Constrained lability in floral evolution: counting convergent origins of hummingbird pollination in Penstemon and Keckiella.

In the clade of Penstemon and segregate genera, pollination syndromes are well defined among the 284 species. Most display combinations of floral characters associated with pollination by Hymenoptera, the ancestral mode of pollination for this clade. Forty-one species present characters associated with hummingbird pollination, although some of these ornithophiles are also visited by insects. The ornithophiles are scattered throughout the traditional taxonomy and across phylogenies estimated from nuclear (internal transcribed spacer (ITS)) and chloroplast DNA (trnCD/TL) sequence data. Here, the number of separate origins of ornithophily is estimated, using bootstrap phylogenies and constrained parsimony searches. Analyses suggest 21 separate origins, with overwhelming support for 10 of these. Because species sampling was incomplete, this is probably an underestimate. Penstemons therefore show great evolutionary lability with respect to acquiring hummingbird pollination; this syndrome acts as an attractor to which species with large sympetalous nectar-rich flowers have frequently been drawn. By contrast, penstemons have not undergone evolutionary shifts backwards or to other pollination syndromes. Thus, they are an example of both striking evolutionary lability and constrained evolution.