Biogeographical affinity
Biogeographical affinity (i.e., the random effect in our models) explained only a small fraction of the variance for the native taxa. This can be attributed to the fact that a large portion (~40%) of the present Aegean flora has reached the Aegean islands as a result of human action in prehistoric or early historic times (Greuter 1979). The presence of these naturally or artificially distance-insensitive species clearly justifies the less significant role of biogeographical affinity in the diversity patterns of the native plant taxa occurring in the Aegean archipelago. On the other hand, the largest proportion of variance for all the endemic species richness metrics is attributed to biogeographical affinity. This is in line with previous studies stating that in the Aegean, bioregionalization is primarily a result of the region’s complex paleogeographical history (Kougioumoutzis et al. 2017) This is also due the high proportion of narrowly-ranged species and consequently of high species turnover in Aegean island plant communities: nearly ~45% of the endemic taxa occurring in the Aegean are SIEs, the vast majority of which occur in Crete and Evvia (Panitsa et al. 2018). Other factors such as climate and geodiversity probably play an important role in shaping current endemic diversity patterns in the Aegean, as is actually the case for the central (Kougioumoutzis and Tiniakou 2014), eastern (Panitsa et al. 2010, Panitsa and Tzanoudakis 2010) and southern (Kagiampaki et al. 2011) Aegean islands. In addition, favourable climatic conditions most probably permitted a relict flora to persist in the southern (i.e., Crete, Karpathos, Rodos) and eastern (e.g., Ikaria: Christodoulakis 1996a, b) Aegean archipelago (Runemark 1969, 1971). In topographically complex islands, some of the old MIEs formed neo-endemic SIEs through allopatric speciation (Runemark 1969, 1971, Bittkau and Comes 2005, 2009, Comes et al. 2008, Jaros et al. 2018). Geographical isolation through sea-level oscillations may have supported the recent diversification of neo-endemic species, especially in the central Aegean where several non-adaptive radiations occurred (e.g. Campanula , Nigella , Erysimum – e.g. Comes et al. 2008, Jaros et al. 2018), as a result of a turbulent and labyrinthine sequence of island-splitting events in a short time-period, since most of the geographic configuration change occurred during marine transgression in the Aegean within 5000 years between 16 and 11 Ka BP (Simaiakis et al. 2017). Especially when considering the repetitive character of fragmentation over the last 2 My, the fragmented and disconnected state existed during every glacial-interglacial interval for ~20 ka disrupting the longer lasting glacially connected state, leading to cumulative genetic divergence between populations (Aguilée et al. 2009). One prominent example for this is the differentiation of the Nigella arvensis species complex which is due to non-adaptive radiation and random genetic drift as a result of several vicariant events during the Pliocene/Pleistocene (Bittkau and Comes 2005, Bittkau and Comes 2009).