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.
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.