Biogeographic Evolution of Madagascar's Microendemic Biota

Abstract

The endemic species richness on Madagascar, relative to landmass area, is unparalleled in the world. Many organisms on the island have restricted geographical ranges. A comprehensive hypothesis explaining the evolution of this microendemism has yet to be developed. Using an analysis of watersheds in the context of Quaternary climatic shifts, we provide a new mechanistic model to explain the process of explosive speciation on the island. River catchments with sources at relatively low elevations were zones of isolation and hence led to the speciation of locally endemic taxa, whereas those at higher elevations were zones of retreat and dispersion and hence contain proportionately lower levels of microendemism. These results provide a framework for biogeographic and phylogeographic studies, as well as a basis for prioritizing conservation actions of the remaining natural forest habitats on the island.

Palaeo-precipitation is a major determinant of palm species richness patterns across Madagascar: a tropical biodiversity hotspot

“…Over 90 per cent of species diversity of Madagascar palms falls within these two genera, which are both estimated to have diverged from their closest relatives relatively recently compared with other palm genera (ca 13 Ma; [36]) and may have undergone rapid radiations, especially Dypsis [63]. Glacial refuge locations have been proposed to explain patterns of micro-endemism on Madagascar [28], notably the habitat isolation caused by aridification of lowelevation river catchments during Quaternary glacial periods will have acted as barriers to gene flow, and consequently would have stimulated speciation of local endemics that are now concentrated mainly in these refuge sites [28]. However, we do not yet understand the evolutionary history of Madagascar's palms well enough to assess how much speciation has occured during the Quaternary.…”

Geographical patterns of deep mitochondrial differentiation in widespread Malagasy reptiles

“…In one of the most recent such proposals Wilmé et al (2006) proposed a number of areas relevant as refugia and for speciation processes. We here overlaid these areas with the general biogeographic regions of Angel (1942) and Glaw and Vences (1994) in order to facilitate referring to particular areas of Madagascar in the text (Fig.…”

“…Multi-species phylogeographic studies (Pastorini et al, 2003) have emphasized the importance of three large rivers as barriers in species distribution. Wilmé et al (2006) postulate that river basins at intermediate altitude functioned as stable refugia during cold glacial periods. Climatic oscillations have caused forests to move up and down the slopes, enabling and severing gene flow and thereby promoting speciation (cf.…”

“…Biogeographic regions of Madagascar as used in this paper. Lines delimit centers of endemism and refuges from the hypothesis laid out by Wilmé et al (2006). Major rivers are depicted in blue.…”

Incorporating evolutionary history into conservation planning in biodiversity hotspots

“…We attempt at identifying a process (or processes) that could generate a high level of endemism within a very short period of time. Based on lemur distribution data, Wilmé et al [13] argued that watersheds provided shelters for species during the Quaternary climatic shifts (by allowing species to escape arid environments in lowlands) and also triggered speciation processes. The watershed hypothesis relies on the idea that river catchments with sources at relatively low elevations were zones of isolation and led to the speciation of locally endemic taxa, whereas those at higher elevations were zones of retreat and dispersion and hence contain proportionately lower levels of endemism.…”