As suggested by the investigations into brown lemur taxonomy, the evolutionary history of the brown lemur group is not currently resolved. Indeed, subfossil evidence may have little utility in clarifying the origins and differentiation of this complex due to the very close relationships of these taxa and the lack of diagnostic morphological features. In an analysis of craniodental characters among Eulemur and Varecia species and subspecies, Tattersall (1993) found extensive homoplasy and could not identify clear apomorphies to resolve brown lemur (or Eulemur) phylogeny.
Nonetheless, combining evidence from karyology, molecular genetics, and bio-geographic patterns, tentative evolutionary relationships can be discerned in the brown lemur complex. There is strong consensus that the brown lemur group forms a monophyletic clade (Tattersall, 1993; Tattersall and Sussman, 1998; Wyner et al., 1999; Pastorini et al., 2000). There is evidence from analysis of metachromism that E. collaris may be basal to the brown lemur complex (Shedd and Macedonia, 1991). Pastorini et al. (2000) found the E. albocollaris/E. collaris clade to be the most primitive clade. Yet, chromosomal studies strongly suggest that it is more likely the E. collaris-E. albocollaris clade diverged from E. fulvus more recently and subsequently evolved into the two present forms (Hamilton et al., 1980; Rumpler, 1989; Djlelati et al., 1997). Among E. fulvus taxa, it appears that E. f. fulvus may be the basal form (Tattersall and Sussman, 1998); this population appears to be the least differentiated, at least in terms of gross morphological or pelage characters (Tattersall, 1982, 1993; Shedd and Macedonia, 1991). If this is indeed the case, based on current and likely recent distributions, the brown lemur group would have first appeared in central-north Madagascar (Meyers and Absher, 1994, in Tattersall and Sussman, 1998), perhaps in humid forests (Tattersall and Sussman, 1998). This population would have subsequently expanded into drier western habitats, southeastern rainforests, and the mosaics of the central plateau, diversifying into a northern clade which gave rise to E. f. albifrons and E. f. sanfordi and a southern group consisting of E. f. rufus and later E. albocollaris/E. collaris (Tattersall, 1992). It is important to note that these patterns, based on the currently recognized brown lemur subspecies, remain speculative without confirmation from further systematics investigations. Again, the only study to date that potentially resolved E. fulvus subspecies phy-logeny identified clades that crosscut traditional taxonomic units (Pastorini et al., 2000).
The timing and mechanisms for brown lemur diversification remain obscure. It is perhaps most likely that these taxa originally diverged in allopatry (Tattersall, 1982). Paleoenvironmental studies of the late Pleistocene and Holocene indicate fluctuating periods of increased aridity in Madagascar (Gasse and van Campo, 1998, 2001), associated with forests contracting into isolated refugia (Tattersall, 1982; Richard and Dewar, 1991; Ganzhorn, 1998). With migration routes through the central plateau limited, rivers could have further served (and continue to serve) to subdivide lemur communities. While it appears that these rivers were barriers for already differentiated species (Tattersall, 1982; Richard and Dewar, 1991; Ganzhorn, 1998), it is possible that closely related populations, such as E. f. rufus and the ancestor to the E. collaris-E. albocollaris group, may have diverged during these more recent drying cycles. Subfossil and extant lemur species assemblages suggest that, while major biogeographic regional distinctions (e.g., between the dry west and humid east and north-south gradients) have long existed, there were numerous routes for faunal exchange among regions prior to the arrival of humans (Ganzhorn, 1998; Godfrey et al., 1999). Present-day parap-atry of brown lemur taxa may then have arisen during forest expansion subsequent to the last major period of aridity (e.g., Tattersall, 1982; Richard and Dewar, 1991). Among the significant questions remaining, however, is why the brown lemur group has diversified so extensively while other species have not. For example, the ecologically similar E. rubriventer (Overdorff, 1993) is monotypic but maintains a broad distribution which it shares with four distinct brown lemur taxa.
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