Evolution and diversification related to rainfall regimes: diversification patterns in the South African genus Metalasia (Asteraceae–Gnaphalieae)

Abstract

AbstractAimThe Cape region is known for its exceptional species richness, although much remains unknown regarding the appearance of the modern Cape flora. One explanation is that floral diversification was influenced by the establishment of winter rainfall/summer arid conditions hypothesized to have occurred towards the end of the Miocene. We studied the evolution and diversification of the plant genus Metalasia (Asteraceae–Gnaphalieae), with the aim of testing whether radiation patterns may have been influenced by the climatic changes.LocationSouth Africa, with emphasis on the south‐west.MethodsThe radiation of Metalasia was investigated using two approaches: a species diffusion approach, which estimated the ancestral areas by means of a relaxed random walk while sampling from extant distributions; and a discrete approach, in which distributions were defined according to the phytogeographical centres of the Cape region. Secondarily derived clock rates from an earlier Gnaphalieae study were used for calibration purposes.ResultsOur analyses date Metalasia to approximately 6.9 Ma, after the Miocene–Pliocene boundary and the establishment of the winter rainfall/summer arid conditions. Metalasia consists of two sister clades: Clade A and Clade B. Clade B, which is endemic to the winter rainfall area, is estimated to have diversified c. 6.4 Ma, whereas Clade A, with a main distribution in the all‐year rainfall area, is considerably younger, with a crown group age estimated to 3.3 Ma. Diversification rates suggest an early rapid speciation, with rates decreasing through time both for Metalasia and for clades A and B separately. Ancestral area estimations show a possible scenario for the radiation of Metalasia to its current diversity and distribution, with no conflict between results inferred from diffusion or discrete methods.Main conclusionsThe diversification of Metalasia is estimated to have begun after the establishment of the winter rainfall/summer arid conditions, consistent with its radiation having been influenced by changes in the climatic regime.