The evolution of complex traits is one of the great wonders of evolution. Traits such as camera eyes, flight and biochemical carbon concentrating mechanisms (CCMs) require the co-ordinated actions of multiple components for function, yet have evolved multiple times in diverse lineages of organisms and environments. In this thesis, I investigate the environmental and genetic enablers of the evolution of CCMs. I performed a literature review of these traits, leading me to focus on Crassulacean Acid Metabolism (CAM) as a study system. Whilst most commonly known in plants dwelling in hot and dry environments, this trait has also evolved in the submerged aquatic lycopod genus Isoëtes, offering a valuable comparison point to infer common evolutionary drivers. Using molecular dating to identify the temporal origins of extant aquatic CAM diversity, I found that aquatic CAM plants evolved and diversified at similar times to their terrestrial counterparts, implicating falling atmospheric CO2 levels as a common environmental driver. To identify if this shared selection pressure invoked similar genetic modifications for CAM evolution in diverse lineages, I used RNA-sequencing to identify CAM genes in two aquatic CAM plants, the lycopod Isoëtes lacustris and the angiosperm Littorella uniflora. I found that the most highly expressed gene lineages in these species strongly overlap, but found that it stemmed from conservation of gene expression levels from the ancestral vascular plant. The similarity of I. lacustris and L. uniflora occurs despite the different mating systems of the angiosperms and lycopods, so I investigated the genetic structure of these species in the British Isles, finding higher levels of genetic structure in Littorella, likely a result of its reliance on emergent flowers for sexual reproduction. These results suggest that common environmental and genetic enablers contribute to trait evolution in divergent environmental and genomic backgrounds, leading to the repeated emergence of complex traits.