Much attention has been paid to the role of developmental information in estimating phylogenetic relationships and, more recently, to the use of phylogenies in understanding the evolution of development. At the moment, however, we lack a sufficiently general theory connecting phylogenetic patterns of character evolution to properties of developmental systems. Here we outline a simple model relating homoplasy to the rate of character change and the number of evolvable states, both of which may reflect developmental constraints. Given a particular rate of character change, the fewer the evolvable states the more homoplasy is expected, and vice versa. The repeated evolution of a limited number of forms of bilateral flower symmetry may reflect constraints imposed by overall flower orientation and underlying mechanisms of differentiation.