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Tragopogon comprises approximately 150 described species distributed throughout Eurasia from Ireland and the UK to India and China with a few species in North Africa. Most of the species diversity is found in Eastern Europe to Western Asia. Previous phylogenetic analyses identified several major clades, generally corresponding to recognized taxonomic sections, although relationships both among these clades and among species within clades remain largely unresolved. These patterns are consistent with rapid diversification following the origin of Tragopogon, and this study addresses the timing and rate of diversification in Tragopogon. Using BEAST to simultaneously estimate a phylogeny and divergence times, we estimate the age of a major split and subsequent rapid divergence within Tragopogon to be ~2.6 Ma (and 1.7-5.4 Ma using various clock estimates). Based on the age estimates obtained with BEAST (HPD 1.7-5.4 Ma) for the origin of crown group Tragopogon and 200 estimated species (to accommodate a large number of cryptic species), the diversification rate of Tragopogon is approximately 0.84-2.71 species/Myr for the crown group, assuming low levels of extinction. This estimate is comparable in rate to a rapid Eurasian radiation in Dianthus (0.66-3.89 species/Myr), which occurs in the same or similar habitats. Using available data, we show that subclades of various plant taxa that occur in the same semi-arid habitats of Eurasia also represent rapid radiations occurring during roughly the same window of time (1.7-5.4 Ma), suggesting similar causal events. However, not all species-rich plant genera from the same habitats diverged at the same time, or at the same tempo. Radiations of several other clades in this same habitat (e.g. Campanula, Knautia, Scabiosa) occurred at earlier dates (45-4.28 Ma). Existing phylogenetic data and diversification estimates therefore indicate that, although some elements of these semi-arid communities radiated during the Plio-Pleistocene period, other clades sharing the same habitat appear to have diversified earlier.

Summary Tragopogon is a large Eurasian genus of approximately 150 species. Despite the use of 6910 aligned bp of sequence data representing seven loci, relationships within the genus remain largely unresolved. The young age of the genus in combination with rapid diversification may be the best explanation for this poor resolution. Our studies have revealed that Geropogon is well supported as the immediate sister to Tragopogon. Sections Tragopogon, Brevirostris, Chromopappus, and Hebecarpus of traditional taxonomic treatments are largely monophyletic; sections Angustissimi, Majores, Collini, and Profundisulcati are non-monophyletic. The monotypic sections Macropogon, Dasypogon, and Dybjanskya appear within other sections and no longer merit recognition. Our molecular investigations of geographically widespread species in Europe, including T. crocifolius, T. pratensis, T. porrifolius, and T. orientalis, indicate that each may be non-monophyletic, comprising several cryptic species. These widespread diploids...

Early flowering plants are thought to have been woody species restricted to warm habitats. This lineage has since radiated into almost every climate, with manifold growth forms. As angiosperms spread and climate changed, they evolved mechanisms to cope with episodic freezing. To explore the evolution of traits underpinning the ability to persist in freezing conditions, we assembled a large species-level database of growth habit (woody or herbaceous; 49,064 species), as well as leaf phenology (evergreen or deciduous), diameter of hydraulic conduits (that is, xylem vessels and tracheids) and climate occupancies (exposure to freezing). To model the evolution of species' traits and climate occupancies, we combined these data with an unparalleled dated molecular phylogeny (32,223 species) for land plants. Here we show that woody clades successfully moved into freezing-prone environments by either possessing transport networks of small safe conduits and/or shutting down hydraulic function by dropping leaves during freezing. Herbaceous species largely avoided freezing periods by senescing cheaply constructed aboveground tissue. Growth habit has long been considered labile, but we find that growth habit was less labile than climate occupancy. Additionally, freezing environments were largely filled by lineages that had already become herbs or, when remaining woody, already had small conduits (that is, the trait evolved before the climate occupancy). By contrast, most deciduous woody lineages had an evolutionary shift to seasonally shedding their leaves only after exposure to freezing (that is, the climate occupancy evolved before the trait). For angiosperms to inhabit novel cold environments they had to gain new structural and functional trait solutions; our results suggest that many of these solutions were probably acquired before their foray into the cold. © 2014 Macmillan Publishers Limited. All rights reserved.

Nature 506, 89–92 (2014); doi:10.1038/nature12872 corrigendum Nature 514, 394 (2014); doi:10.1038/nature13842 Three readers pointed out that in this Letter we applied the threshold of 0.044 (the size at which freezing-induced embolisms are believed to become frequent at modest tensions) to the area of the conduit (in mm2) rather than the diameter (in mm).