The genetics of convergent evolution: insights from plant photosynthesis

The tree of life is resplendent with examples of convergent evolution, whereby distinct species evolve the same trait independently. Many highly convergent adaptations are also complex, which makes their repeated emergence surprising. In plants, the evolutionary history of two carbon concentrating mechanisms (CCMs) - C₄ and crassulacean acid metabolism (CAM) photosynthesis - presents such a paradox. Both of these modifications of ancestral C₃ photosynthesis require the integration of multiple anatomical and biochemical components, yet together they have evolved more than one hundred times. The presence of CCM enzymes in all plants suggests that a rudimentary CCM might emerge via relatively few genetic changes in potentiated lineages. Here, we propose that many of the complexities often associated with C₄ and CAM photosynthesis may have evolved during a post-emergence optimization phase. The ongoing development of new model clades for young, emerging CCMs is enabling the comparative studies needed to test these ideas.