maternal plants, which should lead to an increase in the frequency of white genes, according to modifier gene theory. Studies of geographical distributions indicate other, as yet undiscovered, disadvantages associated with the white phenotype. The ultimate goal of connecting ecology to molecular genetics through the medium of phenotype is yet to be attained, but this approach may represent a model for analyzing the translation between these two levels of biological organization.
The study of adaptation is a problem that intersects all disciplines of biology. The study of adaptation is also among the most difficult and challenging areas of experimental research because a complete causal analysis of adaptation involves a translation between different levels of biological organization, the ecological, the phenotypic, and the molecular level (Clegg, 2000). In this article we review more than 20 years of research on flower color polymorphisms in the common morning glory [Ipomoea purpurea (L.) Roth] that was aimed at exploring this interface.
When the morning glory research program was initiated in the late 1970s, flower color polymorphisms appeared to be a natural starting point because (i) they represented simple discrete phenotypes that were susceptible to genetic analysis; (ii) a substantial body of work existed on the biochemistry of plant secondary metabolism; (iii) flower color was known to be important in insect pollinator behavior; and (iv) selection on reproductive performance should be among the most effective forms of selection, and, as a consequence, it should be the component of selection most likely to yield to experimental analysis. Virtually nothing was known in the late 1970s about the molecular biology of the genes that determine the anthocyanin pigments responsible for flower color, so nothing was known about the specific mutational changes associated with different flower color polymorphisms. Since that time a great deal of progress has been made in describing the molecular biology of the genes of flavonoid biosynthesis that determine flower color, but we are still some distance from a complete causal analysis that connects ecology to phenotype to genes.
We begin by discussing the natural history of the morning glory and then turn to a brief account of the genetics of flower color variation in the common morning glory. Next, we describe the flavonoid biosynthetic pathway that determines flower color, and we review pertinent work on the molecular genetics of the genes that encode enzymes within this pathway. Finally, we consider progress in the analysis of selection on flower color variation in natural and experimental populations of the common morning glory.