Natural selection on Erysimum mediohispanicum flower shape: insights into the evolution of zygomorphy.

Paleontological and phylogenetic studies have shown that floral zygomorphy (bilateral symmetry) has evolved independently in several plant groups from actinomorphic (radially symmetric) ancestors as a consequence of strong selection exerted by specialized pollinators. Most studies focused on unraveling the developmental genetics of flower symmetry, but little is known about the adaptive significance of intraspecific flower shape variation under natural conditions. We provide the first evidence for natural selection favoring zygomorphy in a wild population of Erysimum mediohispanicum (Brassicaceae), a plant showing extensive continuous variation in flower shape, ranging from actinomorphic to zygomorphic flowers. By using geometric morphometric tools to describe flower shape, we demonstrate that plants bearing zygomorphic flowers received more pollinator visits and had the highest fitness, measured not only by the number of seeds produced per plant but also by the number of seeds surviving to the juvenile stage. This study provides strong evidence for the existence of significant fitness differences associated with floral shape variation in E. mediohispanicum, thus illuminating a pathway for the evolution of zygomorphy in natural populations.