Submitted to: American Journal of Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/24/2005
Publication Date: 11/1/2005
Citation: Kay, K.M., P.A. Reeves, R.G. Olmstead and D.W. Schemske. 2005. Rapid speciation and the evolution of hummingbird pollination in neotropical Costus subgenus Costus (Costaceae): Evidence from NRDNA ITS and STA sequences. American Journal of Botany 92(11):1899-1910. Interpretive Summary: The tropical regions of North and South America hold higher plant diversity than any other tropical regions in the world. The mechanism by which this plant diversity has been generated is not well understood, although many theories exist. In this study, the evolutionary relationships among a diverse plant genus of the new world tropics, Costus, were estimated using DNA sequence data. Based on these relationships, it appears that bee pollination was the most ancient form of pollination in Costus. Numerous shifts to hummingbird pollination have occurred since the arrival of Costus in the new world from Africa. If one assumes that the DNA sequences analyzed in this study changed at a relatively constant rate over time, the approximate date when new species evolved can be estimated. The data collected for this study suggest that most of the present day Costus species evolved during a short period of time in the Pliocene and Pleistocene. During this same period, large scale geological changes, such as the rise of the Andes, may have promoted both isolation among populations, and increased environmental variability. Likewise, the closing of the Panama isthmus may have allowed ancient Costus species to spread into new habitats in South America. These factors, in concert with changing climatic conditions and a diverse pollinator fauna, may have promoted rapid speciation within new world Costus.
Technical Abstract: We estimate phylogenetic relationships and the biogeographic and pollination history of Costus subgenus Costus (Costaceae) using sequence data from the internal and external transcribed spacer (ITS and ETS) regions of 18S–26S nuclear ribosomal DNA. The African members of the subgenus form a series of lineages basal to a monophyletic neotropical species radiation. The neotropical species have large, showy flowers visited by either euglossine bees or hummingbirds. The hummingbird pollination syndrome is supported as a derived character state from the bee pollination syndrome, and we estimate that it has evolved independently seven or more times in the neotropics. A molecular clock approach suggests that diversification of the neotropical clade has been recent and rapid and that it coincides with dramatic climatic and geologic changes, Andean orogeny, and the closing of the Panama isthmus that occurred in the Pliocene and Pleistocene epochs. We propose a scenario for the diversification of Costus, in which rapid floral adaptation in geographic isolation and range shifts in response to environmental changes contribute to reproductive isolation among close relatives. We suggest that these processes may be common in other recently diversified plant lineages centered in Central America or the Northern Andean phytogeographic region.