Relative Abundance and the Species-Specific Reinforcement of Male Mating Preference in the Chrysochus (Coleoptera: Chrysomelidae) Hybrid Zone

Abstract

Most studies of reinforcement have focused on the evolution of either female choice or male mating cues, following the long‐held view in sexual selection theory that mating mastakes are typically more costly for females than for males. However, factors such as conspecific sperm precedence can buffer females against the cost of mating mistakes, suggesting that in some hybrid zones mating mistakes may be more costly for males than for females. Thus, the historical bias in reinforcement research may underestimate its frequency. In this study, we present evidence that reinforcement has driven the evolution of male choice in a hybrid zone between teh highly promiscuous lealf beetles chyrsochus cobaltinus and C. auratus, the hybrids of which have extremely low fitness. In addition, there is evidence for male choice in these beetles and that male mating mistakes may be costly, due to reduced opportunities to mate with conspecific females. The present study combines laboratory and field methods to quantify the strenght of sexual isolation, test the hypothesis of reproductive character displacement, and assess the link between relative abundance and the strenght of selection against hybridization. We document that, while sexual isolation is weak, it is sufficient to produce positive assortative mating. In addtion, reproductive character displacement was only detected in the relatively rare species. The strong postzygotic barriers in this system are sufficient to generate the bimodality that characterizes this hybrid zone, but the weak sexual isolation is not, calling into question whether strong prezygotic isolation is necessary for the maintenance of bimodality. Growing evidence that the cost of mating mistakes is sufficient to shape the evolution of male mate choice suggests that the reinforecement of male mate choice may prove to be a widespread occurrence.

On the scent of speciation: the chemosensory system and its role in premating isolation

“…These studies show patterns of reproductive character displacement affecting mating signals and/or mate preferences in various species: for example, between D. pseudoobscura and D. persimilis (Noor, 1995), between two subspecies of the house mouse Ganem, 2005, 2008), between the leaf-beetle species Chrysochus cobaltinus and Chrysochus auratus (Peterson et al, 2005a) or among walking-stick populations (Nosil et al, 2003). Evidence for reproductive character displacement, coupled with evidence for lower fitness of hybrids in the contact areas, argues in favour of reinforcement: selection against hybridization may have favoured divergence in mating signals and/or preference in contact zone populations to reduce the cost of production of unfit hybrid offspring.…”

Fission and fusion of Darwin's finches populations

“…Any factor contributing to the prevention of mating between species would be at a selective advantage, and would increase in frequency in a manner dependent upon the relative fitness of the hybrids, as determined by their survival and reproductive success (Liou & Price 1994;Hostert 1997). Evidence for reinforcement and the divergence of traits involved in mate choice (character displacement) is taxonomically widespread (Saetre et al 1997;Marshall & Cooley 2000;Pfennig 2003;Coyne & Orr 2004;Peterson et al 2005). Reproductive isolation in Darwin's finches appears to be entirely prezygotic as there is no evidence of genetic incompatibilities (Grant 1999).…”

Ecological and Reproductive Character Displacement of an Environmental Gradient

“…Five articles in our literature review (Dunham et al 1979;Frier 1979;Saloniemi 1993;Pfennig and Murphy 2003;Tynkkynen et al 2004; see also Nosil et al 2003;Peterson et al 2005) use quantitative data on observed abundances, with the qualitative expectation that displacement of the trait in one species should be greater where the relative abundance of the other species is greater. In our models, the log of the abundance ratio (log[n i /n j ], where n i Ն n j ) and the difference in phenotypic values (ͦz i Ϫ z j ͦ) show a roughly positive, linear relationship, though linearity breaks down near the region of equal abundance (n 1 ഠ n 2 ).…”