Biogeography and evolution of Abies (Pinaceae) in the Mediterranean Basin: the roles of long-term climatic change and glacial refugia

Aim The genus Abies exemplifies plant diversification related to long-term climatic, geological and evolutionary changes. Today, the Mediterranean firs comprise nine species, one natural hybrid and several varieties. Here I summarize current knowledge concerning the origin and evolution of the genus Abies in the Mediterranean Basin and propose a comprehensive hypothesis to explain the isolation and speciation pattern of Mediterranean firs. Location The Mediterranean Basin. Methods The literature on Abies was reviewed, focusing on the morphology, fossil records, molecular ecology, phytosociology and biogeography of the genus in the Mediterranean Basin. Results Abies fossils from the western Mediterranean indicate a wide Tertiary circum-Mediterranean distribution of the Abies ancestor. Palaeogeographical data also suggest a single eastern Mediterranean Tertiary ancestor. Following the Miocene to Pliocene climate crisis and marine transgressions, the ancestor of the northern Mediterranean firs is hypothesized to have separated into two eastern groups, one on the Balkan Peninsula and the other in Asia Minor. However, land bridges may have permitted gene flow at times. A southward migration of A. alba to refugia, where older fir species may have remained isolated since the Miocene, could explain recent findings indicating that morphologically distant species are more closely related than expected based on such morphological classification. Main conclusions The Abies genus appears to have undergone significant morphological differentiation that does not necessarily imply reproductive isolation. That is, long-term Mediterranean Basin dryness along a south-eastern to north-western gradient may have caused an initial Miocene-Pliocene speciation sequence. Pleistocene glacial cycles probably forced migrations to occur, leading to repeated contact between fir species in glacial refugia.