A Deterministic Genetic Model for Sympatric Speciation by Sexual Selection

Abstract

Abstract. A deterministic haploid genetic model confirms and explores in more detail the results of our previous individual-based simulation model for sympatric speciation by sexual selection. With the deterministic model, we are able to elucidate parameter dependence by phase plane analysis. We clarify how and why sympatric speciation by sexual selection can happen in a number of ways: (1) Female preferences for or against particular types of males have different effects. Whereas the former affects how readily speciation is invoked, the latter changes the stability of speciation equilibrium. (2) When there is no cost on male ornamentations, speciation is triggered regardless of initial haplotype frequencies if sufficient female preference is provided. (3) There exists a threshold for female initial frequencies for speciation to be invoked, but male initial frequencies have little effect. (4) A small cost on female mate choice does not cancel speciation, but when large, it greatly reduces the possibility of speciation.Key words. Cichlids, disruptive sexual selection, interbreeding, mate recognition system, separatrix, sexual selection, speciation.Received November 12, 1999. Accepted July 14, 2000.Sexual selection was first suggested by Darwin (1871) to explain exaggerated male secondary sexual characters and female preference for them. Fisher (1930) argued that the coadaptation between male secondary sexual characters and female preference occurs through a runaway process, and this argument was confirmed by theoretical models (O'Donald 1967;Lande 1981;Kirkpatrick 1982). Paterson (1985) coined the term, mate recognition system (MRS) for such a co-adapted mating system and reasoned that it assures species' identities by providing a concrete species recognition mechanism and preventing mating with other close species. Because the direction of runaway process, which is thought to be initiated by a random increase of female preference, is also random (Lande 1981), the resulting MRSs will differ among geographically isolated populations. Sexual selection has therefore been considered to be a strong force that causes allopatric speciation (Andersson 1994). A question that arises here is whether two different MRSs are established in one population sympatrically, if some initial variabilities of male secondary sexual characters and female preference are provided.This idea has been examined by computer simulation models (Wu 1985;Turner and Burrows 1995;Higashi et al. 1999). We and Turner and Burrows attained stable sympatric speciation. Turner and Burrows, starting from a population with an already established MRS, demonstrated the evolution of an alternative MRS while the first MRS remains in the population. Their analysis, however, is limited. They did not examine the dependence of their key parameter: the cost of the male secondary sexual character (Higashi et al. 1999). Simulations of our individual-based model started from evolutionary established (stationary) trait distributions, where the divergence of mating trait...

Sexual selection can constrain sympatric speciation

“…This procedure departs from several previous studies based on the 'hypergeometric' model that constrains allele frequencies to be equal at all loci (e.g. Doebeli 1996;Kondrashov & Shpak 1998;Kondrashov & Kondrashov 1999;Takimoto et al 2000). That assumption can artificially stabilize the equilibrium (Barton & Shpak 2000).…”

“…As the number of loci grows, the force of selection on each diminishes, so stabilizing selection does not cause fixation in the case of an infinite number of loci. A third group of studies is based on the hypergeometric model, in which allele frequencies at all loci are constrained to be identical (Doebeli 1996;Kondrashov & Shpak 1998;Kondrashov & Kondrashov 1999;Takimoto et al 2000). This assumption is made to speed numerical simulations, but the equilibria identified can be unstable when allele frequencies at different loci are allowed to vary (Barton & Shpak 2000).…”

“…Stabilizing sexual selection generated by assortative mating can work against sympatric speciation by causing fixation at individual loci and by reducing the associations between alleles at different loci (linkage disequilibria) that are the genetic basis for sympatric speciation. Many authors have noted that mating may penalize rare phenotypes and make sympatric speciation less likely (Karlin & Scudo 1969;Kondrashov & Mina 1986;Noest 1997;Kondrashov & Shpak 1998;Van Doorn et al 1998;Dieckmann & Doebeli 1999;Kondrashov & Kondrashov 1999;Drossel & McKane 2000;Takimoto et al 2000). The impact of this effect on polymorphism and the implications for sympatric speciation under Darwin's hypothesis, however, seem not to be fully appreciated.…”

Disruptive sexual selection on male nuptial coloration in an experimental hybrid population of cichlid fish

“…Models of sympatric speciation through mate choice make the assumption that heritable variation in female preferences within a single population exerts disruptive selection on male traits, such that males with intermediate trait value obtain fewer matings than males with high trait value of either sign ( Turner & Burrows 1995;Higashi et al 1999;Kondrashov & Kondrashov 1999;Takimoto et al 2000;Arnegard & Kondrashov 2004;van Doorn et al 2004). Yet, surprisingly few empirical tests of this assumption can be found in the literature.…”

Genetic differentiation among host-associated Alebra leafhoppers (Hemiptera: Cicadellidae)

“…Most sympatric speciation models demand that there is intraspecific genetic variation in traits that differentially affect the fitness of individuals that colonise new habitats or hosts (Dieckman and Doebeli, 1999;Hawthorne and Via, 2001 but see Higashi et al, 1999 andTakimoto et al, 2000). They assume that selection acting on these traits can prevent genetic exchange between populations (Bush, 1975;Tauber and Tauber, 1977;Diehl and Bush, 1989).…”