Westward range expansion from middle latitudes explains the Mississippi River discontinuity in a forest herb of eastern North America

It is often expected that temperate plants have expanded their geographic
ranges northward from primarily southern refugia. Evidence for this
hypothesis is mixed in eastern North American species, and there is
increasing support for colonization from middle latitudes. We studied
genome-wide patterns of variation in RADseq loci to test hypotheses
concerning range expansion in a North American forest herb (Campanula
americana). First, spatial patterns of genetic differentiation were
determined. Then phylogenetic relationships and divergence times were
estimated. Spatial signatures of genetic drift were also studied to
identify the directionality of recent range expansion and its geographic
origins. Finally, spatially explicit scenarios for the spread of plants
across the landscape were compared, using variation in the population
mutation parameter and Tajima's D. We found strong longitudinal
subdivision, with populations clustering into groups west and east of the
Mississippi River. While the southeastern region was likely part of a
diverse Pleistocene refugium, there is little evidence that range
expansion involved founders from these southern locales. Instead, declines
in genetic diversity and the loss of rare alleles support a westward
colonization wave from a middle latitude refugium near the southern
Appalachian Mountains, with subsequent expansion from a Pleistocene
staging ground in the Mississippi River Valley (0.51 - 1.27 Mya). These
analyses implicate stepping stone colonization from middle latitudes as an
important mechanism of species range expansion in eastern North America.
This study further demonstrates the utility of population genetics as a
tool to infer the routes traveled by organisms during geographic range
expansion.