New Phytol. http://doi.org/6t5 (2015)
The expansion of dryland habitats in the late Miocene and early Pliocene, around 10 to 5 million years ago, coincided with the divergence of dryland plant lineages and the emergence of adaptive traits, such as the carbon-concentrating forms of photosynthesis. Ruth Bone and colleagues show that the evolution of crassulacean acid metabolism (CAM) photosynthesis in Afro-Madagascan orchids can be linked to the emergence of new niches around this time.
By temporally separating the processes of carbon capture and fixation by Rubisco, CAM plants are able to keep their stomata closed during the day, and thereby conserve water in arid environments. Bone et al. examined the evolution of CAM in Eulophiinae, a species-rich group of largely terrestrial orchids found in Africa and Madagascar. Combining stable isotope measurements of herbarium specimens, phylogenetic reconstructions and climate data they show that CAM evolved in four independent lineages of this orchid subtribe from the late Miocene onwards.
The researchers find that each of these CAM lineages occupies a different niche along an environmental gradient spanning dry forests and seasonally dry grasslands, suggesting that CAM evolution facilitated the spread of these orchids into newly emerging arid environments.