Abstract
Theoretical and empirical research on the evolution of reproductive isolation have both indicated that the effects of sexual selection on speciation with gene flow are quite complex. As part of this special issue on the contributions of women to basic and applied evolutionary biology, I discuss my work on this question in the context of a broader assessment of the patterns of sexual selection that lead to, versus inhibit, the speciation process, as derived from theoretical research. In particular, I focus on how two factors, the geographic context of speciation and the mechanism leading to assortative mating, interact to alter the effect that sexual selection through mate choice has on speciation. I concentrate on two geographic contexts: sympatry and secondary contact between two geographically separated populations that are exchanging migrants and two mechanisms of assortative mating: phenotype matching and separate preferences and traits. I show that both of these factors must be considered for the effects of sexual selection on speciation to be inferred.
Highlights
The fascination that biologists have long had with speciation has increased in recent decades as we have gained a better and better understanding of the complexity of this engine of biodiversity
What can we conclude about the effects of sexual selection on speciation in these geographic contexts from these theoretical models? In sympatric speciation, it is very difficult to argue that sexual selection is doing anything but inhibiting divergence in most cases
Sexual selection is unlikely to contribute to divergence except in the late stages of sympatric speciation or in secondary sympatry, when there is already substantial divergence present (e.g., Doebeli 1996; Matessi et al 2001; Arnegard and Kondrashov 2004; Gourbiere 2004; Otto et al 2008; Pennings et al 2008)
Summary
The fascination that biologists have long had with speciation has increased in recent decades as we have gained a better and better understanding of the complexity of this engine of biodiversity. In Servedio and Bu€rger (2014), we considered a situation much more favorable to speciation—one which quite a number of speciation researchers that I informally polled (including myself!) thought would lead to sexual selection promoting speciation, despite gene flow In this model, we assumed that the male trait was, itself, under divergent selection, so that allopatric populations had evolved a preference for a locally adapted male trait before migration began between them. Because this indirect selection on preference alleles is mediated by the strength of linkage disequilibrium, it is generally weak (e.g., Kirkpatrick and Barton 1997), and low preference differentiation results These homogenized preferences will in turn cause sexual selection to tend to be a homogenizing force between populations, and the stronger it is, the more it will counter local adaptation to bring trait frequencies closer together (Fig. 3). The dynamics in models including sexual selection by separate preferences and traits are quite complex, and it is difficult to make general statements about their results; much may depend on the underlying biology of a specific pair of species (e.g., Bank et al 2012)
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