciation.

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Figure 1: Examples of sympatric speciation generated by our model.
Figure 2: Three possible evolutionary outcomes: no change, trait shift and speciation.
Figure 3: The likelihood (represented here by the number of occurrences out of 30 simulations runs) of each of the three evolutionary outcomes, speciation (defined here as a case in which the interbreeding rate is less than 0.05), trait shift and no change, when either α or ωM is changed from a value for generating initial stationary unimodal distributions to another value.


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M.H. thanks J. Lawton and the NERC Centre for Population Biology for their hospitality. This work was supported by a MESSC grant to M.H.

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Correspondence to M. Higashi.

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Higashi, M., Takimoto, G. & Yamamura, N. Sympatric speciation by sexual selection. Nature 402, 523–526 (1999) doi:10.1038/990087

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