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.

Rapidin-situdiversification rates in Rhamnaceae explain the parallel evolution of high diversity in temperate biomes from global to local scales

“…Expansion of temperate biomes is thought to have provided ecological opportunities for lineages to colonize and diversify (Simpson, 1953;Donoghue, 2008;Stroud & Losos, 2016). Indeed, many temperate-adapted lineages evolved and diversified around and after Oligocene, such as drought-and cold-adapted C4 grasses (Poaceae), succulents (e.g., Aizoaceae, Cactaceae), and orchids (Orchidoideae), leading to the subsequent spread of grasslands and deserts (Spriggs et al, 2014;Palazzesi et al, 2022;Thompson et al, 2023a;Thompson et al, 2023b). In most of these cases, it is thought that lineages already possessed traits needed for colonizing a new region, and that the niche was largely conserved as lineages radiated after arrival (Wiens & Donoghue, 2004;Donoghue, 2008;Crisp et al, 2009;Donoghue & Edwards, 2014).…”