[1] Photosynthesis in most plants proceeds by a pathway, designated C3, whose initial carbon-fixing enzyme is prone to a wasteful reaction with oxygen — a process called photorespiration that intensifies as temperature rises and as leaves close their pores to conserve water. [2] A minority of species have evolved an elaborate alternative, the C4 pathway, which concentrates carbon dioxide around that enzyme and so suppresses the oxygen reaction, conferring an advantage under hot, bright, or arid conditions. [3] What makes the C4 pathway remarkable is not merely its efficiency but its distribution: it has arisen independently more than sixty times across unrelated plant lineages, a striking case of convergent evolution.
[4] Such repeated invention poses a puzzle, for the C4 system requires the coordinated modification of leaf anatomy, cellular biochemistry, and gene regulation, and a partial version might be expected to confer little benefit. [5] One resolution holds that the transition proceeded through intermediate stages, each independently advantageous, so that the full pathway was assembled incrementally rather than in a single improbable leap. [6] A complementary explanation invokes the environmental backdrop: a long-term decline in atmospheric carbon dioxide, together with the spread of open, seasonally dry habitats, is thought to have raised the cost of photorespiration and so repeatedly favored whatever lineages had drifted partway toward the C4 solution. [7] On this account the recurrence of C4 photosynthesis reflects less an intrinsic ease of evolving it than a persistent selective pressure that many lineages happened to meet with the same anatomical raw materials. [8] The two explanations are not rivals so much as complements: the first specifies a feasible path, the second the conditions that made traversing it worthwhile.
