The lemurs of Madagascar provide an excellent model for exploring evolutionary diversification. This study investigates genetic divergence among most extant lemur taxa in relation to potential geographical boundaries to gene flow. For this purpose, Ϸ2,400 bp of mitochondrial DNA (part of the COIII gene; ND3, ND4L, and ND4 genes; and five tRNAs) were sequenced in a total of 131 lemurs from 5 families, 12 genera, 25 species, and 18 subspecies to reconstruct phylogenetic relationships among them. The comprehensive range of taxa makes this a particularly suitable molecular data set to examine lemur evolution. Those data clearly reveal that the Betsiboka River acts as an isolating barrier between populations of lemurs in north-western Madagascar. The Tsiribihina River similarly serves as a barrier to gene flow between northern and southern populations of lemurs in central western Madagascar, whereas the Mahavavy River does not seem to lead to genetic isolation of lemur populations. Several discrepancies among molecular data, current taxonomy, and geographic distribution along the western coast emerged. Examination of geographical distribution of the taxa concerned in comparison with distribution boundaries of other lemur taxa in that region yielded explanations for these inconsistencies. Eulemur fulvus and Eulemur mongoz are the only lemur taxa that also occur outside Madagascar, on the Comoro Islands. Genetic data show no significant differentiation between Malagasy and Comorian populations of these species, supporting the interpretation that both were introduced only recently to the Comoro Islands.

The evolution and origin of primate social organisation has attracted the attention of many researchers, and a solitary pattern, believed to be present in most nocturnal prosimians, has been generally considered as the most primitive system. Nocturnal prosimians are in fact mostly seen alone during their nightly activities and therefore termed 'solitary foragers', but that does not mean that they are not social. Moreover, designating their social organisation as 'solitary', implies that their way of life is uniform in all species. It has, however, emerged over the last decades that all of them exhibit not only some kind of social network but also that those networks differ among species. There is a need to classify these social networks in the same manner as with group-living (gregarious) animals if we wish to link up the different forms of primate social organisation with ecological, morphological or phylogenetic variables. In this review, we establish a basic classification based on spatial relations and sociality in order to describe and cope properly with the social organisation patterns of the different species of nocturnal prosimians and other mammals that do not forage in cohesive groups. In attempting to trace the ancestral pattern of primate social organisation, the Malagasy mouse and dwarf lemurs and the Afro-Asian bushbabies and lorises are of special interest because they are thought to approach the ancestral conditions most closely. These species have generally been believed to exhibit a dispersed harem system as their pattern of social organisation ('dispersed' means that individuals forage solitarily but exhibit a social network). Therefore, the ancestral pattern of primate social organisation was inferred to be a dispersed harem. In fact, new field data on cheirogaleids combined with a review of patterns of social organisation in strepsirhines (lemurs, bushbabies and lorises) revealed that they exhibit either dispersed multi-male systems or dispersed monogamy rather than a dispersed harem system. Therefore, the concept of a dispersed harem system as the ancestral condition of primate social organisation can no longer be supported. In combination with data on social organisation patterns in 'primitive' placentals and marsupials, and in monotremes, it is in fact most probable that promiscuity is the ancestral pattern for mammalian social organisation. Subsequently, a dispersed multi-male system derived from promiscuity should be regarded as the ancestral condition for primates. We further suggest that the gregarious patterns of social organisation in Aotus and Avahi, and the dispersed form in Tarsius evolved from the gregarious patterns of diurnal primates rather than from the dispersed nocturnal type. It is consequently proposed that, in addition to Aotus and Tarsius, Avahi is also secondarily nocturnal.

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