Pre Pleistocene History The Diversification and Mixing of Different Lineages

A general picture suggests that humid and mesothermic climates predominated throughout most of the Late Cretaceous and Tertiary in Mediterranean ecoregions where subtropical forests were dominating. The onset of Mediterranean climates is relatively young, and pale-oclimatic reconstructions demonstrate that cooling and drying, with the combination of summer drought and mild winter temperatures rose quite rapidly in Late Miocene or Early Pliocene (c. 5-3 Ma). In the Mediterranean Basin, a gradual but deep climatic change occurs during the Pliocene (3.5-2.4 Ma), with a significant drop in temperature and a marked seasonality in thermal and rainfall regimes; the stabilization of the summer drought arises here at c. 2.6 Ma. Concomitant with the appearance of Mediterranean-climate conditions, a massive ecological radiation was initiated in the Late Miocene-Pliocene. However, there are also phylo-genetic evidences of earlier radiation events during the Oligocene involving geographic speciation; this is the case of the shrub genus Protea (Proteaceae) in South African fynbos and the geophytic genus Androcymbium (Colchicaceae) in xerophytic grasslands of South Africa and the Mediterranean Basin. The deterioration of Tertiary warm climates resulted in the extinction of several subtropical and warm-temperate species during Plio-Pleistocene, but historical discrepancies exist between the Mediterranean ecoregions, from the point of view of the impact of environmental changes and the macroscale process of species recolonization after drastic climatic events.

Mediterranean ecoregions closely situated to subtropical regions and without major geographical barriers between these two biomes have significantly experienced moderate cases of species extinctions, because of possible latitudinal shifts in species range or a higher climatic stability. This is the case of the eastern lowlands of the Cape Floristic Region, where patches of subtropical thickets and warm-temperate forests are still dominated by trees and shrubs of paleotropical and Gondwanian affinities. These highly species- and genus-rich formations are extremely ancient, with many elements phylogenetically basal to the western Cape species. The precise timing of this lineage diversification is still unknown but geographic speciation was probably the key process.

In south-central Chile, the actual coastal forests at mid-latitudes remained fairly stable despite major climatic and tectonic changes in southern South America during the Pleistocene. These forests are indeed characterized by a notable evolutionary stability, favoring the conservation of ancient species assemblages. The nearest ancestor of the Mediterranean sclerophyllous vegetation of Chile corresponds probably to the Neogene subtropical paleoflora that occured in the Proto-Andean foothills of this area during the lower to mid-Miocene (c. 20-15 Ma), and the pre-Pleistocene paleofloras were developed under a more humid and warmer paleoclimate induced by an Andean rain shadow effect.

In California, shrublands (chaparrals) are composed of temperate, subtropical, and desert elements. Many of these Californian chaparral genera appeared in the Early Tertiary, following the trends since the Eocene toward cooler and drier climates. This sclerophyllous flora seems to have arisen as an understory component of evergreen woodlands and form only the chaparral in response to the latter appearance of the Mediterranean climate and related increased disturbance by fire. During unfavorable periods, southward migration of species in Baja California or central Mexico, or their location in southern coastal refugia and subsequent recolonization explain the actual persistence in California of several outstanding trees (Sequoia, Sequoiadendron, Tsuga, Umbellularia, etc.), whereas these genera have disappeared around the Mediterranean sea because of the existence of several east-west oro-graphic and maritime barriers which prevent putative latitudinal migrations of species.

Moreover, in the Mediterranean ecoregion, climatic coolings during the Neogene have provoked severe extinctions, for example, of 45 genera of megathermous and warm-temperate ligneous species distributed in the northwestern part of the Mediterranean sea. The main diversification of the Mediterranean Basin species took place during the Miocene, notably in conjunction with the collision of the African and Eurasian platforms through the Arabian Plate. Recurrent episodes of species dispersal and vicariance have occurred between the eastern and western Mediterranean regions due to several marine regressions/transgressions and the ongoing rise of Alpine and Atlas orogenesis. Another major biogeo-graphic event of the end of the Tertiary is the closure of the Mediterranean-Atlantic gateways (now the Gibraltar Strait). This episode known as the Messinian salinity crisis (5.77-5.33 Ma) induced a considerable evaporation of the Mediterranean Sea and the formation of several land bridges suitable for dispersal-vicariance events and species radiations. Thus, the Mediterranean species pool arises from diverse biogeographic origins and if it includes still some subtropical species, mostly originated from African and Asian lineages, the extratropical species of autochthonous and northern lineages predominate.

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