The Grasses: A Case Study in Macroevolution

Macroevolution in the grasses has often involved change in the position (heterotopy) of developmental programs, possibly via ectopic gene expression. Heterotopy apparently has been involved in the evolution of unique epidermal morphology in the grasses and their sister genus, Joinvillea; in the origin of the grass flower and possibly in the spikelet as well; in the formation of unisexual flowers in the panicoid grasses, and in the repeated origin of C4 photosynthesis. Change in timing of development (heterochrony) may explain the novel morphology of the grass embryo. Changes in the structure and size of the nuclear genome correlate with phylogenetically informative cytogenetic characteristics. Most of the 10,000 species of grasses evolved tens of millions of years after the common ancestor of the family, indicating that the origin of novel morphologies did not lead to immediate radiation.