Molecular Studies

In recent years, approaches to lemur systematics have been dominated by molecular comparisons that have mostly yielded findings that support the monophyly of the entire lemur fauna. Anne Yoder and her colleagues (e.g., Yoder et al., 1996; Yoder, 2003; Yoder and Yang, 2004) have particularly vociferously rejected the notion that there is a special affinity between the cheirogaleids and the lorisoids. Most of this work has consisted of comparative studies of the mitochondrial cytochrome b gene, but lately certain nuclear elements have been added to the mix. Yoder and co-workers find that the basal split among the strepsirhine primates is between lorisiforms on the one hand, and lemuriforms including Cheirogaleidae on the other. Within Lemuriformes, they find that the basal split is between Daubentoniidae and all the rest.

Numerous lower-level problems of relationship among the species and genera of lemurs have been clarified by the molecular studies undertaken so far; but the rather fast-evolving mitochondrial genome is generally considered unreliable for assessing ancient splits, and DelPero et al. (2001) have found among the lemurs that while the 12S rRNA mitochondrial gene is useful for gauging within-family affinities, relationships among families separated by large genetic distances (<12% divergence) defy consistent resolution. This is most clearly the case among lorisids, galagids, daubentoniids and the apparent lemurid/indriid clade.

A recent study by Roos et al. (2004) that combined cytochrome b results with an analysis of nuclear short interspersed elements in a variety of strepsirhines situated the cheirogaleids within the lemuriform radiation, with a basal split between the aye-aye and all the others. This is another pointer toward the conclusion that the deeply entrenched notion of lemur monophyly may well be accurate—even though it suggests enormous levels of convergence and primitive retention respectively between and within the lorisoids and cheirogaleids. For the time being, definitive demonstration perhaps still awaits; but the evidence of historical biogeography, together with the fact that the other groups of Malagasy terrestrial mammals also appear most likely to be monophyletic, suggests that the external probabilities are on the side of lemur monophyly as well.

A further ramification of molecular studies is the estimation of divergence dates for the various higher taxa recognized (e.g., Yoder et al., 1996; Porter et al., 1997; Yoder and Yang, 2004). The most recent estimated date for the basal split among a monophyletic lemuriform group is 47 Ma (Porter et al., 1997), and Yoder has lately raised her estimate from >54 Ma (Yoder et al., 1996) to 62-65 Ma (Yoder and Yang, 2004), based on a variety of both mitochondrial and nuclear gene loci. Calibration was from the fossil record, with all the consequent uncertainties enumerated by Grauer and Martin (2004). Still, current molecular and morphological estimates of the divergence time seem to be in (very) approximate agreement. For if the lemurs are in fact monophyletic, the ancestral strepsirhine having given rise very early in the Tertiary (and most probably in Africa) to the ancestor of the Malagasy group on the one hand, and to the ancestor of the lorisids/galagids on the other; and if the lorisids and galagids had indeed differentiated by the mid-to-late Eocene (ca. 41-37 Ma) as Karanisia and Saharagalago seem to indicate, then some stretching of the molecular time scale would seem to be plausible. This is especially the case given the sheer scale of the diversification that has taken place among the lemurs subsequent to the time of their common ancestor, and it is supported by molecular comparisons that suggest that much of this diversification took place at an early stage—earlier than that of the living lorisoids (see, e.g., Yoder and Yang, 2004).

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