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Robinson’s mouse opossum (Marmosa robinsoni) typically inhabits xeric shrublands, savannas, and deciduous forests from Panama through Colombia and Venezuela, to the islands of Trinidad, Tobago, and Grenada. We assessed its phylogeographic structure in the 1st such study based on dense geographic sampling of any vertebrate from dry habitats in this region. We sequenced the cytochrome-b gene and the X-linked intron Olinked N-acetylglucosamine transferase, largely from dried skins and residual tissue on osteological material of museum specimens. Phylogenetic analyses revealed the existence of 2 well-supported phylogroups primarily distributed to the east and west of the Cordillera de Merida. The estimated time since divergence between these phylogroups postdates the Miocene; therefore, Andean uplifts, changes in the course of the R´ıo Orinoco, and marine transgressions of that epoch cannot be implicated as causal vicariant agents. Instead, expansion of humid forest or marine transgressions, or both, during the Pliocene and Pleistocene more likely led to this differentiation. We encountered little structure among populations east of the Cordillera de Merida, suggesting recent range expansion in this region. Surprisingly, isolated populations from the Pen´ınsula de Paraguana (northwestern Venezuela) are not closely related to...
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