Geomorphological changes have been demonstrated to have had profound impacts on biodiversity, often leading to demographic expansions and contractions and allopatric divergence of taxa. We examined DNA sequence variation at two nuclear and one maternally inherited plastid locus among 10 populations of Schiedea globosa on the Hawaiian Islands to assess the primary factors shaping genetic structure, phylogeographic patterns, and the importance of geographic isolation to population divergence. Schiedea globosa has characteristics that may promote gene flow, including wind pollination and rafting of plants in ocean currents. However, we detected significant differentiation among populations on all islands except Hawaii, with the maternally inherited plastid locus having the greatest genetic structure (FST = 0.81). Migration rates across all loci are less than one migrant per generation. We found evidence of growth in several populations and on the islands of Molokai and Maui, which supports population expansion associated with the formation of Maui Nui during the last glacial maximum. Similar to data for many other Hawaiian taxa, these data suggest S. globosa originated on Oahu and subsequently colonized Molokai, Maui, and Hawaii in progression. Given the high level of genetic structure, allopatric divergence will likely contribute to further divergence of populations.
