Evolution of average [O III]/Hβ versus [N II]/Hα of SF-dominated GAEA-LC galaxies at different redshift intervals, as indicated by the legend (coloured lines). Shown are simulated SDSS-like galaxies with masses greater than 10⁹ M_⊙. Overplotted is the mean relation found by Steidel et al. (2014) in their sample of star-forming galaxies at redshift 2.3 (thick grey line) and, as in Fig. 3, the SDSS sample (grey contours) and empirical criteria distinguishing between ionising sources (Kewley et al. 2001; Kauffmann et al. 2003, black lines).