The genus Adenia provides a natural experiment to investigate the evolution of growth form. Within the group, trees, shrubs, herbs, vines, and lianas are present. Using ITS1 5.8S ITS2 sequence data, a phylogeny of 67 of ~100 species of Adenia is inferred using parsimony and Bayesian analyses. Specific hypotheses of monophyly are also tested to couch the analyses of growth form evolution within a phylogenetic framework. Within the context of this phylogeny, synapomorphies for major clades are discussed, as are patterns of growth form evolution. Absolute divergence times of nodes are estimated using penalized likelihood, and speciation rate based on these times is inferred to be fast relative to some other lineages of life. By reconstructing evolutionary history on a sample of trees from the posterior distribution of the Bayesian analysis, it is estimated that succulent stems evolved ca. four times, and tubers ca. eight. Transitions between markedly different growth forms occur on the scale of a few hundred thousand to a few million years, and close relatives frequently have different forms with no intermediates. The rapid diversification rates, fast morphological transitions, and multiple origins of water storage tissue in roots and shoots are hypothesized to result from a shared developmental program for water storage tissue that is flexibly turned on and off during evolution in stems and roots.
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