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CASE 2: DEFINING GEOGRAPHIC BOUNDARIES AMONG SPECIES AND RECONSTRUCTING THE HISTORY OF TRIDENT BATS IN MADAGASCAR

An ongoing study of trident bats (genus Triaenops, family Hipposideridae) demonstrates the ways in which extensive sampling within Madagascar yields biogeographic insights both within and beyond the island’s physical limits (A.L.R., J.R., E. Palkovacs, S.M.G., and A.D.Y., unpublished work). On the basis of a recent morphological study, three species are currently recognized: Triaenops rufus, Triaenops furculus, and Triaenops auritus (J.R. and S.M.G., unpublished work). As illustrated in Fig. 11.2, this result is supported by molecular phylogenetic analysis. Moreover, when the Malagasy species are analyzed with their African congener (Triaenops persicus), the phylogeny reveals that the Malagasy members of this genus are paraphyletic with respect to the African species. Thus, two dispersal events between Africa and Madagascar must be invoked to explain this distribution. The unanswered question at present is whether Africa served as the center of origin, with two dispersal events to Madagascar, or whether Madagascar served as the center of diversification, with (presumably) a back migration to Africa.

We are presently employing population genetic methods to address these competing dispersal hypotheses, as well as to test the hypothesis that one of the northern rivers in western Madagascar may act as a biogeographic barrier separating T. auritus and T. furculus. Neutrality tests, FS (Fu and Li, 1997) and R2 (Ramos-Onsins and Rozas, 2002), and mismatch distributions (Slatkin and Hudson, 1991) support a history of population expansion in both T. rufus and T. furculus, with the strong indication that expansion was much more recent in T. rufus. Results from T. auritus are consistent with a history of constant population size through time, and may represent an older lineage that is at mutation-drift equilibrium. These results therefore seem to support two allochronic dispersals from Africa to Madagascar. The more northern populations of T. furculus (Namoroka and Anjohibe) are significantly differentiated from those in the south, but genetic variation within the two regions, respectively, is considerably lower, lending support to the north/south biogeographic structuring observed in some other Malagasy mammals (Yoder et al., 2000). Conversely, analyses of genetic structure within T. rufus show a complete lack of geographic structure. Pastorini et al. (2003) found that the Betsiboka River formed a major barrier separating populations and species in several different lemur groups. The Triaenops data, however, are not consistent with that pattern. Given the vastly different life history and dispersal characteristics in lemurs and bats, it should not be surpris-



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