Diversification Slowdown in the Alliance (, Orchidaceae): Insights From the Evolutionary Dynamics of Crassulacean Acid Metabolism.

Evolutionary slowdowns in diversification have been inferred in various plant and animal lineages. Investigation based on diversification models integrated with environmental factors and key characters could provide critical insights into this diversification trend. We evaluate diversification rates in the alliance (, Orchidaceae subfam. Epidendroideae) using a time-calibrated phylogeny and assess the role of Crassulacean acid metabolism (CAM) as a hypothesised key innovation promoting the spectacular diversity of orchids, especially those with an epiphytic habit. An explosive early speciation in the alliance is evident, with the origin of CAM providing a short-term advantage under the low atmospheric CO concentrations (CO) associated with cooling and aridification in the late Miocene. A subsequent slowdown of diversification in the alliance is possibly explained by a failure to keep pace with CO dynamics. We further demonstrate that extinction rates in strong CAM lineages are ten times higher than those of C lineages, with CAM not as evolutionarily labile as previously assumed. These results challenge the role of CAM as a "key innovation" in the diversification of epiphytic orchids.