Morphometric architecture of the most taxon‐rich species in the U.S. flora: Astragalus lentiginosus (Fabaceae)

• Premise of the study : The study of infra‐taxa has historically been considered the study of incipient species. Astragalus lentiginosus Douglas ex Hooker (Fabaceae) is the most taxonomically diverse species in the U. S. flora. To gain insight into the nature of infra‐taxa, I ask whether an optimal number of groups exists within A. lentiginosus , whether the varieties represent discrete entities, and whether morphological diversity within A. lentiginosus can be explained by climate. • Methods : Morphometric measurement was made on herbarium specimens. Seasonality was inferred by fitting sine waves to climatic data. Statistical clustering, ordination, and multiple regression analyses were employed to test hypotheses. • Key results : K ‐means clustering resulted in no clear optimal number of groups within the species. Principal components analysis demonstrated that taxa occupy contiguous regions of morphospace but do not form distinct clusters. Multiple regression demonstrated significant relations among morphology and climatic parameters, which are generally stronger than geographic relations. • Conclusions : The bewildering amount of diversity contained within A. lentiginosus begs for decomposition, yet its clinal nature precludes it from division into discrete groups. As a species, A. lentiginosus exists as an array of populations in a delicate balance between homogenizing forces, such as gene flow and retention of ancestral characters, and diversifying forces, such as local adaptation and drift. The effect of these forces has become evident in the phenotype of A. lentiginosus , resulting in more infra‐taxa than any other species in the U. S. flora, but has failed to fracture this group into distinct entities.