Fig. 6.

Evolutionary divergence (A) and divergence in mating system characteristics (B) within clades does not correlate with gene expression distance (Spearman ρ = −0.8, P = 0.95; ρ = 0.7, P = 0.12, respectively). However, when expression variation due to phylogeny is removed (using phylogenetically independent contrasts) (C), we find a significant relationship between mating system and neural gene expression across clades (linear regression r² = 0.6, t = 3.8, P = 0.005). Mating system score was calculated as the sum of the mating system characteristics where higher values indicate more elaborated monogamy (i.e., males form pair bonds, provide direct and indirect parental care, exhibit high levels of territoriality, and are less sexually dimorphic; Fig. 2 and SI Appendix, Fig. S1 and Tables S1 and S2). A principal component analysis was performed on the differentially expressed (±1 log₂ fold difference in at least one clade) and variable (Upper quartile of variance across all species) OGGs. This subset of 401 OGGs was also used for OGG expression divergence in A and B. See SI Appendix.