THE EVOLUTIONARY HISTORY OF THE SILVERSWORD ALLIANCE
High on the Hawaiian volcanoes of Haleakala and Mauna Kea is a remarkable plant that attracts a steady stream of admirers from around the world. Argyroxiphium sandwicense, commonly known as the silversword, is for most of its life a globe-shaped plant with rigid leaves covered by dense silver hairs. After living for anywhere from 15 to 50 years, the plant produces a spectacular stalk up to six feet tall covered with hundreds of maroon flowers; soon after producing a new generation of seeds, it dies. Near extinction in the 1920s because of destruction by humans, goats, and cattle, the silversword is now protected and is gradually increasing in numbers.
The silverswords that grow on the flanks of Haleakala and Mauna Kea represent one of five separate silversword species found on Maui and the Big Island (one of these species is now extinct). Recognizing similarities in their leaves and flowers, botanists have known for decades that these five species are related to 25 other Hawaiian plant species. Two of these other species belong to the genus Wilkesia and grow only on the island of Kauai. The other 23 species belong to the genus Dubautia and grow from Kauai to the Big Island.
In the late 1980s, botanists Bruce Baldwin, Donald Kyhos, and Gerald Carr, after comparing the chromosomes and DNA of the silversword species with that of various California species, succeeded in producing vigorous hybrids between the Hawaiian plants and a California tarweed, demonstrating that they are closely related. Further genetic research has demonstrated that all the members of the silversword alliance are descended from a single ancestral species that colonized the Hawaiian islands. Since then, biologists have used the genetic differences within the alliance to derive an evolutionary phylogeny showing how the species are related. These data suggest that there was a single species from which all the current members of the alliance evolved and that this species lived on what is now the island of Kauai about five million years ago.