Timing and rate of speciation in<i>Agave</i>(Agavaceae)

The Agave (Agavaceae) are keystone species of semiarid to arid regions where the geographic center of origin is Mexico but whose populations spread from the southwestern U.S. through Central America, the Caribbean, and into northern South America. Our analyses indicate that Agave is a young genus, between 7.8 and 10.1 million years old, and yet it harbors the most species of any genera in the family. Of the eight genera in the family, Agave is paraphyletic with respect to three of them, and these four genera are often grouped into a genus termed Agave sensu lato, which harbors 208 of the 293 recognized species in the Agavaceae. In this article, we examine the phylogenetic limits of Agave sensu lato and present analyses elucidating the origin and rate of speciation in the group. These analyses lead to some new insights into the phylogenetic limits of Agave, indicate an estimated age of the family between 20 and 26 million years and an age of the Agave sensu lato of </=10 million years. Furthermore, we estimate a high mean per-lineage rate of diversification for the genus and find that rates of speciation were significantly elevated between 8 and 6 million years ago and then again between 3 and 2.5 million years ago. We discuss the potential for both monocarpy and the evolution of a generalist pollination system, largely dependent on nectarivorous bat species, as possible driving factors in the radiation of the group.