| Diversity of floral form and color within a group of closely related species is often taken as evidence for adaptive radiation to different pollinators. Such adaptive hypotheses can be testing by examining the evolution of floral traits and pollination systems in the context of a well-supported phylogeny. In this study, I investigated the evolutionary history and pollination biology of Iochrominae, a clade of Solanaceae which represents a putative case of pollinator-mediated diversification of floral form and color. Iochrominae is a group of about 35 Andean species, nested within the large Neotropical Solanoid radiation of some 1700 species and 64 genera. Despite its small size, Iochrominae contains a great diversity of flower forms and colors, many of which are rare in Solanaceae as whole. In order to place the evolution of these floral traits into an historical framework, I estimated the phylogeny of Iochrominae using three nuclear regions: the internal transcribed spacer (ITS), the second intron of LEAFY, and exons 2 through 9 of the granule-bound starch synthase gene. These analyses revealed the floral morphology is extremely labile, with multiple convergences on similar combinations of form and color. Field studies were undertaken to examine whether the observed patterns of floral evolution relate to differences in pollination ecology. However, despite enormous floral diversity among the study taxa, few major shifts in pollination were found, and no shifts appeared related to the appearance of particular floral traits, with the exception of nectar reward. Correlated evolution between pollinators and floral traits was then examined in more detail in a statistical phylogenetic framework. These analyses confirmed that changes in reward, but not corolla length or color, are strongly correlated with shifts in pollination system. Thus, mechanisms beyond a simple matching of pollinator taxa with floral traits must be considered. In particular, I focus on two alternative explanations for the pattern of floral variation: (1) phylogenetic constraint (inertia) and (2) diversifying selection due to interactions among species. |
