Rapid advances in plant genomics and in developmental genetics are providing new tools for plant systematists. Understanding the developmental and genetic basis of morphology will help to understand homology and character state delimitation, and to suggest the sorts of selective pressures that might have been responsible for diversification. A developmental genetic framework has been used to investigate macroevolution in the grasses, in which major morphological change has often involved change in the positionof developmental programs (heterotopy), possibly via ectopic gene expression. Heterotopy may have been responsible for 1) long-short cell alternation in the leaf epidermis of the grasses and their sister genus, Joinvillea; 2) diversification of function of C-class genes in the grass flower, and possibly suppression of portions of the perianth; 3) acquisition of perianth-like characteristics in leaf-like inflorescence organs ; 4) formation of staminate flowers in the panicoid grasses; and 5) repeated origin of C4 photosynthesis in multiple lineages. Not all macroevolutionary changes fit this model. For example, the novel morphology of the grass embryo can be described as a change in timing of development (heterochrony) rather than a change in position. As our understanding of morphological characters improves in coming years, it will be intriguing to see if other morphological novelty is created by simply moving old genes to new places.

Key words: development, grasses, heterochrony, heterotopy, macroevolution