Molecular biogeography and diversification of the endemic terrestrial fauna of the Hawaiian Islands

Oceanic islands have played a central role in biogeography and evolutionary biology. Here, we review molecular studies of the endemic terrestrial fauna of the Hawaiian archipelago. For some groups, monophyly and presumed single origin of the Hawaiian radiations have been confirmed ( achatinelline tree snails, drepanidine honeycreepers, drosophilid flies, Havaika spiders, Hylaeus bees, Laupala crickets). Other radiations are derived from multiple colonizations ( Tetragnatha and Theridion spiders, succineid snails, possibly Dicranomyia crane flies, Porzana rails). The geographic origins of many invertebrate groups remain obscure, largely because of inadequate sampling of possible source regions. Those of vertebrates are better known, probably because few lineages have radiated, diversity is far lower and morphological taxonomy permits identification of probable source regions. Most birds, and the bat, have New World origins. Within the archipelago, most radiations follow, to some degree, a progression rule pattern, speciating as they colonize newer from older islands sequentially, although speciation often also occurs within islands. Most invertebrates are single-island endemics. However, among multi-island species studied, complex patterns of diversification are exhibited, reflecting heightened dispersal potential ( succineids, Dicranomyia). Instances of Hawaiian taxa colonizing other regions are being discovered ( Scaptomyza flies, succineids). Taxonomy has also been elucidated by molecular studies ( Achatinella snails, drosophilids). While molecular studies on Hawaiian fauna have burgeoned since the mid-1990s, much remains unknown. Yet the Hawaiian fauna is in peril: more than 70 per cent of the birds and possibly 90 per cent of the snails are extinct. Conservation is imperative if this unique fauna is to continue shedding light on profound evolutionary and biogeographic questions.SUMMARY: The progression rule pattern provides a predictive theoretical framework against which lineage age, bifurcation order and colonization timing can be better understood, especially when applied within a comparative approach. Overall, many groups examined to date follow, at least roughly, a progression rule pattern of diversification, wherein the most basal taxa occur on the oldest island in the distribution of the lineage. However, a number of biogeographic patterns in terrestrial Hawaiian radiations are complex (Funk & Wagner 1995; Holland & Hadfield 2004). While some endemic groups clearly originated on Kauai, others appear to have first colonized either Oahu (achatinelline tree snails), Maui Nui (Laupala cerasina group, crickets) or the island of Hawaii (Hylaeus bees and one lineage of succineid snails: clade B, figure 2). In a few groups (Havaika spiders, Banza crickets, Dicranomyia crane flies), neither the original island colonized nor the subsequent pattern of diversification is clear.