Integrative species delimitation methods infer species boundaries in the Lomatium foeniculaceum complex and indicate an evolutionary history from the Southwest towards the Northeast in western North America

Delimiting species boundaries is a perennial challenge in the field of systematics. Resolving whether morphological variation is the result of environmental parameters, incipient speciation, or complete speciation is especially challenging when the variation is subtle. Within the perennial endemic North American clade of Apiaceae (PENA) there are numerous examples in which widespread species have subtle geographically defined morphological variants that have typically been recognized at the subspecific rank. The Lomatium foeniculaceum (Nutt.) Coult & Rose species complex is a salient case that has long been treated as a single species with five infrataxa, spanning much of the western United States and western Canada in a morphological continuum: L. foeniculaceum var. foeniculaceum, L. foeniculaceum var. daucifolium (Torr. & A. Gray) Cronquist, L. foeniculaceum var. macdougalii (J.M. Coult. & Rose) Cronquist, L. foeniculaceum var. fimbriatum (W.L. Theob.) B. Boivin, and L. foeniculaceum var. inyoense (Mathias & Constance) B. Boivin. We utilized the Angiosperm353 baits kit to sample nuclear loci from these five taxa to determine if the subspecific taxa formed discrete genetic groups with the molecular data. Groups that were identified were then examined for a combination of morphological and ecological traits that corresponded to these groups and could be used to determine the most appropriate taxonomic ranks of recovered groups. Molecular data recovers six well-supported monophyletic clades and a seventh clade of a single individual. Samples initially identified as L. foeniculaceum var. macdougalii are in two clades with one sister to L. foeniculaceum var. foeniculaceum/L. foeniculaceum var. daucifolium the other sister to L. foeniculaceum var. fimbriatum. Most samples of L. foeniculaceum var. foeniculaceum are in the same clade; others are in the clade with L. foeniculaceum var. daucifolium depending on the analysis. Each clade can be defined with a distinct morphological diagnostic character state. We conclude that molecular data and morphology support the recognition of five distinct species within the complex: L. inyoense Mathias & Constance, L. macdougalii J. M. Coult. & Rose, L. fimbriatum (W.L. Theob.) Botello & J.F. Sm., L. semivaginatum Botello & J.F. Sm., and L. foeniculaceum with two varieties retained, L. foeniculaceum var. foeniculaceum and L. foeniculaceum var. daucifolium. The data suggest that the Continental Divide has been influential in the evolution of these species, acting as an effective barrier facilitating speciation.