A commonly accepted evolutionary principle is that adaptive change constrains the potential directions of future evolutionary change. One manifestation of this is Dollo's law, which states that character elimination is irreversible. Although the common occurrence of irreversibility has been documented by phylogenetic analyses of phenotypic transitions, little is known about the underlying causes of this phenomenon. One explanation for evolutionary irreversibility relies on the fact that many characteristics result from interactions between multiple gene products. Such characteristics may often be eliminated by inactivation of just one gene in the network. If they serve no other functions, other genes of the network are then free to accumulate mutations or evolve new functions. Evolutionary change after character loss results in the accumulation of redundant loss-of-function mutations. Such pathway degeneration makes it very unlikely that the characteristic will re-evolve, because multiple simultaneous mutations would be required. Here we describe what appear to be the initial stages of such degeneration in the anthyocyanin pigment pathway associated with an adaptive change from blue to red flowers in the morning glory Ipomoea quamoclit.