Evolutionary plant radiations: where, when, why and how?

Radiations generating exceptionally diverse clades are a fundamental component of evolutionary diversification across all organismal groups. For plants, radiations have occurred in many different geographical and ecological settings, many different plant lineages, and atmany different times over the last 400 million years. This prevalence means that working out the causes, mechanisms and outcomes of radiations is central to understanding the evolution of plant diversity. This Special Issue of New Phytologist focuses on plant radiations and contains 19 papers spanning a vibrant mix of conceptual, methodological and empirical contributions. These papers result from the Symposium,  Where, When, Why and How?, which took place in Z€ urich (Switzerland), 13–14 June 2014 (http://www. systbot.uzh.ch/static/congresses/radiations/). The complementary primary data that inform us about evolutionary diversification – fossils and time-calibrated molecular phylogenies – are amply represented in this Special Issue, with landmark studies using either both in combination or just one type of information. Macro-evolutionary studies have proliferated massively in recent years with the development of time-calibrated molecular phylogenies. For the first time these are revealing what had been suspected for a long time: the existence of extensive diversification rate heterogeneity through time, among lineages, and in different geographical and ecological settings. As a result, in the last decade, investigating the patterns and processes of radiations has become both possible and highly topical, and research in this area has been developing very rapidly. This Special Issue summarizes the current state of play aboutWhere,When,Why and How plant radiations happened, and the significant progress that has beenmade over the last few years since these questions were last posed (Linder, 2008). In light of these advances it is interesting to reflect upon what constitutes a radiation. This is an old and well-trodden debate (Givnish, 1997, in this issue,  1998; Donoghue & Sanderson, pp. 260–274) not least because the term radiation can encompass a wide spectrum of concepts. Few would disagree that most radiations involve elements of both adaptive (phenotypic trait or ecological) diversification and lineage (species) diversification (Sanderson, 1998; Donoghue & Sanderson, pp. 260–274; Losos & Mahler, 2010; but see Givnish, ). Somewould argue that radiationsmust constitute rapid episodes of species and/or trait diversification. The ability to quantify rates of evolution and locate rate shifts across phylogenies more precisely (reviewed by Stadler, 2013; Morlon, 2014), opens up opportunities to understand the interplay between species and trait diversification on a scale not previously envisaged (e.g. Venditti et al., 2011; Rabosky et al., 2013). With these more powerful insights come possibilities to define radiations more objectively and quantitatively (e.g. Drummond et al., 2012), but also in more specific and restrictive ways (Bouchenak-Khelladi et al., pp. 313–326; Donoghue & Sanderson, pp. 260–274). This new era of quantitative analyses argues for retaining a broad concept of what constitutes a radiation – as adopted in this Special Issue – whilst recognizing the finer conceptual distinctions, many particular types of radiations (e.g. adaptive radiation, non-adaptive radiation,mixedmodel radiations, explosive species diversification, super radiation, semi-replicated radiations, progressive radiations, convergent radiations), the potential continuities across these definitional spectra (Olsen & Arroyo-Santos, 2009), and the diverse evolutionary processes underlying radiations (Givnish, ).