The temporal course of quaternary diversification in the European high mountain endemic Primula sect. Auricula (Primulaceae)

Primula sect. Auricula is one of only a few endemics of the European high mountains with a comparatively large number of species. We explored the section's geographical origin, time of origin, and temporal course of diversification via parsimony, genetic distance, and lineages-through-time analyses based on the internal transcribed spacer ( ITS) regions of nuclear ribosomal DNA. The taxa analyzed included 52 individuals representing all 25 species of the section, representatives of seven other European, Asian, and/or North American sections of the genus, and two species of Douglasia. We present evidence that sect. Auricula likely originated from an Asian ancestor at the end of the Late Tertiary, followed by its primary diversification into an eastern'' and western'' lineage at the Plio-/Pleistocene boundary. Comparison with a constant rates null model of stochastic diversification-extinction (birth-death) demonstrates that diversification has proceeded nonrandomly through Quaternary times in both lineages. They display a pulse of speciation events occurring soon after their origin and relatively few such events occurring since. This pattern contrasts with the predictions of the Late Pleistocene origins hypothesis and implies unpredictability of the evolutionary response of sect. Auricula to the recurrent abiotic conditions of the Quaternary glacial-interglacial cycles. We conclude that it is not necessary to invoke an increase in extinction rate. Rather, the observed slowdown of interspecific ( and intraspecific) diversification in sect. Auricula toward the present likely results from a decrease in diversification rate due to ecological and/or geographical space-filling processes.