Sympatric Speciation by Sexual Selection Alone Is Unlikely

Abstract

Abstract. According to Darwin, sympatric speciation is driven by disruptive, frequency-dependent natural selection caused by competition for diverse resources. Recently, several authors have argued that disruptive sexual selection can also cause sympatric speciation. Here, we use hypergeometric phenotypic and individual-based genotypic models to explore sympatric speciation by sexual selection under a broad range of conditions. If variabilities of preference and display traits are each caused by more than one or two polymorphic loci, sympatric speciation requires rather strong sexual selection when females exert preferences for extreme male phenotypes. Under this kind of mate choice, speciation can occur only if initial distributions of preference and display are close to symmetric. Otherwise, the population rapidly loses variability. Thus, unless allele replacements at very few loci are enough for reproductive isolation, female preferences for extreme male displays are unlikely to drive sympatric speciation. By contrast, similarity-based female preferences that do not cause sexual selection are less destabilizing to the maintenance of genetic variability and may result in sympatric speciation across a broader range of initial conditions. Certain groups of African cichlids have served as the exclusive motivation for the hypothesis of sympatric speciation by sexual selection. Mate choice in these fishes appears to be driven by female preferences for extreme male phenotypes rather than similaritybased preferences, and the evolution of premating reproductive isolation commonly involves at least several genes. Therefore, differences in female preferences and male display in cichlids and other species of sympatric origin are more likely to have evolved as isolating mechanisms under disruptive natural selection.

Frequency-Dependent Selection and the Evolution of Assortative Mating

“…(H ns ¼ 0) and yet the mating preferences would lead to complete assortative mating above the thick curve (Figure 4), solely because of the sexual selection induced (as noted by Arnegard and Kondrashov 2004).…”

Multiple Cases of Striking Genetic Similarity Between Alternate Electric Fish Signal Morphs in Sympatry

“…By contrast, disruptive selection on acoustic signals functioning in both echolocation and mate recognition appears to have occurred between some species of horseshoe bats (Kingston and Rossiter 2004). Arnegard and Kondrashov (2004) reason that relatively cryptic initial responses to ecological selection (e.g., behavioral resource partitioning) can accompany sympatric divergence in species-packed communities, such as the electric fish assemblage that inhabits Loa-Loa rapids. In addition, other causes of divergence under gene flow (e.g., Gavrilets and Waxman 2002;Seehausen and Schluter 2004) might not lead to immediate changes in external morphology and should not be ruled out for this intriguing system.…”

Females learn from mothers and males learn from others. The effect of mother and siblings on the development of female mate preferences and male aggression biases in Lake Victoria cichlids, genus Mbipia

“…Several developmental mechanisms can produce such assortative preferences, ranging from a fully genetically controlled development to fully environmentally determined preferences (Shaw 2000;Riebel 2003). Some of these mechanisms, however, will not produce assortative behaviour in all circumstances, while other mechanisms may particularly easily produce species-assortative interactions under many circumstances (Dieckmann and Doebeli 1999;Servedio 2000;Arnegard and Kondrashov 2004;van Doorn et al 2004;Beltman and Metz 2005;Verzijden et al 2005). The developmental mechanism behind preferences and biases in social interactions may therefore play a pivotal role in the evolution of species-assortative behaviour and, therefore, speciation.…”