Patterns and Mechanisms of Evolution of Island Plants
Ballard, H. E. .
Emerging revelations on evolutionary processes and products in the Hawaiian violets (Viola).
As a modest radiation of ca. 10 endemic taxa in Hawaii, the genus Viola possesses features that are amenable to evolutionary studies. Taxa are few, many are readily accessible, most are not of conservation concern, and their morphologies represent putatively simple evolutionary changes. Previous phylogenetic studies have identified the Arctic tundra bog species Viola langsdorffii as sister to the Hawaiian species and place Kauai taxa in basal positions, suggesting that the successful colonist was an herbaceous bog plant arriving first on Kauai. Studies of growth form, leaf morphology and anatomy confirm the expectations of an adaptive radiation affecting the plant body. Recent investigations of ecological differentiation in light availability, soil moisture and pH, and soil nutrients (Ca, K, Mg and P) for three species on Kaui (V. tracheliifolia of dry forests, V. waialenalenae of swamps and V. kauaensis of bogs), plus a bog species (V. maviensis) on Maui, demonstrate differentiation of ecophysiological tolerances among sister species on Kauai. The three species inhabit consistently distinct niches based on soil moisture, organic matter and nitrogen. Conversely, the non-sister bog violets on Kaui and Maui thrive in essentially identical niches. These results and the eco-geographic location of all Hawaiian species suggest recurrent ecological speciation, probably initially along a moisture gradient, subsequent to each dispersal event, with morphological and anatomical phenotypes presumably differentiating later under locally strong environmental selection. Imminent future studies revolve around two key objectives. The first is identification and characterization of pre- and post-mating isolation mechanisms that maintain integrity among Kauai species. The second is identification and characterization of "ecological" gene systems underlying the divergent ecophysiological tolerances confirmed in the Hawaiian violets, and elucidation of the specific molecular steps initially driving the process of ecological speciation.
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1 - Ohio University, Dept. of Environmental & Plant Biology, and Molecular & Cellular Biology Program, 317 Porter Hall, Athens, Ohio, 45701, USA
Presentation Type: Symposium or Colloquium Presentation
Location: Salon J - Austin Grand Ballroom/Hilton
Date: Wednesday, August 17th, 2005
Time: 10:45 AM