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. While allopatry seems to be the main driver in
speciation among islands, within-island speciation occurs in sympatry.
Contrary to our expectation, phylogenetically closely related species
tend to occupy similar ecological niches, suggesting 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 ecology and may
putatively exacerbate divergence among populations. Thus, allopatry and
sympatry are complementary speciation mechanisms on oceanic islands,
jointly spurring this enigmatic radiation.