had split, the maize genome became polyploid, which accounts for much of the difference in DNA content between these related species. The polyploid event was followed by diploidization and much rearrangement of the genome, so that maize is now a diploid. But there remains much “extra” DNA in maize, mostly consisting of multiple repetitions of retrotransposons that account for 50% of the nuclear genome. This multiplication has occurred within the last 5–6 million years and has also contributed to the genome differentiation between maize and sorghum. The evolutionary complexities of cultivated maize extend to individual genes that have been variously impacted by domestication and intensive breeding.
Michael T. Clegg and Mary L. Durbin (“Flower Color Variation: A Model for the Experimental Study of Evolution, ” Chapter 12) trace the development of flower color in the morning glory, from the molecular and genetic levels to the phenotype, as a model for analyzing adaptation. Most mutations determining phenotypic differences turn out to be due to transposon insertions. Insect pollinators discriminate against white flowers in populations where white flowers are rare. This would provide an advantage to white genes through self-fertilization in white maternal plants. The pattern of geographic distribution of white plants indicates that such advantage is counteracted by definite, but undiscovered disadvantages of the white phenotype. The authors conclude by proposing that floral color development is an area of special promise for understanding the complex gene interactions that impact the phenotype and its adaptation, precisely because “the translation between genes and phenotype is tractable . . . [and] the translation between environment and phenotype is more transparent for flower color than in most other cases.”
Barbara A. Schaal and Kenneth M. Olsen point out, in “Gene Genealogies and Population Variation in Plants” (Chapter 13), that it was largely due to Stebbins that the investigation of individual variation within populations become part and parcel of the study of plant evolution. For many years beyond 1950, the focus of investigation was the phenotype: morphology, karyotype, and fitness components. Protein electrophoresis opened up the identification of allozyme variation and thus the study of allelic variation at individual genes. Restriction analysis and DNA sequencing have added the possibility of reconstructing the intraspecific genealogy of alleles. The mathematical theory of gene coalescence has provided the analytical tools for reconstruction and interpretation. Schaal and Olsen put all these tools to good use in several model cases: the recent rapid geographic expansion of Arabidopsis thaliana, with little differentiation between populations; the recolonization of European tree species from refugia created by the Pleistocene glaciation; the origin and domestication of cassava (manioc), the main carbohydrate source for 500 million people in the world tropics.