Southeast Asia as a cradle of early flowering plant diversification
Darwin's 'abominable mystery'
Flowering plants (angiosperms) represent one of the greatest terrestrial radiations in recent geological times, with over 250,000 species growing in all types of ecosystems across the globe. Although significant progress has been made in describing this diversity, and inferring evolutionary relationships among the major lineages of angiosperms (APG III, 2009), their origin and rapid rise to dominance remains one of the most enduring unsolved mysteries in evolutionary biology.
Darwin referred to this unsolved question as an 'abominable mystery' in his famous letter to Joseph Hooker. The father of evolutionary theory could not fathom this rapid diversification and rather invoked the idea that 'there was during long ages a small isolated continent in the S. hemisphere, which served as the birthplace of the higher plants'. I became interested in this topic during a Marie Curie postdoctoral fellowship (funded by the European Union) conducted at Kew’s Jodrell Laboratory two years ago. The vast collections and knowledge of plants at Kew provided the optimal environment to brainstorm the key environmental factors that could have triggered the rapid diversification of angiosperms during the Cretaceous.
What I report here is the development of a hypothesis that islands located in the region today occupied by South-East (SE) Asia played a major role in the diversification of angiosperms. This was recently published as a point of view in a special issue of the Botanical Journal of the Linnean Society entitled Ecology and evolution on oceanic islands: broadening the botanical perspective and builds upon my recent studies on the soapberry or litchi family, (Sapindaceae; Buerki et al., 2013), the coffee family (Rubiaceae; Barrabé et al., 2012) and the screw-pines (Pandanaceae; Buerki et al., 2012).
The earliest fossils of angiosperms date back to the Early Cretaceous (136-130 million years ago (hereafter Ma)) and were found in deposits in China, Israel and England (Friis et al., 2011). Interestingly, fossils of more derived groups (e.g. Ranunculales) are globally relatively common almost at the same geological time (ca. 130-112 Ma) (Friis et al., 2011). These findings provide evidence for a major event of spread and diversification in angiosperm history closely following their appearance in the fossil record, in other words over a relatively short period of time. This pattern of rapid global diversification, together with the absence of 'archaic' fossil lineages, makes it difficult to pinpoint the geographic area that may have acted as the cradle of angiosperms. So can we use other lines of evidence to provide further insights?
Evolutionary trees, dating and biogeography
Studies of the evolution of angiosperms have identified a group of early-diverging plants known as the ANA grade (APG III, 2009). Based on analyses of DNA data in which the branches of the evolutionary tree are dated, this ancient group is estimated to have arisen sometime between the Early Cretaceous (130Ma) and the Late Jurassic (160Ma) (see e.g. Magallon & Castillo, 2009; Bell et al., 2010). Five of these seven early plant families are distributed in East and SE Asia, Australia and the Pacific islands.
Similar conclusions also apply to the rest of the early lineages of angiosperms occurring in SE Asia, since most of the islands in this region (mainly the Wallacea region) are the product of the collision between the Australian and Eurasian plates, which started to take place at the Eocene-Oligocene boundary (ca. 33.9 Ma).
Palaeogeography of SE Asia
- APG III. (2009). An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III. Botanical Journal of the Linnean Society 161: 105-121. Available online
- Barrabé, L., Buerki, S., Mouly, A., Davis, A.P., Munzinger, J. & Maggia, L. (2012). Delimitation of the genus Margaritopsis (Rubiaceae) in the Asian, Australasian and Pacific region, based on molecular phylogenetic inference and morphology. Taxon 61: 1251-1268. Available online
- Bell, C.D., Soltis, D.E. & Soltis, P.S. (2010). The age and diversification of the angiosperms revisited. American Journal of Botany 97: 1296-1303. Available online
- Buerki, S., Forest, F., & Alvarez, N. (2014). Proto-South-East Asia as a trigger of early angiosperms diversification. Part of the Special Issue: Ecology and evolution on oceanic islands: broadening the botanical perspective. Botanical Journal of the Linnean Society 174: 326-333. Available online
- Buerki, S., Forest, F., Stadler, T. & Alvarez, N. (2013). The abrupt climate change at the Eocene-Oligocene boundary and the emergence of Southeast Asia triggered the spread of sapindaceous lineages. Annals of Botany 112: 151-160. Available online
- Buerki, S., Callmander, M.W., Devey, D.S., Chappell, L., Gallaher, T., Munzinger, J., Haevermans, T. & Forest, F. (2012). Straightening out the screw-pines: a first step in understanding phylogenetic relationships within Pandanaceae. Taxon 61: 1010-1020. Available online
- Friis, E. M., Crane, P. R. & Pedersen, K. R. (2011). Early flowers and angiosperm evolution. Cambridge University Press, Cambridge.
- Magallon, S. & Castillo, A. (2009). Angiosperm diversification through time. American Journal of Botany 96: 349-365. Available online