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Each of >20 independent evolutions of C₄ photosynthesis in grasses required reorganization of the Calvin–Benson‐cycle (CB‐cycle) within the leaf, along with coordination of C₄‐cycle enzymes with the CB‐cycle to maximize CO₂ assimilation. Considering the vast amount of time over which C₄ evolved, we hypothesized (i) trait divergences exist within and across lineages with both C₄ and closely related C₃ grasses, (ii) trends in traits after C₄ evolution yield the optimization of C₄ through time, and (iii) the presence/absence of trends in coordination between the CB‐cycle and C₄‐cycle provides information on the strength of selection. To address these hypotheses, we used a combination of optimality modelling, physiological measurements and phylogenetic‐comparative‐analysis. Photosynthesis was optimized after the evolution of C₄ causing diversification in maximal assimilation, electron transport, Rubisco …