The phylogeographic history of a range disjunction in eastern North America: the role of post‐glacial expansion into newly suitable habitat

Abstract

PREMISE A disjunct distribution, where a species’ geographic range is discontinuous, can occur through vicariance or long‐distance dispersal. Approximately 75 North American plant species exhibit a ~650 km disjunction between the Ozark and Appalachian regions. This disjunction is attributed to biogeographic forces including: (1) Eocene–Oligocene vicariance by the formation of the Mississippi embayment; (2) Pleistocene vicariance from interglacial flooding; (3) post‐Pleistocene northward colonization from separate glacial refugia; (4) Hypsithermal vicariance due to climate fluctuations; and (5) recent long‐distance dispersal. We investigated which of these pathways most likely gave rise to the Appalachian‐Ozark disjunction in Delphinium exaltatum. METHODS We genotyped populations of D. exaltatum from five Ozark and seven Appalachian localities, analyzed genetic structure, tested the order and timing of divergences using DIYABC, and conducted niche reconstructions up to 21,000 years before present (YBP). RESULTS Populations fell into five main genetic clusters, i.e., a group in the central Appalachians, and four “lowland” groups. DIYABC analyses showed the central Appalachian and lowland lineages diverging 11,300 to17,000 YBP, and the lowland groups diverging 6800 to 10,900 YBP. Niche reconstructions showed that suitable climate for the central Appalachian lineage experienced large spatial discontinuity starting 14,000 YBP, such that divergence and persistence before this period is less plausible than divergence thereafter. CONCLUSIONS Our results did not fully support any of the original hypotheses. Rather, the oldest divergence likely occurred after 13,500 YBP through expansion into newly opened habitat in the Appalachians. The Appalachian‐Ozark disjunction likely resulted from northward dispersal of the lowland lineage as climate warmed during the Holocene.

Revisiting the comparative phylogeography of unglaciated eastern North America: 15 years of patterns and progress

“…ENM has also been used to identify past glacial refugia (Waltari et al, 2007 ). One example of how ENM can help infer the phylogeographic history of a group is illustrated in a study of Delphinium exaltatum (Mohn et al, 2021 ), where ENM analyses indicated that a divergence occurred due to postglacial northward range expansion. ENM also helped narrow estimates of the divergence time between the two lineages by showing that one lineage showed a major shift in niche availability over a 500‐year period; this gap in niche space would have required very rapid migration rates for this lineage, suggesting that it likely did not exist prior to the shift in niche availability.…”

Population Genetic Analysis of the Threatened Plant Leavenworthia exigua var. laciniata (Brassicaceae) Reveals Virtually No Genetic Diversity and a Unique Mating System

“…laciniata is that it is an artifact of the particular microsatellite markers developed for this study, possibly related in some way to how we isolated the microsatellite loci. However, we employed well-established marker-development protocols that we have used successfully to identify polymorphic microsatellites in a wide diversity of plant species (Swift et al, 2016;Edwards et al, 2019Edwards et al, , 2021Mohn et al, 2021). Furthermore, while developing the markers, we attempted to avoid ascertainment bias by incorporating material from multiple maternal individuals of L. exigua var.…”

Integrative demographic modelling reduces uncertainty in estimated rates of species' historical range shifts

“…However, SDMs rely on numerous assumptions, including temporal stability of species' niches, and the absence of both dispersal constraints and species interactions (Wiens et al., 2009). Finally, genetic data can help infer population histories (Hu et al., 2009; McLachlan et al., 2005; Mohn et al., 2021), including timing of population splits and distinct migration pathways (Avise, 1992; Laricchia et al., 2015; Shafer et al., 2010). Nevertheless, genetic inferences suffer from identifiability issues.…”