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Molecular inference of a Late Pleistocene diversification shift in Nigella s. lat. (Ranunculaceae) resulting from increased speciation in the Aegean archipelago

Authors


  • This paper stems from a contribution initially presented at the conference Origin and Evolution of Biota in Mediterranean Climate Zones: an Integrative Vision, held in Zurich on 14–15 July 2007.

*Christiane Bittkau, Institut für Spezielle Botanik und Botanischer Garten, Johannes Gutenberg-Universität, D-55099 Mainz, Germany. E-mail: bittkau@uni-mainz.de

Abstract

Aim  To infer the temporal course and geographical mode of speciation in Mediterranean/Southwest Asian Nigella s. lat.

Location  Mediterranean Basin, Aegean archipelago.

Methods  Phylogenies for Nigella L. and Garidella L. (= Nigella s. lat.) were obtained from maximum-likelihood analyses of internal transcribed spacer (ITS) sequences. Diversification through time was analysed by log lineages-through-time (LTT) plots and survival analyses. Relative node age estimates were regressed against the degree of sympatry between sister clades to infer the predominant mode of geographical speciation in Nigella s. lat.

Results  The Late Pleistocene radiation of the Nigella arvensis complex in the Aegean region caused a significant departure from a stochastic speciation/extinction process of diversification during the evolution of Nigella s. lat., a lineage of (at least) Late Miocene origin. Speciation within Nigella s. lat. predominantly took place in allopatry.

Main conclusions  No significant effect on diversification rate was found regarding the establishment of a Mediterranean-type climate, or the onset of the Quaternary climatic oscillations. Rather, the accelerated rate of speciation in the N. arvensis complex is plausibly related to increased opportunities for allopatric speciation afforded by the (palaeo)geographical complexity of the Aegean archipelago combined with Late Pleistocene changes in climate and sea level. The evolution of self-pollination and associated changes in habitat preference and flowering time further augmented speciation and niche differentiation within the complex, but these changes did not act as the primary promoters of the radiation process.

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