Fig. 4: Simulations of temporal dynamics of hybridization where alleles for a reproductive barrier trait have partially dominant expression in one population, and with incomplete assortative mating.

Throughout the figure, red and blue colors indicate mating trait and selectively neutral clines, respectively. a Position of cline centers (Y axis) over a course of 500 generations of hybridization (X axis). The eight scenarios vary in the strength of assortative mating, degree of partial dominance and a number of loci underlying mating trait. All clines have centers initially positioned at 0.5 and are allowed to move towards a trait encoded by recessive homozygote (0) or dominant homozygote (1). Clines moving towards dominant homozygote in mating trait and selectively neutral loci are highlighted. b Frequency of cline centers displaced towards recessive homozygote (RH, 0–0.44), staying relatively stable in space (S, 0.45–0.55) and displaced towards dominant homozygote (DH, 0.56–1) by the end of the simulation. c Distribution of cline center positions for selectively neutral and mating trait loci for a subset of mating clines moved towards dominant homozygote (DH). Mean centers are indicated by dashed lines. P-values stand for paired one-sided Wilcoxon test. Note that under the tested parameter combinations, the mating cline will rarely stay stable in space and will be moving towards recessive or dominant homozygote genotype. The latter scenario, matching the pattern observed in the wagtail hybrid zone, was an especially likely outcome under the moderate partial dominance and moderate assortative mating.