Adaptive Radiation in the Hawaiian Drosophila (Diptera: Drosophilidae): Ecological and Reproductive Character Analyses!

The entomologist R. C. L. Perkins pioneered observations of breeding site ecology for the endemic Hawaiian Drosophilidae, a renowned group of flies that has undergone explosive speciation and adaptive radiation into a wide variety of breeding niches. Females of the various species groups and subgroups oviposit their eggs in either fungi, flowers, fruits, leaves, stems, bark, sap fluxes, or other novel substrates. Varied selective forces in these alternative breeding sites have apparently molded female reproductive characters and strategies into diverse outcomes; some species mature and oviposit only one egg at a time, whereas others oviposit hundreds. Here, we have analyzed the pattern of shifts in breeding substrate, and the associated evolution of selected ovarian, egg, and ovipositor traits, by mapping the various ecological and female reproductive character states on an independently derived phylogenetic hypothesis based on nuclear and mitochondrial DNA sequences. This comparative phylogenetic approach demonstrates a number of strong historical associations among female reproductive traits and between particular traits and the breeding substrate, although the overall pattern is complex and more data are needed. Identification of certain apomorphic traits associated with shifts in breeding substrate suggests an adaptational origin for some of the changes in egg load per fly, in the length of the respiratory filaments of the egg, and in the length and shape of the ovipositor. Although these hypotheses need further testing, it appears that the ecological diversification in breeding substrates has been an integral component in the radiation of drosophilids in Hawai'i. species. In a preliminary taxonomic study of the Hawaiian Drosophilidae, Hardy (1965) described 400 species from Hawai'i, noting that the Hawaiian drosophilid fauna was without doubt one of the most remarkable of any area of the world and "by far the largest known group of animals in Hawai'i." With current estimates of more than 800 endemic species (Hardy and Kaneshiro 1981; Kaneshiro, pers. comm.), the Drosophilidae must certainly be considered a dominant component of the Hawaiian insect fauna. What is most remarkable is that this morphologically and behaviorally diverse group of flies probably evolved from one original founder or at most two founder individuals from a distant continent (Throckmorton 1966). Indeed, Perkins (1913) understood that many well-developed groups in Hawai'i had evolved from a single successful immigrant and was ahead of his time in enunciating some of the basic aspects of founder effect speciation (Mayr 1954, Carson