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among representatives of the families Fabaceae, Lamiaceae and Orchidaceae. The Sicilian
flora makes no exception to these general trends and the remarkable climatic heterogeneity of
Sicily, ranging from semi-arid to humid ombrotypes and from thermo- to cryo-oromediterra-
nean thermotypes (BRULLO et al. 1996, BAZAN et al. 2006), favoured over time the floristic
enrichment and differentiation of the island.
A regional assessment of the distribution of vegetation types per bioclimatic units has been
recently published by BAZAN et al. (2015). However, the evolutionary consequences of such
a remarkable climate variability and heterogeneity are acting at local scale within each single
vegetation patch: relatively short generation time and intense gene flows may easily segregate
new forms or varieties, particularly among annual plants, which can efficiently occupy new
peculiar niches, that given to the patchiness and heterogeneity of the Sicilian landscape are
very common. Most of this intraspecific variability is still poorly investigated in the Sicilian
flora, but noticeable examples of local speciation as the ending result of micro-evolutionary
processes that promote population differentiation are provided, for instance, by the species
complex of Silene sect. coloratae (BRULLO et al. 2012, 2014 a-b, 2015). Random gene fixa-
tion and subsequent divergence may be a common feature, so far poorly investigated, in the
population biology of many Sicilian annuals and, more in general, of insect-pollinated herba-
ceous species, particularly when climate change affects population size and isolation, in eco-
logically marginal or geographically peripheral populations.

3. Geological patchiness
The geographical position of Sicily, its complex orogeny and high topographic diversity
make the island one of the most heterogeneous Mediterranean territories (CATALANO et al.
2013). The main points of the geomorphological evolution of Sicily are the following (Fig. 2):
1) Strong conflict between tectonic uplifting (which creates relief) and subsidence processes
(which destroy it);
2) Relief is created in the south and destroyed in the north, where it collapses towards the Tyr-
rhenian Sea;
3) An ever-changing relief in central Sicily, where soft rocks crop out;
4) An increasingly old terrain, as you move from south (newly emerged areas) to north (hea-
vily eroded areas);
5) Conservation of the oldest terrains in mountain districts of the north, where more “erosi-
on-resistant” rocks crop out;
The single most relevant landmark of the island is Mt. Etna, the biggest volcano in the
Mediterranean region. It dominates the Eastern side of Sicily, with multiple layers of erupted
materials that cover an area of 1190 km², with a basal circumference of 140 km.
Apart from Etna, the main elevations of Sicily (ranging from 1400 to 1979 m) are aligned
along the so-called Sicilian Apennine, ranging along the NE-coast from the Strait of Messina
up to the valley of the Torto River. Three sectors can be recognized, from east to west: Pelori-
tani-, Nebrodi- and Madonie Mountains. Peloritani are constituted by the oldest outcrops of
Sicily: a complex of different metamorphic rocks (gneiss, schistose and phylladic alternati-
ons) partially covered by sedimentary sandstones and limestones. Nebrodi are mostly consis-
ting of quartz sandstones, clayey and siltose depositions belonging to the Numidian Flysch.
Madonie are formed by carbonatic, dolomitic and quartzitic outcrops, frequently interrupted
by outcroppings of salty clays and layers of halite. Carbonatic and dolomitic rocks are for-
ming, as well, the reliefs in the western part of Sicily, overlapping a basal complex constituted
by carbonate sands and clays.
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