Evolutionary and ecological aspects of photosynthetic pathway variation

C₄ and CAM photosynthesis are evolutionarily derived from C₃ photosynthesis. The morphological and biochemical modifications necessary to achieve either C₄ or CAM photosynthesis are thought to have independently arisen numerous times within different higher plant taxa. It is thought that C₄ photosynthesis evolved in response to the low atmospheric CO₂ concentrations that arose sometime after the end of the Cretaceous. Low CO₂ concentrations result in significant increases in photorespiration of C₃ plants, reducing productivity; both C₃-C₄ intermediate and C₄ plants exhibit reduced photorespiration rates. In contrast, it may be argued that CAM arose either in response to selection of increased water-use efficiency or for increased carbon gain. Globally, all three pathways are widely distributed today, with a tendency toward ecological adaptation of C₄ plants into warm, monsoonal climates and CAM plants into water-limited habitats. In an anthropogenically altered CO₂ environment, C₄ plants may lose their competitive advantage over C₃ plants.