Premise of research.  Plant communities assemble through historical, adaptive, and stochastic processes, with their relative contributions varying among communities. This study traces the origin of the highly threatened Florida scrub ecosystem, a series of relic sand dunes with exceptionally high endemism. We used dated phylogenies with four endemic angiosperms to test two hypotheses on the origins of the Florida scrub: the western hypothesis, which emphasizes the role of mid-Pliocene fragmentation of the xeric belt between southwestern North America and Florida, and the eastern hypothesis, which emphasizes the glacial cycles of the Pleistocene.

Methodology.  Augmenting existing phylogenies, we sequenced the nuclear internal transcribed spacer region and several plastid loci of species of Prunus (Rosaceae), Polygala (Polygalaceae), Persea (Lauraceae), and Ilex (Aquifoliaceae) to phylogenetically place the Florida scrub endemics and identify their putative sister taxa. We used topology tests, ancestral area reconstruction analyses, and time-calibrated trees to evaluate the western and eastern hypotheses both geographically and temporally.

Pivotal results. Results for Polygala and Ilex support the eastern hypothesis for the origin of the central Florida endemics, while data for Persea and Prunus are ambiguous. However, all species show an eastern ancestral distribution to some degree. Molecular dating analyses suggest that extant Ilex opaca populations diverged in the Pleistocene, with older Pliocene origins for the North American Persea clade, Polygala lewtonii, and Prunus geniculata.

Conclusions.  Taken together, our results support eastern North America as the dominant origin of the plant species of the central Florida scrub. However, contrary to the current eastern hypothesis, molecular dating suggests that the origins of the four endemics predate the last glacial cycles. These results have implications for the age of the Florida scrub itself as well as its component species, suggesting that the Florida scrub or its precursor existed before Pleistocene glaciation. Furthermore, the more ancient age of this assemblage of species than envisioned by most accounts suggests that interspecific interactions in this community are not recently formed (i.e., since the end of the Pleistocene) and may have been established over much longer periods of geological history. These fundamental associations argue for enhanced conservation efforts of both the individual species and the persistent remnants of the scrub community.