Speciation across the Earth driven by global cooling in orchidoid orchids

Abstract

Though climate change has been implicated as a major catalyst of diversification, its effects are thought to be inconsistent and much less pervasive than localised climate or the accumulation of species with time. But, we need focused analyses of highly specious clades to disentangle the consequences of climate change, geography and time. Here, we show that global cooling shapes the biodiversity of terrestrial orchids. Employing a phylogenetic framework of 1,450 species of Orchidoideae, the largest terrestrial orchid subfamily, we find that speciation rate is causally linked with historic global cooling, not time, habitation in the tropics, altitude, variation in chromosome number, or other types of historic climate change. Relative to the gradual accumulation of species with time, models specifying speciation driven by historic global cooling are 328 times more likely. Evidence ratios estimated for 212 other plant and animal groups reveal that the orchidoids represent one of the best-supported cases of temperature-spurred speciation yet reported. Employing >1.4 M georeferenced records, we find that global cooling drove contemporaneous diversification in each of the seven major orchid bioregions of the earth. With current emphasis on understanding and predicting the immediate impacts of global warming, our study provides a clear case study of the longterm impacts of global climate change on biodiversity. Significance statementThe staggering biodiversity of angiosperms has been difficult to reconcile with the gradual Darwinian process thought to create it. Changes in climate through the Earths history could have instigated this diversification, but perceived variability across clades and geography has restrained generalisation. In this paper, we reconstruct the evolutionary history of a rich terrestrial orchid subfamily favoured by Darwin (Orchidoideae, ~5,000 species), and use >1.4 million georeferenced records to test how and where those orchid species arose. We find that global cooling between the Oligocene and present day spurred an avalanche of speciation in orchidoid assemblages across the Earth. This work resolves the orchidoid phylogeny and provides a clear example of how historic climate change drives global patterns of biodiversity.