Comparison of Rates of Speciation in Web-Building and Non-Web-Building Groups within a Hawaiian Spider Radiation

The isolation of the Hawaiian archipelago has resulted in a fauna that shows high levels of endemism. I examined the role of lifestyle, as inoltyferred from web-building versus non-web-building behavior, in dictating the rate of differentiation and species formation within a lineage of spiders in the genus Tetragnatha from the Hawaiian Islands. This genus comprises a group of morphologically, ecologically and behaviorally diverse taxa. Included in the radiation is a 'spiny-leg' clade which never builds webs and is relatively loosely associated with a specific habitat, and a large group of web-building species which are generally more tightly associated with a given substrate and habitat. Sequences of mitochondrial cytochrome oxidase DNA provided relative estimates of the age of a clade. Both linear and logarithmic models were used to estimate rates of speciation and the relative time required for speciation for each clade. The results showed that several small clades of web-building species have a greater rate of speciation as compared to the 'spiny-leg' clade. One explanation is that the web-building species may be capable of differentiation between more closely contiguous habitats, which would be consistent with the hypothesis that ecological differentiation promotes diversification and species formation. Possible alternative explanations for the results include differences in rates of molecular evolution, for example as a consequence of differences in metabolic activity. The Hawaiian archipelago provides a natural laboratory for studies of speciation (Simon 1987). First, the extreme isolation of the islands has caused accentuation and acceleration of evolution in the archipelago, with numerous examples of rampant species proliferation (for reviews see Wagner & Funk 1995; Roderick & Gillespie 1998). Further, the islands are a series of volcanoes arranged within an identifiable chronological time frame (Carson & Clague 1995). The currently high islands range from Kauai, the oldest and most eroded, to Hawaii, the youngest, highest, and largest, with five separate volcanoes. Each volcanic mountain therefore shows a different stage in the evolutionary history of a clade, and allows determination of the nature of the relationship between evolutionary time and the abundance and distribution of a set of species. Radiations of spiders in the Hawaiian Islands include the genera Tetragnatha (as described below), Argyrodes (Simon 1900), Theridion (Simon 1900), and species in the family Thomisidae (Simon 1900; Suman 1970; J.E. Garb, this volume), among others (Gillespie et al. 1998). I have been examining patterns of speciation (Gillespie 1991a, b, 1992a, Gillespie 1993; Gillespie & Croom 1995; Gillespie et al. 1994) and extinction (Gillespie 1992b, Gillespie & Reimer 1993) in a radiation of Hawaiian spiders in the long-jawed orb-weaving genus Tetragnatha (Tetragnathidae). Outside the archipelago, Tetragnatha is of worldwide distribution (Levi 1981), yet it is also one of the most homogeneous genera of spiders, in both morphology (elongate form [Kaston 1948]) and ecology (Caraco & Gillespie 1986; 1987a, b; Gillespie & Caraco 1987). Until 1991, information on the endemic Hawaiian tetragnathids was based on descriptions of only nine species (Karsch 1880; Simon 1900) in the genus Tetragnatha and one Doryonychus. Over the last 11 years I have collected native Tetragnatha in every native habitat type on all of the Hawaiian Islands. I have now described an additional 25 species of Hawaiian Tetragnatha (Gillespie 1991 a, 1992a, 1994) and more than 60 additional new taxa have been collected, of which descriptions for many are near completion. The tetragnathid radiation spans a tremendous spectrum of col-