LEAF SHAPE EVOLUTION IN THE SOUTH AFRICAN GENUS PELARGONIUM L' HÉR. (GERANIACEAE)

Leaf shapes reflect complex assemblages of shape-determining elements, yet evolutionary studies tend to treat leaf shape as a single attribute, for example cordate or linear. As with all complex structures, individual elements of a leaf could theoretically evolve independently and at different rates to the extent permitted by genetic and functional limitations. We examined relative evolutionary lability of shape-determining elements in the highly diverse South African plant genus Pelargonium (Geraniaceae). We used SIMMAP to calculate Bayesian posterior probabilities for ancestral states of leaf-shape characters for major nodes across multiple phylogenetic trees. Trees were derived from a Bayesian analysis of DNA sequence data from four partitions. We found that shape elements differed in rates of character-state transformations across the tree. Leaf base, apex, and overall outline had low rates. Transformations in venation occurred at slightly higher rates and were associated with shifts in venation among major clades. Leaf margin type and overall leaf size showed intermediate rates, whereas high rates were observed in the extent of lamina lobing and functional leaf size. The results indicate that suites of elements characteristic of the recently evolved xerophytic lineage, for example pinnate venation, dissected lamina, and entire margins, were acquired piecemeal over nested levels of the phylogeny.