The role of historical and contemporary processes in structuring Central American tree genetic diversity
Poelchau, Monica F.
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In Central America, two major historical events, the uplift of the Central American land bridge during the Pliocene, and Pleistocene glacial epochs, may have significantly influenced the ecological and evolutionary histories of the region’s biota, especially its flora. However, the relative impact of these events is still poorly understood. I ask whether 1) strong phylogeographic structure observed in three focal tree species can be attributed to colonization of the uplifting Pliocene landbridge, and/or species’ range contractions during Pleistocene glacial cycles; 2) the three species’ distributions, and distributions of wet- or dry forest obligate species, contracted into proposed refugia during the last glacial maximum in the Pleistocene (LGM, 21,000 years ago); and 3) current, indirectly measured patterns of gene flow reflect historical vicariance, or current barriers to gene flow. In the first study, I use a comparative phylogeographic approach to determine the relative importance of Pleistocene climate change and Pliocene colonization patterns on contemporary patterns of chloroplast sequence variation in three widespread lowland tree species (Bursera simaruba, Brosimum alicastrum, and Ficus insipida). Dating and spatial arrangement of three of the breaks placed population divergence in the Pleistocene. Signatures of range expansion, however, were idiosyncratic across species, and did not match refugial expectations. While much of the evidence points to common population divergence during the Pleistocene, it is possible that concordant phylogeographic patterns are pseudo-congruent. In the second study, I use a paleoniche modeling approach to ask whether the focal species’, and wet and dry forest obligate species’, distributions overlap with proposed Pleistocene refugia. My results suggest that these species’ distributions were not fragmented at the LGM, and call into question whether forest taxa in general were restricted into refugia during the Pleistocene. Finally, I use a landscape genetic approach to identify current barriers to gene flow in the three focal species. I find that, contrary to prior expectations, differential barriers to gene flow occur: only one species has limited gene flow due to elevation-mediated temperature differences. A second species’ gene flow is restricted solely by distance, whereas the third species’ genetic variation may still be contingent on historical effects.