Taxonomic delimitation and drivers of speciation in the Ibero‐North African <i>Carex</i> sect. <i>Phacocystis</i> river‐shore group (Cyperaceae)

<jats:p>• <jats:italic>Premise of the study:</jats:italic> The Ibero‐North African <jats:italic>Carex</jats:italic> sect. <jats:italic>Phacocystis</jats:italic> river‐shore group is a set of perennial helophytic species with poorly defined taxonomic boundaries. In the present study, we delimited the different taxonomic units, addressed the phylogeographic history, and evaluated the drivers of differentiation that have promoted diversification of these plants.</jats:p><jats:p>• <jats:italic>Methods:</jats:italic> We analyzed molecular data using statistical parsimony for plastid sequences (26 samples from 26 populations) and principal coordinate analysis, neighbor joining, and Bayesian analysis of population structure for AFLPs (186 samples from 26 populations). Chromosome numbers from 14 samples (9 populations) are newly reported.</jats:p><jats:p>• <jats:italic>Key results:</jats:italic> Three species can be distinguished (<jats:italic>C. acuta</jats:italic>, <jats:italic>C. elata</jats:italic>, and <jats:italic>C. reuteriana</jats:italic>). Unexpectedly for rhizome‐growing helophytes, the vegetative reproduction detected was incidental. The widespread <jats:italic>C. elata</jats:italic> was found to be a genetically poorly differentiated taxon, whereas the local <jats:italic>C. reuteriana</jats:italic> displayed geographical structuring. Geographical factors seem to be the main driver of differentiation for both taxa.</jats:p><jats:p>• <jats:italic>Conclusions:</jats:italic> Despite apparent morphological and ecological similarities, <jats:italic>C. elata</jats:italic> and <jats:italic>C. reuteriana</jats:italic> have disparate genetic structures and evolutionary histories, which may have originated from small ecological differences. <jats:italic>Carex elata</jats:italic> is broadly distributed throughout Europe, and its northern populations were recently founded, probably after the last glacial maximum. In contrast, <jats:italic>C. reuteriana</jats:italic> is an Ibero‐North African endemic, with long‐standing populations affected by isolation and limited gene flow. It is likely that high‐density blocking effects and different gene‐flow barriers act together to delimit its distribution and promote its relatively high population differentiation.</jats:p>