Species Delimitation In An Intractable Syngameon: Bringing Order To The Polyphyletic Heuchera Americana Group

Species are the fundamental analytical units of evolutionary processes; thus evidence-based species delimitation is a crucial step for understanding species radiations. However, the task of delimiting species is particularly challenging in the context of a syngameon--a group of distinct, but closely related species that have incomplete reproductive isolation and frequently hybridize in nature. This problem is further exacerbated by the presence of cryptic species--species that are phenotypically distinct, though difficult to distinguish with gross morphology alone. Heuchera subsect. Heuchera comprises both clear and cryptic species within a syngameon that has seen study from morphological, experimental, and phylogenetic aspects. This group has long been recognized for its taxonomic complexity, namely two recognized hybrid zones with extreme morphological variation and persistent non-monophyly among parental populations. Here, we reassess species limits within Heuchera subsect. Heuchera, focusing on the hybrid complex between H. americana and H. richardsonii and adjacent H. americana populations. We use a multipronged approach with deep population-level sampling to 1) assess the genetic structure of 655 individuals across the geographic range of the H. americana group to identify genetic lineages and 2) assess the phenotypic diagnosability of these lineages. Despite extensive admixture and gene tree conflict, we find multiple cohesive lineages with diagnosable phenotypes. We recognize five species and three varieties within the H. americana group, one new and four resurrected. Our results demonstrate that even highly reticulate syngameons can be partitioned into meaningful taxonomic units with multiple lines of evidence.