Glaciations, gradients, and geography: multiple drivers of diversification of bush frogs in the Western Ghats Escarpment

Abstract

The historical processes underlying high diversity in tropical biodiversity hotspots like the Western Ghats of Peninsular India remain poorly understood. We sampled bush frogs on 13 massifs across the Western Ghats Escarpment and examined the relative influence of Quaternary glaciations, ecological gradients and geological processes on the spatial patterns of lineage and clade diversification. The results reveal a large in situ radiation (more than 60 lineages), exhibiting geographical structure and clade-level endemism, with two deeply divergent sister clades, North and South, highlighting the biogeographic significance of an ancient valley, the Palghat Gap. A majority of the bush frog sister lineages were isolated on adjacent massifs, and signatures of range stasis provide support for the dominance of geological processes in allopatric speciation. In situ diversification events within the montane zones (more than 1800 m) of the two highest massifs suggest a role for climate-mediated forest-grassland persistence. Independent transitions along elevational gradients among sub-clades during the Miocene point to diversification along the elevational gradient. The study highlights the evolutionary significance of massifs in the Western Ghats with the high elevations acting as centres of lineage diversification and the low- and mid-elevations of the southern regions, with deeply divergent lineages, serving as museums.

Keywords: Earth processes; Western Ghats; bush frogs; diversification.

Figure 1.

Alternative processes driving diversification in a mountain setting and expected patterns of sister-lineage distributions (summarized in the electronic supplementary material, table S1). (Online version in colour.)

Figure 2.

Geographical ancestral range reconstruction on a chronogram of Western Ghats bush frogs shows two sister clades, North and South, divided by an ancient barrier, the Palghat Gap (PG), with limited dispersal (see map). The range on the map does not incorporate dispersal across the PG and an isolated lineage from one of the eastern massifs. Inset map shows global geographical extent of the lineages used in the reconstruction.

Figure 3.

Modes of speciation among the sister lineages of bush frogs in the Western Ghats. Allopatric speciation was found to be dominant at the terminal nodes. Parapatric speciation showed an increase with the addition of internal nodes.

Figure 4.

Zone of origin of bush frogs and historical transitions along an elevational gradient. Colours refer to the elevational zones (see (b)). In the case of transitions inferred using both six and three zones, the branches are coloured in red and transitions seen using only six zones are shown in brown. The shaded region corresponds to Late and Mid-Miocene. (a) Geographical range of bush frog lineages indicated by a number corresponding to major massifs in (b). Distribution: A, allopatric; P, parapatric; W, widespread; N and E, northern Western Ghats (WG) and eastern isolated massifs (not illustrated in (b)). (b) Elevational (cross-section) profile of the WG Escarpment.

Figure 5.

Congruence in diversification along elevational gradients (C and G in figure 4a). Both clades are characterized by a low-mid elevation widely distributed lineage, which is sister to a clade composed of high elevation allopatric lineages.