Niche evolution during allopatric and sympatric diversification of Aeonium (Crassulaceae) on the Canary Islands

Although species radiations on island archipelagos are broadly studied, the geographic and ecological modes of speciation that underlie diversification are often not fully understood. Both allopatry and sympatry play a role during radiations, particularly on islands with profound habitat diversity. Here, we use the most diverse Canary Island plant radiation, Aeonium (Crassulaceae), to phylogenetically test two hypotheses: (1) allopatric speciation, which predicts that closely related taxa are ecologically similar but do not co-occur, and (2) sympatric speciation, whereby closely related taxa co-occur geographically but are ecologically distinct. We fitted niche and spatial distribution models based on extensive field surveys to quantify geographic and ecological divergence among taxa integrated in a phylogenetic context. Contrary to our expectation, phylogenetically closely related species tend to occupy similar ecological niches, revealing that ecological niche divergence among species accumulates slowly, even in sympatry. This suggests that evolutionary young taxa may be partially reproductively isolated due to subtle phenotypic differences, such as reproductive morphology and phenology, rather than by ecological niche and may putatively exacerbate divergence among populations. Thus, allopatry and sympatry are complementary speciation mechanisms on oceanic islands, jointly spurring this enigmatic radiation.

Methods
We surveyed the archipelago, covering all 38 Aeonium species and subspecies following Bañares-Baudet (2015). Surveys were performed between February and September in 2018–2020 and aimed at capturing the ecological breadth of the islands through high-resolution presence/absence observation points. To guarantee a dense and stratified sampling, we divided each sampled island into climatic quarters based on the average of the mean annual temperature (bio01) and annual precipitation (bio12): 1) dry and hot; 2) dry and cold; 3) wet and hot; 4) wet and cold. The quarters of each island were sampled through extensive driving and hiking. A total of 10’180 data points were collected using a Global Positioning System receiver (GPS 72H, Garmin, Taiwan) with a positional accuracy of ~ 5 m, including 9’719 presences (including data points with multi-taxa occurrences) and 2’450 absences. Points were registered at c. 50 – 200 m intervals, noting all occurring Aeonium species. For each island, we observed all Aeonium taxa known to occur. Fuerteventura was not visited because its only species was scored from Lanzarote.