Did succulents diversify in response to aridity? Evolutionary analyses of major succulent lineages around the world

Abstract

The succulent syndrome is one of the most iconic life strategies in angiosperms, maximising water storage through a suite of adaptations to water-scarcity. Though succulence is considered a classic case of convergent evolution driven by shared environmental drivers, we lack a full understanding of whether the timing and drivers of the diversification of succulent lineages are, in fact, concordant. Here we analyse time-calibrated phylogenetic reconstructions of the seven richest lineages of succulents, and study diversification dynamics in relation to abiotic variables. Our analyses reveal different levels of synchronicity and relation with aridity. The impact of atmospheric CO₂ on succulent macroevolution is varied. While transitions and radiations are especially concentrated in recent time, following a collapse of atmospheric CO₂ ∼15 million years ago, CO₂-dependent diversification is not supported in most lineages. With the exception of Euphorbia, we find that succulence elevates net diversification, though the effects on underlying speciation and extinction disagree. The phylogenetic distribution of transitions to succulence and rate shift increases suggest these phenomena are decoupled, indicating that succulence might not adhere to a classic key innovation model. We discuss that these evolutionary patterns reveal the ecological complexity of the succulent strategy, beyond simplistic interpretations as adaptations towards aridity.