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C4 photosynthesis results from anatomical and biochemical characteristics that together concentrate CO2 around ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), increasing productivity in warm conditions. This complex trait evolved through the gradual accumulation of components, and particular species possess only some of these, resulting in weak C4 activity. The consequences of adding C4 components have been modelled and investigated through comparative approaches, but the intraspecific dynamics responsible for strengthening the C4 pathway remain largely unexplored. Here, we evaluate the link between anatomical variation and C4 activity, focusing on populations of the photosynthetically diverse grass Alloteropsis semialata that fix various proportions of carbon via the C4 cycle. The carbon isotope ratios in these populations range from values typical of C3 to those typical of C4 plants. This variation is statistically explained by a combination of leaf anatomical traits linked to the preponderance of bundle sheath tissue. We hypothesize that increased investment in bundle sheath boosts the strength of the intercellular C4 pump and shifts the balance of carbon acquisition towards the C4 cycle. Carbon isotope ratios indicating a stronger C4 pathway are associated with warmer, drier environments, suggesting that incremental anatomical alterations can lead to the emergence of C4 physiology during local adaptation within metapopulations. Summary Statement: Intraspecific analyses of photosynthetically diverse Alloteropsis semialata populations show that strengthening of the C4 photosynthetic pathway is associated with changes in multiple leaf anatomical traits. Stronger C4 activity is correlated with warmer, drier habitats, indicating local adaptation. [ABSTRACT FROM AUTHOR]