Phylogenetic evolution and biogeography of Southeast Asian shrews (genusCrocidura: Soricidae)

Abstract

Genetic variation of 20 species of shrews from the Malay peninsula, Sumatra, Java, Borneo, the Philippines and Sulawesi was assessed by allozyme electrophoresis at 32 loci. According to Mantel's tests, the genetic differentiation of these species of shrews is not a function of the geographic distance separating them (r = 0·09, NS), but is correlated to the water depth surrounding the islands where they live (r = 0·49,P < 0·01). The results are just the reverse if the correlations are computed for the Sunda Shelf taxa only. In this case, the sampled populations show an isolation-by-distance relationship (r = 0·32;P < 0·01), while no significant correlation with water depth was detected (r = 0·20;P = 0·07). Qualitative predictions based on eustatic sea level variation and water depth were formulated as a model of historic connections between the islands. This palaeogeographic model was tested through Brooks Parsimony Analysis. The assumption of a simple vicariant evolution of the shrews was rejected, but several concordant patterns indicate that the phylogeny of these mammals was indeed shaped by these events. Homoplasies demonstrated that the SE Asian species ofCrocidurainclude composite zoogeographic histories. Sulawesi, for example, supports at least six species, five of which are closely related, while the last one,C. nigripes, is more closely related to a Bornean taxon. This pattern was interpreted as the result of a first wave of colonizers which subsequently radiated, followed by a more recent, second colonization event from Borneo. The overall small genetic distance found within the assemblage of the five old endemics (DN = 0·151 ± 0·041) suggests that the radiation was not accompanied by extensive differentiation, although from a karyological point of view, they exhibit unusual variations when compared to other IndomalayanCrocidura. By contrast, the four species found on Sumatra are more differentiated (DN = 0·221 ± 0·063) and never form sister-group relationships in any phylogenetic reconstruction; each one is more closely related to different taxa living outside Sumatra. This suggests that they are probably remnants of an important centre of dispersal for the entire Malay Archipelago. The standard genetic distance averaged among all Southeast Asian species (DN = 0·235 ± 0·094) is about half that measured within Palearctic or African taxa. Such an overall lower mean level of genetic variability is consistent with the hypothesis of a relatively recent colonization of the Malay Archipelago by shrews of the genusCrocidura.