Genetics and Morphological Evolution in Plants

The genetic basis of differences in morphology within and between flowering plant species is reviewed in order to elucidate how many genetic changes are responsible for the evolution of new characters. Two broad morphological categories are evident. Differences in structure, shape, orientation, and presence versus absence are frequently discrete and appear to be governed by one or two genes. Differences in dimensions, weight, and number usually exhibit continuous variation and are influenced by numerous genes, though many of them probably act only indirectly via general effects at the whole organ or whole plant levels. Although it is difficult to specify the relative contributions of the two morphological categories during evolutionary divergence, it is clear that discrete character differences are more common in plants than in animals. I propose that their prevalence in plants is a direct consequence of the open, less integrative, and plastic patterns of plant morphogenesis which permit large changes in morphology on the basis of relatively few genetic changes. Morphological divergence among genera or families of flowering plants may reflect many fewer genetic changes than is the case for similar taxonomic levels of higher animals. Accurate estimates of the number of genes responsible for character divergence require knowledge of the ontogenetic and anatomical details of character development and these must be coordinated with genetic analyses. Until this knowledge becomes available, general conclusions about the number of genetic changes responsible for morphological diversity are premature.