Deciphering the drivers of speciation in Alpine Noccaea (Brassicaceae) reveal an evolution predominantly shaped by geography

The process of species delimitation, defined as the act of recognising biologically meaningful taxonomic units, enables the detection of overall biodiversity and the exploration of speciation processes. Given the complex nature of the origin of biodiversity, a significant number of cryptic Alpine plant groups exhibit ambiguity regarding their phylogenetic relationships and species circumscription. In this study, we attempt to apply the principles of integrative taxonomy to a complex of cryptic alpine plants from the genus Noccaea, which are often described as substrate specialists. We benefited from an extensive sampling across the Alps combined with high-throughput genotyping to apply a modern and standardised method of species delimitation. We find that the Noccaea rotundifolia complex is represented by five species within the Alps, which exhibit a restricted geographic distribution. Four of these species diverged approximately 350 ka ago (259–429), indicating a recent diversification. Instead of recognizing species with different substrate affinities, as done by current taxonomy, we rather find that the most widespread species are substrate generalists but with allopatric distributions. Our findings indicate that N. rotundifolia is growing exclusively in the west of the well-known biogeographic barrier of the Aosta Valley. N. corymbosa is found to grow between the Aosta Valley barrier and the Lake Como one. N. cepaeifolia, in turn, is observed in the eastern Alps, separated by the Brenner valley. Overall, this study provides a detailed systematic review, supporting the pivotal role of the recognized Alpine biogeographic barriers, in shaping species distribution and speciation.