Fig. 5. Hybrid song instability, character displacement, and asymmetric introgression.

Double-hierarchical linear mixed model results illustrating a genome-wide ancestry effects on IOI and its variance (mean and 95% CI for 25,000 independent posterior draws), with higher pusillus ancestry associated with faster, and more stable songs, with the hypothesis of unstable songs in hybrids (H1) supported for those individuals with mixed ancestry (represented by quadratic term) specifically at b NRXN1 and COQ8A (smaller panels). Support for character displacement c in song stability (H2) in pure parental species ancestry individuals (>99%), based on significant interaction of species (pusillus) and sympatry, showing that the difference in stability between the species is significantly greater in sympatry (represented by σ pusillus x sympatry) than in allopatry (represented by σ pusillus), where it is not significantly different. σ terms indicate estimates for the variance part of the models, with * denoting statistically significant effects. Positive mean estimates and 95% CI (based on 37,500 independent posterior draws) are represented in blue, and negative in red. For exact estimates and 95% CI for the models in (a–c), please refer to Supplementary Tables 11, 13. Parental ancestries d of 95 females (heterogametic sex) in the contact zone, determined by assigning Z chromosome ancestry to fathers and calculating the proportion of the autosomal ancestry estimate attributed to mothers after accounting for paternal ancestry estimates. Pure pusillus mothers (>0.97 pusillus ancestry, red lines) mate assortatively with (<0.8) pusillus fathers, but pure extoni mothers (<0.03 pusillus ancestry, yellow lines) mate with males across the spectrum of pusillus ancestry.