A key question remains whether the radiation pre-dates or post-dates the establishment of a summer-arid climate in the south-western tip of southern Africa in the mid-Miocene (10–15 Mya, Cowling, Procheş & Partridge, 2009). (2004), little progress has been made with regard to understanding the precise tempo and causes of the radiation of core Ruschioideae. Nearly a decade since the publication of the ice plant dating analysis of Klak et al. This recent and fast radiation is considered one of the most rapid recorded in angiosperms (Valente, Savolainen & Vargas, 2010b) and rivals some of the fastest radiations in the world, such as that of cichlid fish ( Verheyen et al., 2003). (2004) dated the radiation of a subclade of the ruschioids (core Ruschioideae) at 3.8–8.7 Mya, with a diversification rate of 0.77–1.75 species Myr −1. With subfamily Mesembryanthemoideae, species of Ruschioideae are commonly referred to as the ‘mesembs’. Represented by 1585 species in 112 genera, Ruschioideae are one of the most species-rich and diverse clades of angiosperms in southern Africa ( Smith, 1998 Goldblatt & Manning, 2002). The most charismatic and perhaps most spectacular of the recent ‘explosive’ radiations in the GCFR is that of subfamily Ruschioideae of Aizoaceae (ice plant family). The current consensus is that high diversity in the GCFR is due to a combination of the gradual accumulation of species from old plant lineages ( Linder, 2008 Verboom et al., 2009 Valente et al., 2010a, 2011), and of recent and rapid radiations that are thought to have been triggered by the establishment of a summer-dry climate in the south-western tip of southern Africa in the Miocene ( Richardson et al., 2001 Linder, 2003 Verboom et al., 2003 Klak, Reeves & Hedderson, 2004). Considerable research has been conducted with the aim of disentangling the major forces that have driven diversification in the region ( Linder, 2003 Verboom et al., 2009 Schnitzler et al., 2011). The Greater Cape Floristic Region (GCFR) of southern Africa is the most biodiverse temperate region of the globe in terms of plant species (Born, Linder & Desmet, 2007 Kreft & Jetz, 2007). We also statistically examine environmental and morphological correlates of richness in ice plants and find that diversity is strongly linked with precipitation, temperature, topographic complexity and the evolution of highly succulent leaves and wide-band tracheids. Our estimates of the age of the clade are even younger than initially thought (stem age 1.13–6.49 Mya), supporting the hypothesis that the radiation post-dates the establishment of an arid environment in the GCFR and firmly placing the radiation among the fastest in angiosperms (diversification rate of 4.4 species per million years). Here, we reassess the tempo of radiation of ice plants by using the most complete generic-level phylogenetic tree for Aizoaceae to date, a revised calibration age and a new dating method. However, a more recent analysis found an older age for the Ruschioideae lineage (17 Mya), suggesting that the group may in fact have originated much before the aridification of the region 10–15 Mya. A previous phylogenetic study of Aizoaceae subfamily Ruschioideae dated the radiation of this clade of > 1500 species in the GCFR to 3.8–8.7 Mya, establishing it as a flagship example of a diversification event triggered by the onset of a summer-arid climate in the region. The exceptionally high plant diversity of the Greater Cape Floristic Region (GCFR) comprises a combination of ancient lineages and young radiations.
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