Sympatric speciation by sexual conflict

Abstract

It is well established that sexual conflict can drive an endless coevolutionary chase between the sexes potentially leading to genetic divergence of isolated populations and allopatric speciation. We present a simple mathematical model that shows that sexual conflict over mating rate can result in two other general regimes. First, rather than "running away" from males, females can diversify genetically into separate groups, effectively "trapping" the males in the middle at a state characterized by reduced mating success. Female diversification brings coevolutionary chase to the end. Second, under certain conditions, males respond to female diversification by diversifying themselves. This response results in the formation of reproductively isolated clusters of genotypes that emerge sympatrically.

Fig 1.

Endless coevolutionary chase between the sexes. Parameters: μ = 10⁻⁵, r = 0.5, Δ = 10⁻⁶, v_m,j = 1, Ψ_ij = exp[−(i − j)²/(2α²)] with α = , and w_f,i = exp[−S(P_i − P_opt)²] with S = 1, P_opt = 0.4. Initial genotype frequencies are Φ₅₆ = 0.99, Φ₅₅ = 0.01. The equilibrium value of P̄ is ≈0.86.

Fig 2.

Buridan's Ass scenario. Parameters are the same as in Fig. 1 except Δ = 10⁻⁸, P_opt = .6 and initial genotype frequencies are Φ₂₃ = 0.99, Φ₂₂ = 0.01. The equilibrium value of P̄ ≈ 0.64.

Fig 3.

Sympatric speciation. Parameters are the same as in Fig. 2 except P_opt = 0.4. The equilibrium value of P̄ ≈ 0.42.

Fig 4.

Complex genetic structure. (a) Sympatric emergence of four species. Parameters are the same as in Fig. 3 except P_opt = 0.25 and S = 0.5. P̄ ≈ 0.27. (b) Genetic diversification without distinct cluster formation. Parameters are the same as in Fig. 3, except Ψ_ij = exp[−/(2α²)], P_opt = 0.7, α = 2, and initial genotype frequencies are Φ₁₀ = 0.45, Φ₋₁₀ = 0.55. P̄ ≈ 0.71.