cal power, it may not prove feasible to measure each component of fitness with sufficient accuracy to identify all of the factors that comprise a balance. It is also important to note that, despite much experimental effort over two decades, no evidence has emerged for differential selection among other flower color phenotypes such as those determined by the P/p and I/i loci.
We began this work with the conviction that a full understanding of the mechanisms of adaptation would entail an integration of the ecological and the genetic dimensions of biology. This wisdom was by no means original but, rather, was embodied in Ledyard Stebbins ' monumental contributions to plant evolutionary biology. As Stebbins emphasized, the point of contact between these two levels of biological organization is the phenotype. Whether a phenotype is adapted to a particular environment depends on a host of biotic and abiotic factors that collectively translate into survival and reproductive success. It is often difficult to identify the environmental elements that occasion phenotypic success. We began with the notion that flower color provided a simple model for the study of adaptation because one aspect of the environment seemed, a priori, to be predominant—the behavior of insect pollinators. To a large extent a substantial body of experimental work has validated this assumption, but along the way we have been forced to begin to confront the full complexity of plant reproductive biology. In a similar vein, the phenotypic bases of flower color variation appeared to be susceptible to analysis because much was already known about the biochemistry and genetics of flower color determination and because the tools for molecular analysis were rapidly appearing on the horizon. What we did not anticipate was the remarkable ecology of plant genomes in which most genes are redundant and in which mobile elements are a major player in the generation of phenotypic diversity.
One complexity that may be more apparent than real is the problem of genetic redundancy. As noted in this article, most of the genes of flavonoid biosynthesis occur in multiple copies, and sorting through this redundancy to find the particular genes responsible for individual flower color phenotypes appeared daunting at first sight. The actual findings are encouraging, in that particular gene copies of CHS and DFR are shown to be causally responsible for particular flower color phenotypes. In the case of CHS, we know that the other redundant copies are predominantly expressed at other stages in development (e.g., CHS-E in the floral tube) or they have diverged in catalytic properties (e.g., CHS-A, -B, and -C).
Another complexity is the pleiotropic nature of biosynthetic path-