Evolutionary or genealogical relationships in flowering plants are often poorly understood at taxonomic levels above the rank of genus. These difficulties presumably reflect the more frequent occurrence of convergent or parallel evolution in morphological characters at higher taxonomic levels. Although numerous examples of this exist, one of the most problematic is the Saxifragaceae family, including the gooseberries, hydrangeas, and saxifrages. As originally defined, this family consists of a diverse array of herbaceous and woody plants distributed throughout the world and placed into 15-17 subfamilies. Little agreement exists among plant systematists regarding the circumscription, taxonomic rank (subfamily, family), or relationships of these various groups. The taxonomic complexity involved in dealing with the Saxifragaceae also requires a thorough analysis of the large order Rosales, of which it is a member. It has become clear in recent years that DNA sequence comparisons can be particularly helpful in determining evolutionary relationships at higher taxonomic levels. Dr. Douglas Soltis of Washington State University is studying nucleotide sequence variation among a large sample of species of Saxifragaceae, focusing upon the photosynthesis-related gene rbcL. A broad analysis of rbcL sequence data will help clarify relationships within Saxifragaceae and in conjunction with data being generated in several other laboratories will contribute to determination of phylogenetic affinities among the rosalean dicots. The resulting phylogeny will then provide an evolutionary framework for interpreting change in morphological and chemical characteristics of these plants, and for informing their historical biogeography.
