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Rapid speciation and the evolution of hummingbird pollination in neotropical Costus subgenus Costus (Costaceae): evidence from nrDNA ITS and ETS sequences¹

²Department of Plant Biology, Michigan State University, 166 Plant Biology Building, East Lansing, Michigan 48824 USA; W. K. Kellogg Biological Station, 3700 East Gull Lake Drive, Hickory Corners, Michigan 49060 USA; ³USDA-ARS National Center for Genetic Resources Preservation, 1111 South Mason, Fort Collins, Colorado 80521-4500 USA; ⁴Department of Biology, University of Washington, Seattle, WA 98195 USA

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.

Key Words: Costaceae • ETS • euglossine bees • hummingbirds • ITS • neotropics • pollination syndromes • speciation

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