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The Hawaiian Archipelago is the most isolated oceanic island system in the World, separated from the nearest source areas by more than 4000 km. Five independent colonization events have resulted in diversification of a native carabid beetle fauna in excess of 400 known species. This diverse assemblage is disharmonic, with the major radiations restricted to the platynine genus  Blackburnia  Sharp (139 species), the subgenus  Nesocidium  Sharp of  Bembidion  Latreille (21 species), and the moriomorphine genus  Mecyclothorax  Sharp (239 species). Biogeographical, ecological, and evolutionary attributes of these three radiations are compared in order to determine factors crucial to carabid beetle diversification in this most-isolated situation. Biogeographical attributes include the age of origin of the constituent radiation, the island likely colonized by its common ancestor, and the biological characteristics, where known, of the colonizing ancestors for each independent radiation. Ecological attributes include the amount of habitat specialization undergone during each radiation, taking into account the primordial habitat colonized and the subsequent pattern of occupation of different habitat types during diversification. Evolutionary attributes include brachyptery, body-size evolution, sexual selection, and the evolution of specialized body conformations. It is shown that ecological specialization—i.e., occupation of a diverse array of ecological zones and microhabitats—in concert with reduced dispersal ability brought on by evolution of brachyptery are positively associated with enhanced levels of diversification. Comparing sympatric island faunas, it is shown that the latter operates synergistically with body size, as the smaller-bodied  Mecyclothorax  beetles speciate much more rapidly than the larger-bodied  Blackburnia  on Maui and Hawai῾i Island .  Nonetheless, small body size does not gaurantee high diversity, as  Bembidion  beetles attain body sizes similar to  Mecyclothorax  beetles. Age of origin  of a radiation is a subsidiary criterion for diversification given that the  Mecyclothorax  radiation commenced only 1.2 Ma, whereas it is hypothesized that  Blackburnia  have been resident in the Hawaiian archipelago for upwards of 28 Ma. Thus especially for  Blackburnia  we are constrained in our ability to know all of the evolutionary products of the radiation due to extinction of presumably all or nearly all species that occupied the now-sunken islands northwest of the oldest high island of Kauai. We are fortunate to know of several extinct  Blackburnia  species discovered in lowland subfossil deposits in Kauai, and these species provide crucial information now regarding future patterns of extinction. Sexual selection can be demonstrated for the  Bembidion  subgenus  Nesocidium , and is a likely explanation for genitalic evolution over parts of the  Mecyclothorax  radiation, but it is not a phenomenon pervasively associated with increased levels of speciation.

Evolution in Extremis: The When, How, and Why of Hawaiian Carabid Beetles