Abstract
The Irano-Turanian floristic region spans a topographically complex and climatically continental territory, which has served as a source of xerophytic taxa for neighboring regions and is represented by a high percent of endemics. Yet, a comprehensive picture of the abiotic and biotic factors that have driven diversification within this biota remains to be established due to the scarcity of phylogenetic studies. Acantholimon is an important component of the subalpine steppe flora of the Irano-Turanian region, containing c. 200 cushion-forming sub-shrubby pungent-leaved species. Our recent molecular phylogenetic study has led to enlarging the circumscription of this genus to include eight mono- or oligospecific genera lacking the characteristic life-form and leaves. Using the same molecular phylogeny, here we investigate the tempo and mode of diversification as well as the biogeographic patterns in this genus, to test the hypothesis that a combination of key morphological innovations and abiotic factors is behind Acantholimon high species diversity. Molecular dating analysis indicates that Acantholimon s.l. started to diversify between the Late Miocene and the Pliocene and the biogeographic analysis points to an Eastern Iran–Afghanistan origin. Macroevolutionary models support the hypothesis that the high diversity of the genus is explained by accelerated diversification rates in two clades associated with the appearance of morphological key innovations such as a cushion life-form and pungent leaves; this would have favored the colonization of water-stressed, substrate-poor mountainous habitats along the newly uplifted IT mountains during the Mio-Pliocene. Given the apparent similarity of mountain habitats for most species of Acantholimon, we hypothesize that its current high species diversity responds to a scenario of non-adaptive radiation fueled by allopatric speciation rather than evolutionary radiation driven by ecological opportunity. Similar scenarios might underlie the high diversity of other speciose genera in the topographically complex Irano-Turanian landscape, though this remains to be tested with fine-grained distribution and climatic data.
Highlights
The disparity of organismal diversification rates and its causes are a hot topic in evolutionary biology (Ricklefs, 2006; Linder, 2008; Losos, 2010; Morlon et al, 2011; Pyron and Burbrink, 2013; Stadler, 2013; Morlon, 2014; Alexander et al, 2016; López-Estrada et al, 2018)
In defining the boundaries of our operational areas, we considered the current distribution patterns of Acantholimon s.l. species, i.e., geographic areas defined by the congruence in distribution of two or more species, and the existence of significant geological features that could have acted as barriers to dispersal, or whose appearance could have resulted in the formation of new species due to vicariance (Sanmartín, 2014)
Acantholimon s.l. diverged from its sister genus 7.48 Mya (4.91– 11.72 95% high posterior density (HPD)) and started to diversify 4.08 Mya (2.61– 6.22 95% HPD)
Summary
The disparity of organismal diversification rates (speciation minus extinction) and its causes are a hot topic in evolutionary biology (Ricklefs, 2006; Linder, 2008; Losos, 2010; Morlon et al, 2011; Pyron and Burbrink, 2013; Stadler, 2013; Morlon, 2014; Alexander et al, 2016; López-Estrada et al, 2018). Most studies on radiations have focused on adaptive radiation, defined as “the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage” (Schluter, 2000), a process in which ecological opportunity and key innovations are important features (Givnish, 2010; Yoder et al, 2010) While there is both empirical (Seehausen, 2006; Fior et al, 2013; Joly et al, 2013; Lagomarsino et al, 2016) and theoretical (Gavrilets and Vose, 2005) support for the occurrence of this model, which elements are central to the hypothesis of adaptive radiation remain controversial (Glor, 2010; Losos and Mahler, 2010; Givnish, 2015). Others openly advocate the role than nonadaptive radiation, i.e., involving niche conservatism, may play in radiative processes (Gittenberger, 1991; Kozak et al, 2006) Such alternative models and views on how radiations may be generated suggest that theoretical preconceptions should be minimized when examining new empirical data that involve rapid diversification in species-rich groups
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