Adaptive radiation involves the diversification of species each adapted to exploit different ecological roles. I have studied a radiation of spiders in the genus Tetragnatha (Tetragnathidae) in the Hawaiian Islands to elucidate processes involved in such diversification. The temporal framework of the Hawaiian Islands allows examination of the changing pattern of adaptive radiation over time, as lineages have generally progressed down the island chain from older to younger islands. Species of Tetragnatha in the spiny-leg clade on any one island are typically most closely related to others on the same island, and the same set of ecological forms (ecomorphs) has evolved repeatedly on different islands. These results indicate that adaptive radiation frequently involves ecological divergence between sister taxa to allow multiple close relatives to co-occur in the same habitat. The current study examines the geographical context within which these species arose. I focus on a clade of 5 species that occur on the volcano of East Maui; at any given site 3 species can co-occur, one of each of 3 different ecomorphs. Mitochondrial DNA sequences from populations of these 5 species from throughout their distribution (Maui, Lanai and Molokai) were used to infer the geographic history of the species on East Maui and to determine whether diversification likely occurred in situ, or alternatively whether diversification occurred in allopatry on different volcanoes. Although ecological differentiation between taxa is evident, allopatry is clearly implicated in the initial divergence of taxa. Further study is required to understand the nature of the interplay between allopatry and ecological divergence in species formation.
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