Silversword alliance

The silversword alliance refers to an adaptive radiation of around 34 species in the tarweed subtribe, Madiinae, of the sunflower family, Asteraceae. The group is endemic to Hawaii, and is derived from a single immigrant to the islands. The ancestor of the silversword alliance was a species of Pacific Coast tarweed, which differ significantly in appearance from members of the alliance. For radiating from a common ancestor at an estimated 3.5±1.5 Ma, the clade is extremely diverse, composed of trees, shrubs, subshrubs, mat-plants, cushion plants, rosette plants, and lianas. Consequently, the silversword alliance has been described as a model system for the study of plant evolution and adaptive radiations.

The silversword alliance is named for its most famous and visually striking members, the silverswords. The species of the clade belong to three genera: Wilkesia, Argyroxiphium, and Dubautia. There are five species, including two species known as greenswords, in the genus Argyroxiphium, confined to the islands of Maui and Hawaiʻi, and two species of Wilkesia (iliau) on Kauaʻi. The bulk of the species are placed in the genus Dubautia, which is widespread on all the main islands. Similar species frequently occur in the same habitat and are often difficult to tell apart. Hybrids frequently occur between Dubautia species, and between Dubautia and Argyroxiphium.

Many members of the silversword alliance are threatened or endangered, and are highly vulnerable to browsing by introduced feral animals.