Functional traits reveal unaccounted diversity within Sonchus asper (L.) Hill (Asteraceae) in the northern Mediterranean and enable the reinstatement of ancient taxa

Abstract Functional traits express the ecological and evolutionary attributes of individuals that compose taxa, providing robust criteria to detect speciation when integrated with phylogenies. We tested this approach in Sonchus asper (L.) Hill, a putatively self-fertilising annual weed that has recently been reported to exhibit extensive floral and life-form variation across the northern Mediterranean. This variability was already documented in the nineteenth century, but it is incongruent with current subspecific classifications based on leaf arrangement. We sampled 40 populations encompassing this variation, assigning each to (a) one of the four morphotypes defined by floral traits and life forms, and (b) a leaf arrangement group. To assess their taxonomic relevance, we tested grouping characters as functional traits and analysed the consistency of these assignments with the phylogeny (ITS/ETS and chloroplast DNA markers), and genetic structure (AFLP markers). Phylogenetic analysis using floral traits and life forms identified three well-supported clades that contradict leaf-arrangement taxonomy. The earliest-diverging clade comprises the common, annual, self-fertilising S. asper. The remaining two clades consist of self-incompatible plants: one contains the rhizomatous morphotype, whereas the other groups annual and biennial plants. These life-history strategies exhibited distinct AFLP-based genetic profiles, alongside complex cpDNA phylogeographical patterns. The congruence between DNA analyses and functional value of floral and life-form traits corroborates the proposed morphotypes as distinct evolutionary lineages. This reveals unaccounted-for biodiversity and supports reinstating historical taxa. Our study shows that combining functional traits with genealogical relationships provides a powerful operational taxonomic criterion that integrates ecological and evolutionary species concepts.