The Galloway lab has published a study in Molecular Ecology that demonstrates Campanula americana expanded its range in a two-step process after the last glacial period, first moving from the eastern edge of the range and then from near the Mississippi River (see figure). 78% of the variation in genetic diversity among populations is associated with this colonization pattern! You may read more about it here.
Citation:
Prior, C. J., N. C. Layman, M. H. Koski, L. F. Galloway and J. W. Busch. 2020. Westward range expansion from middle latitudes explains the Mississippi River discontinuity in a forest herb of eastern North America. Molecular Ecology, doi:10.1111/mec.15650.