A new paper in Annals of Botany demonstrates the importance of the combined effect of geomorphological (the emergence of most islands in SE Asia approx. 30 million years ago) and climatic (the dramatic Eocene–Oligocene climate change that shifted the tropical belt and reduced sea levels) factors in shaping plant species distribution.
Although not considered one of the ‘Big five’ mass extinctions, the abrupt cooling near the Eocene–Oligocene (E–O) boundary 33.7 million years ago had great impacts on biodiversity. During this period, Earth’s climate shifted from a relatively ice-free world to one with glacial conditions in polar regions characterized by substantial ice sheets. In a relatively short time span, high-latitude temperatures decreased from approximately 20 °C to about 5 °C. Explanations for this cooling include changes in ocean circulation due to the opening of Southern Ocean gateways, a decrease in atmospheric CO2 and a decrease in solar insulation. This period also coincided with drought in southern regions and subsequent reduction of the tropical belt. As a consequence, this abrupt cooling seemed to be related to a decrease in species diversity as shown, for instance, in the decline of the Neotropical floras. During the same geological period, intensive volcanic activities were recorded in South-East (SE) Asia as a result of the collision of the Eurasian and Australian plates. Wallacea is at present a region of high endemism for plants and animals.
The data in this paper suggest suggest an origin of the sapindaceous clade in Eurasia sometime during the Late Cretaceous, with subsequent dispersals into the southern hemisphere during the Late Paleocene mediated by the Gondwanan break-up and the emergence of proto-SE Asia. Well-known members of Sapindales include citrus; maples, horse-chestnuts, lychees and rambutans; mangos and cashews; frankincense and myrrh; mahogany and neem.
The abrupt climate change at the Eocene–Oligocene boundary and the emergence of South-East Asia triggered the spread of sapindaceous lineages. Annals of Botany (2013) 112 (1): 151-160. doi: 10.1093/aob/mct106
Paleoclimatic data indicate that an abrupt climate change occurred at the Eocene–Oligocene (E–O) boundary affecting the distribution of tropical forests on Earth. The same period has seen the emergence of South-East (SE) Asia, caused by the collision of the Eurasian and Australian plates. How the combination of these climatic and geomorphological factors affected the spatio-temporal history of angiosperms is little known. This topic is investigated by using the worldwide sapindaceous clade as a case study. Analyses of divergence time inference, diversification and biogeography (constrained by paleogeography) are applied to a combined plastid and nuclear DNA sequence data set. Biogeographical and diversification analyses are performed over a set of trees to take phylogenetic and dating uncertainty into account. Results are analysed in the context of past climatic fluctuations. An increase in the number of dispersal events at the E–O boundary is recorded, which intensified during the Miocene. This pattern is associated with a higher rate in the emergence of new genera. These results are discussed in light of the geomorphological importance of SE Asia, which acted as a tropical bridge allowing multiple contacts between areas and additional speciation across landmasses derived from Laurasia and Gondwana. This study demonstrates the importance of the combined effect of geomorphological (the emergence of most islands in SE Asia approx. 30 million years ago) and climatic (the dramatic E–O climate change that shifted the tropical belt and reduced sea levels) factors in shaping species distribution within the sapindaceous clade.