Pacific Windfalls

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Science  07 Apr 2000:
Vol. 288, Issue 5463, pp. 13
DOI: 10.1126/science.288.5463.13a

The phylogenetics, systematics, and geography of organisms have been linked inextricably since Darwin's day. However, seemingly straightforward questions—such as that of the relationship between the center of origin and the center of diversity of a given taxon—have been notoriously vexing, especially with plants. The application of molecular techniques, though not always a panacea, is exemplified by a new study of the Pacific wind-dispersed tree genus Metrosideros.

Wright et al. trace the phylogenetic affinities of the 25 to 30 species of Metrosideros using ribosomal DNA sequencing and map the phylogeny onto the current geographical distribution of these species across the central and southwestern Pacific. They find that New Zealand is the most likely candidate for the source of the current Pacific Metrosideros flora, even though neighboring New Caledonia has more species. They construct a chronology for the dispersal events that spread Metrosideros across the Pacific; while it appears that these trees were present in the western Pacific for much of the Miocene epoch, it is likely that the genus reached the Hawaiian islands only during the much more recent Ice Ages.

As might be expected, wind-dispersed plants form an important part of the flora of oceanic islands. With the ability to withstand the subzero temperatures of the upper atmosphere, it is feasible for seeds to be transported thousands of kilometers. Wright et al. suggest that the surprising recency of the Metrosideros colonization of the Hawaii islands is the result of a Pleistocene shift to strong westerly wind patterns at the height of the glaciations. Metrosideros, which is capable of colonizing fresh lava fields, is now one of the dominant tree taxa in the Hawaiian islands. The extension of such studies to other taxa promises to reveal much about the historical and geographical intricacies of biodiversity, its global patterns and local idiosyncrasies.—AMS

Proc. Natl. Acad. Sci. U.S.A., in press

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