ORIGINAL ARTICLE doi:10.1111/evo.12534 Evolutionary bursts in Euphorbia (Euphorbiaceae) are linked with photosynthetic pathway James W. Horn,1 Zhenxiang Xi,2 Ricarda Riina,3,4 Jess A. Peirson,3 Ya Yang, 3 Brian L. Dorsey,3,5 Paul E. Berry,3 Charles C. Davis,2 and Kenneth J. Wurdack1,6 1Department of Botany, Smithsonian Institution, NMNH MRC-166, P.O. Box 37012, Washington, DC 20013 2Department of Organismic and Evolutionary Biology, Harvard University Herbaria, 22 Divinity Avenue, Cambridge, Massachusetts 02138 3Department of Ecology and Evolutionary Biology and University of Michigan Herbarium, 3600 Varsity Drive, Ann Arbor, Michigan 48108 4Real Jardın´ Botanico,´ RJB-CSIC, Plaza de Murillo 2, 28014 Madrid, Spain 5The Huntington Botanical Gardens, 1151 Oxford Road, San Marino, California 91108 6E-mail:
[email protected] Received December 6, 2013 Accepted September 17, 2014 The mid-Cenozoic decline of atmospheric CO2 levels that promoted global climate change was critical to shaping contempo- rary arid ecosystems. Within angiosperms, two CO2-concentrating mechanisms (CCMs)—crassulacean acid metabolism (CAM) and C4—evolved from the C3 photosynthetic pathway, enabling more efficient whole-plant function in such environments. Many an- giosperm clades with CCMs are thought to have diversified rapidly due to Miocene aridification, but links between this climate change, CCM evolution, and increased net diversification rates (r) remain to be further understood. Euphorbia (2000 species) in- cludes a diversity of CAM-using stem succulents, plus a single species-rich C4 subclade. We used ancestral state reconstructions with a dated molecular phylogeny to reveal that CCMs independently evolved 17–22 times in Euphorbia, principally from the Miocene onwards.