Morphometrics as a robust tool for disambiguation in plant taxonomy: the case of Lactuca livida, a commonly accepted but never delimited taxon

An important goal of taxonomy is to clarify the identity of ambiguous taxa. Since lineage divergence usually involves ecological shifts that are associated with plant morphology, we propose that searching for fixed, non-overlapping morphological characters and specific associations of features should be the first task in disambiguation. We applied this idea in an analysis of the taxonomic identity of Lactuca livida, an Iberian endemic relative of L. virosa that, despite an extremely imprecise delimitation, is usually recognized in standard floras and germplasm banks. We analysed 24 possible diagnostic characters across the Iberian Peninsula, drawn from related taxonomic literature. No discontinuities in frequency variability distribution characters were found, even in two bimodal quantitative characters: leaf lobation and number of florets per capitulum. There were no notable patterns of association among characters, and the PCA/PCoA score plots did not show any distinctive groupings. Leaf lobation followed a significant geographic pattern, but there was no effective segregation of leaf shapes. We conclude that the variability found is symplesiomorphic, as it is present throughout the Serriola group of Lactuca, to which L. virosa belongs. The analyses, together with previous biological knowledge, indicates that Lactuca livida should be considered a synonym of L. virosa.