We fit a molecular data set, consisting of the rpL16 cpDNA marker and eight microsatellite loci, to the isolation-with-migration model as implemented in IMa to test a well-supported phylogenetic hypothesis of relationships within the Carex macrocephala species complex (Cyperaceae). The phylogenetic hypothesis suggests C. macrocephala from North America is reciprocally monophyletic and is sister to a reciprocally monophyletic clade of C. kobomugi. The North American C. macrocephala and C. kobomugi clade form a sister clade with a lineage of Asian C. macrocephala, thereby forming a paraphyletic C. macrocephala species. Not only does the phylogenetic hypothesis suggest C. macrocephala is paraphyletic, but it also suggests that the two lineages which share a partially overlapping distribution, Asian C. macrocephala and C. kobomugi, are not the most closely related. To test these relationships, we used coalescent-based population genetic models to infer divergence time for each lineage pair within the species complex. The coalescent-based models account for the stochastic forces which drive population divergence, and can account for the lineage sorting that occurs prior to lineage divergence. A drawback to phylogenetic-based phylogeographical analyses is that they do not account for stochastic lineage sorting that occurs between gene divergence and lineage divergence. By comparing the relative divergence time of the three main lineages within this group, Asian C. macrocephala, North American C. macrocephala, and C. kobomugi, we concluded that the phylogenetic hypothesis is incorrect, and the divergence between these lineages occurred during the Late Pleistocene epoch.