Effects of female preference intensity on the permissiveness of sexual trait polymorphisms

Abstract

Recent developments in sexual selection theory suggest that on their own, mate preferences can promote the maintenance of sexual trait diversity. However, how mate preferences constrain the permissiveness of sexual trait diversity in different environmental regimes remains an open question. Here, we examine how a range of mate choice parameters affect the permissiveness of sexual trait polymorphism under several selection regimes. We use the null model of sexual selection and show that environments with strong assortative mating significantly increase the permissiveness of sexual trait polymorphism. We show that for a given change in mate choice parameters, the permissiveness of polymorphism changes more in environments with strong natural selection on sexual traits than in environments with weak selection. Sets of nearly stable polymorphic populations with weak assortative mating are more likely to show accidental divergence in sexual traits than sets of populations with strong assortative mating. The permissiveness of sexual trait polymorphism critically depends upon particular combinations of natural selection and mate choice parameters.

Keywords: assortative mating; fisher process; null model; polymorphism permissiveness; sexual selection.

Figure 1

Effects of selection parameters (α₁ , α₂ and s) on polymorphic zones. (a) Phase map showing attraction basin of polymorphic equilibria (polymorphic zone), delimited by two thresholds, U and L (thick black curves) for α₁ = α₂ = α = 0.8. The thin black line is the theoretical stable line of polymorphic equilibria. (b) Changes in the polymorphic zone when mate preferences are equal in both female types (α = α₂ = α), varying α. (c) Changes in the polymorphic zone for unequal mating preferences: varying α₁ and holding α₂ constant and strong (α₂ = 0.8). (d) Changes in the polymorphic zone as a function of viability selection strength s for α₁ = α₂ = 0.6. Each combination of α₁ and α₂ or s (vertical axis) in b, c, and d correspond with one upper and one lower boundary and forms one polymorphic zone. Dark black lines on the light gray surface (U) are upper boundaries and those on the dark gray surface (L) are lower boundaries. Note the differences in shape and size of polymorphic zones. Starting frequencies of P₁ and T₁ alleles anywhere inside U and L boundaries maintain polymorphism in T in the future. Starting points outside U and L surfaces lose T polymorphism in the future

Figure 2

Relationship between the area of the polymorphic zone (permissiveness of sexual trait polymorphism) and the mean strength of mate choice (α_mean) under different viability selection (s) regimes Each dot in the panels a–i represents the area of the polymorphic zone for a unique combination of α₁ and α₂ across the entire range of possible values of α₁ and α₂