. "12 Flower Color Variation: A Model for the Experimental Study of Evolution." Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years after Stebbins. Washington, DC: The National Academies Press, 2000.
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Variation and Evolution in Plants and Microorganisms: TOWARD A NEW SYNTHESIS 50 YEARS AFTER STEBBINS
ways. At first sight it appears that each mutation in a gene encoding a pathway component is likely to have many phenotypic effects. Is it possible to associate a main effect with one aspect of phenotype? Perhaps, if gene redundancy and specialization in expression patterns approximates a one-to-one correspondence between gene and phenotype. Whether the unraveling of phenotypic determination will be so simple remains to be seen, but there is some reason to be optimistic based on flower color analyses.
Another fascinating result of this work is the overwhelming importance of mobile elements as generators of phenotypic diversity. All except one of the flower color phenotypes analyzed so far in both I. purpurea and I. nil are the result of transposon insertions. This strongly implies that transposable elements are the major cause of mutations that yield an obvious phenotype in these plant species. Because the distribution of transposons appears to be heterogeneous across plant genomes, it seems reasonable to speculate that some species may experience higher mutation rates and therefore may be more flexible in adapting to environmental changes than other plant species. In the case of I. purpurea, a significant environmental change was the appearance of human plant domesticators who selected and propagated unusual phenotypes for esthetic and perhaps other purposes. The plant has clearly been successful, at least in the short term, by virtue of its ability to adapt to this circumstance.
A number of questions remain to be resolved before this work can be seen as complete. First, the gene that is clearly subject to selection in nature is not a structural gene determining an enzyme in the anthocyanin pathway but, instead, a regulatory gene that determines the floral distribution of pigmentation (W/w locus). To date, this gene has not been characterized at the molecular level, and we do not know the nature of the mutational changes that cause the white phenotype. We do know a lot about the A/a locus, which also determines a white (albino) phenotype, and we can probably assume that this phenotype is also discriminated against by insect pollinators, but the albino phenotype is rare in populations. So major gaps exist in our present knowledge, but we have every reason to expect these gaps to fill in over time. Interestingly, we do know the molecular bases for the P/p locus phenotypes, but so far there is no evidence of selection in nature at this locus, although in a broader sense it is clear that man has selected this polymorphism for propagation.
A second limitation is that most population work has focused on the southeastern U.S., where the plant is introduced, rather than on native populations in the central highlands of Mexico. The southeastern U.S. is not the geographical region in which the species evolved, and it is not possible to make inferences about the longer term ecological circumstances that shaped the evolution of this species based on investigations