Does competitive divergence occur if assortative mating is costly?

Abstract

Most models of sympatric speciation have assumed that assortative mating has no costs. A few studies, however, have shown that the costs for being choosy can prevent such speciation. Here, we investigate the role of the strength of assortment and of the costs for being choosy for a simple genetic model of a single (‘magic’) trait that mediates both intraspecific competition for a continuum of resources and assortative mating, which is induced by choosy females who preferentially mate with males of similar phenotype. Choosiness may be costly if it is difficult to find a mating partner. Such magic trait models are considered to be most conducive of sympatric speciation. We consider a sexually reproducing population of haploid individuals that is density regulated. The trait is determined by a single locus with multiple alleles. The strength of stabilizing selection (caused by a unimodal resource distribution), the strength of competition, the degree of assortment and the costs for being choosy are independent parameters. We investigate analytically and numerically how these parameters determine the equilibrium and stability structure. In particular, we identify conditions under which no polymorphism at all is maintained as well as conditions under which strong competitive divergence occurs, or the population even splits into two reproductively isolated classes of highly diverse phenotypes. If costs are absent or moderate, genetic variability tends to be minimized at intermediate strengths of assortment, and reproductively isolated classes of phenotypes are a likely result of evolution only for intermediate or strong competition and for very strong assortment. The likelihood of divergence depends relatively weakly on the costs as long as they are not high. With high costs, however, increasingly strong assortment rapidly depletes all genetic variation, and strong competitive divergence is prevented.

The counterintuitive role of sexual selection in species maintenance and speciation

“…It is known that sexual selection has the potential to lead to the loss of variation in sexually selected traits across incipient species, eliminating reproductive isolation, and that this is more likely to occur if trait and preference frequencies are increasingly asymmetrical between the diverging types (15)(16)(17)(18)(19). Because our goal is to pinpoint the role of sexual selection in species maintenance when this maintenance occurs, we center our analyses on roughly symmetric starting conditions and focus (although not exclusively) on symmetric parameter values.…”

Limits to the evolution of assortative mating by female choice under restricted gene flow

“…Whether assortative mating could evolve in this situation may depend on the biology of searching and the assumptions of the model; de Cara et al [22] found that the spread of an allele for assortative mating was uniformly inhibited by direct selection when they assumed lower mating success of rare individuals (but see, e.g. [9]; interestingly, a version of de Cara et al's model that does not induce sexual selection finds that assortment always evolves to completion provided that disruptive viability selection on the trait is present, unlike in, e.g. [12]; intermediate levels of assortment are not found in de Cara et al's models).…”

“…positive frequency-dependent selection), making it harder for sympatric speciation to occur. Similarly, Bü rger and coworkers [9,10,25] demonstrated that an intermediate level of assortment tends to deplete genetic variation in the trait that is the target of assortative mating, owing to the generation of locally stabilizing selection when rare types find it difficult to find mates (see also [35]). Although they did not study the evolution of assortment per se, they argue that this loss of variation would prevent further progress towards speciation.…”

“…[6][7][8][9][10][11][12]). Sexual selection in these models generally acts on males, driving changes in allele frequencies, but does not act on females.…”

A Model for the Evolution of Assortative Mating

“…Recently, Schneider and Bürger (2005) showed that slight costs can still allow speciation to occur. Whether this conclusion implies that assortment can evolve with slight costs remains to be shown.…”