Evolution and Adaptive Radiation in Lipochaeta (Compositae) of the Hawaiian Islands

The Wedelia-like ancestors of Lipochaeta were probably dispersed to Hawaii by water, becoming established either on Kauai or Oahu. From the original colonizers, 25 species and four infraspecific taxa evolved. Polypoloidy played a major role in the formation of two independent evolutionary units. Among the diploids, the present distribution suggests that the species have evolved allopatrically on single islands through geographical isolation. The distribution pattern of the tetraploids suggests that they evolved allopatrically on separate islands rather than within different habitats on single islands as with the diploids. An analysis of leaf flavonoid chemistry has shown that the diploids synthesize few compounds, all flavonols. In contrast, the tetraploids elaborate many compounds, both flavones and flavonols. Although the diploids show marked morphological differences, their flavonoid systems have diverged very little. The broader array of compounds in the tetraploids suggests that their chemical systems are still under intense selection pressures. Adaptive radiation describes the process by which a group of organisms adapts to a broad variety of situations (Carlquist, 1970). One can argue that most groups have radiated to some extent, so to be more meaningful the term should be restricted to a small group of organisms that illustrates unexpected diversification. For adaptive radiation to become maximal, an array of habitats must be available to which few of the species initially are adapted. Perhaps the best natural laboratories for the study of this phenomenon are volcanic oceanic islands because: 1) an island mass of volcanic origin often has an environmental range from tidewater marsh through dry lowland to wet forest, or even to an alpine zone, providing a broad variety of habitats concentrated into a relatively small geographical area; and 2) plants that are dispersed to and become established on oceanic islands are isolated from mainland ancestral populations, and therefore, the initial gene pool is limited to very few individuals. If one assumes there are no later introductions of the same taxon, then all of the diversity that develops can be traced to the original colonizers. To further our understanding of adaptive radiation and evolution in an island-centered plant group, Lipochaeta De Candolle, endemic to the I This paper is a portion of a Ph.D. dissertation submitted to The Ohio State University, Columbus. I thank Tod F. Stuessy for his advice throughout the study and acknowledge support for the field work from National Science Foundation Grant GB41175 and from Sigma Xi Grants-in-Aid of research for chemical supplies. 2 Botany, The Ohio State University, Columbus, Ohio 43210. This content downloaded from 207.46. 13.124 on Wed, 22 Jun 2016 06:11:00 UTC All use subject to http://about.jstor. org/terms 384 SYSTEMATIC BOTANY [Volume 1 Hawaiian Islands, was selected for study. In addition to morphological comparisons, many taxa were analyzed for chromosome number (Gardner, 1976) and leaf flavonoid chemistry. Data from the latter were utilized to determine whether flavonoids are correlated with patterns of adaptive radiation. Numerous studies have documented the utility of these compounds for indicating degrees of relationship among plant taxa (Turner &< Alston, 1959; Alston & Turner, 1962; Levin, 1967; Crawford, 1974; Giannasi, 1975), yet very few studies (Gupta c Gillett Sc Lim, 1970) have attempted to use chemical data to help interpret patterns of evolution among island-centered plant groups. Lipochaeta is listed (Carlquist, 1965) as an example of a genus that has undergone considerable adaptive radiation. Detailed analysis, however, including a revisionary study that recognizes 25 species (Gardner, 1976), has shown that although the genus represents an excellent system for determining evolutionary relationships, it illustrates only a moderate degree of adaptive radiation. The purpose of this paper is to discuss the origin of Lipochaeta and indicate evolutionary and adaptive trends within the genus.