Dispersal and Vicariance in Hawaiian Platynine Carabid Beetles (Coleoptera)

Abstract

The monophyletic, native Hawaiian Platynini have diversified on the Hawaiian Island chain through progressive colonization, mixed with vicariance on the various islands. Single-island endemism stands at 97% of the species, with the few widespread species exhibiting distributions largely congruent with the fundamental area cladogram found using cladistic biogeographic methods. The cost of accepting an ad hoc dispersal hypothesis for individual taxa that conflicts with the fundamental area cladogram is weighed against the savings in items of error when taxa are excluded from the biogeographic analysis. Based on this objective assessment, only one back-dispersal from Maui Nui to O'ahu is supported. Vicariance of Maui Nui, leading to the present-day islands of Moloka'i, Lana'i, and Maui, has resulted in seven resolvable species triplets composed of single-island endemics occupying these areas. These seven triplets represent five biogeographic patterns, necessitating explanation by numerous ad hoc hypotheses of extinction to support a single hypothesis of area relationships. In six of the seven triplets, the cladistically basal species exhibits a higher minimum elevational limit of occupied habitat than either of the more apical sister species. This result is consistent with isolation of more persistent, peripheral populations at higher elevations, leading to speciation. Comparison of higher-elevation endemics to lower-elevation widespread species supports this interpretation. Such a finding affirms the importance of understanding geographic distribution on a scale appropriate to the action of vicariant mechanisms.