Temperature‐Dependent Speciation in the Sundaland Region Driven by Interglacial Refugial Isolation in the <i>Bulbophyllum lobbii</i> Complex (Orchidaceae)

ABSTRACT Aim The historical origins of biota in the Sunda Shelf region of Southeast Asia represent a long‐standing contentious issue in evolutionary biology. However, there is still no general agreement on the key factors driving the region's high species diversity and endemism in both plants and animals. Here, we inferred the evolutionary history of a small‐sized orchid radiation from the tropical rainforests (TRFs) of Sundaland and adjacent regions to explore the drivers of plant diversification in one of the world's biodiversity hotspots. Location Southeast Asia: Sundaland and adjacent regions. Taxon Bulbophyllum lobbii Lindl. species complex (Orchidaceae). Methods We estimated a time‐calibrated (nuclear/plastid) phylogeny of the B. lobbii complex (14 spp., 66 accessions) and performed model‐based biogeographical and diversification rate analyses, as well as ecological niche modelling (ENM) for current and four past periods (including a Mid‐Pleistocene Interglacial). Results were placed into the context of the region's palaeogeography. Results Lineage and species divergence in this Quaternary (≤ 2.40 Ma) orchid radiation predominantly occurred on the mainland‐Asian/Sunda Shelf landmass or, to a lesser extent, in the Philippines, and mostly before the first inundation of Sundaland ( c . 400 ka). The group's drastic decline in speciation rate after the ‘Mid‐Pleistocene Transition’ ( c . 1.0 Ma) likely reflects reduced opportunities for population isolation in interglacial mountain refugia due to intensified/lengthened glacial periods of range expansions. The latter, combined with long‐term climatic stability in mountain habitats, likely buffered against extinction. Main Conclusions Our results highlight the importance of Quaternary climate‐induced range dynamics as a key driver of temperature‐dependent speciation in an orchid lineage from the Sundaland region and thus help to better understand the impact of palaeoclimatic change on the region's extraordinary biodiversity. In addition, they strongly support an interglacial refuge theory of TRF plant taxa in this region, indicating that their currently restricted ranges are in a ‘refugial (warm) stage’ of likely high vulnerability.