Fig. 2

Reinforcing versus reversing expression plasticity (RI and RV, respectively). A A schematic showing how to infer RI and RV with samples of the Rufous-capped Babbler populations (coded as in Fig. 1). For genes involved in the bird’s high-altitude adaptation (ACDE genes as defined in the text), we first identified directions of PC and GC expression changes, corresponding to plastic responses of the lowland population and genetic divergence between lowland and highland populations, respectively. We then inferred RI and RV when obtaining the same and opposite PC and GC directions, respectively. Inferences were made for either brain- or liver-expressed ACDE genes, with PC and GC directions identified based on either population L1 or L2 (totally four conditions). The schematic also shows the total expression change between lowland and highland populations in their respective adapted environments (TC), with TC = PC + GC. B Frequencies of RI and RV in the ACDE genes and in their subset that acquire strong support in the parametric bootstrap analyses (≥ 950/1000). Unequal RI and RV frequencies are evaluated by two-sided binomial tests (*** indicates P < 0.001 while the blank indicates P > 0.05. C Differences between RI and RV in the absolute PC magnitude (|PC|) that are evaluated by Kruskal–Wallis tests (*** indicates P < 0.001)