The consequences of variation in leaf form (simple, complex) has long intrigued plant physiologists and ecologists. Here, phylogenetic methods (using recent molecular phylogenies of angiosperms) are used to examine patterns in the direction of change. These analyses suggest, commensurate with paleobotanical studies, that the ancestral angiosperm had simple leaves. However, contrary to the general (implicit) assumption, there is an overall trend of a lower frequency of transitions from simple to complex (‘gains’) than from complex to simple (‘losses’). This finding has wide-ranging molecular developmental, physiological and ecological implications that are briefly discussed.

Key words: development, evolution, leaf