The silversword alliance, also known as the tarweeds, refers to an

adaptive radiation In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic int ...

of around 30 species in the composite or sunflower family,

Asteraceae The family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae ...

. The group is

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found els ...

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only ...

, and is derived from a single immigrant to the islands. For radiating from a common ancestor at an estimated 5.2±0.8 Ma, the clade is extremely diverse, composed of trees, shrubs, subshrubs, mat-plants, cushion plants, rosette plants, and lianas. The silversword alliance is named for its most famous and visually striking members, the silverswords. The species of the clade break down into three genera: '' Wilkesia,

Argyroxiphium ''Argyroxiphium'' is a small genus of plants in the family Asteraceae. Its members are known by the common names silversword or greensword due to their long, narrow leaves and the silvery hairs on some species. The silverswords belong to a larger ...

,'' and '' Dubautia.'' There are three species of silverswords and two greenswords in the genus ''Argyroxiphium'', confined to the islands of Maui and

, and two species of ''Wilkesia'' (''iliau'') on Kauai. The bulk of the species are placed in the genus ''Dubautia'', which is widespread on all the main islands. The genus ''Dubautia'' contains a wide variety of forms, including

cushion plant A cushion plant is a compact, low-growing, mat-forming plant that is found in alpine, subalpine, arctic, or subarctic environments around the world. The term "cushion" is usually applied to woody plants that grow as spreading mats, are limited ...

s, shrubs, trees, and

liana A liana is a long- stemmed, woody vine that is rooted in the soil at ground level and uses trees, as well as other means of vertical support, to climb up to the canopy in search of direct sunlight. The word ''liana'' does not refer to a ...

s. 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''. As a result, there is some disagreement over the number of species, with modern sources giving between 28 and 33 species.

Characteristics

All members of the silversword alliance are perennials, but otherwise occupy a wide range of ecological niches.

Evolution

All Hawaiian tarweeds trace their lineage back to a species of Pacific coast tarweed, very similar to extant species like '' Carlquistia muirii''. The last common ancestor of the silversword alliance was likely a mat and rhizome forming plant not more than tall, with a chromosome number of 2''n'' = 16, and perhaps another similar species. Species of ''Dubautia'' however have 2''n'' = 14 chromosomes. How the silverswords' chromosome number arose is a matter of some uncertainty, but two major scientific theories have been proposed. One is that two ancestor species, one with ''n'' = 6 and one with ''n'' = 8 chromosomes hybridized, resulting in a ''n ='' 7 hybrid. The hybrid then, by

allopolyploid Polyploidy is a condition in which the cells of an organism have more than one pair of (homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contains ...

y doubled its chromosome number spontaneously, leading to the resultant and extant 2''n'' = 14 species. Alternatively, the modern chromosome number could have arisen from an ancestor like '' Anisocarpus scabridus'', with a chromosome complement of ''n ='' 7, and then arisen by

autopolyploidy Polyploidy is a condition in which the cells of an organism have more than one pair of (homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contains ...

, instead of needing to first hybridize.

References

Endemic flora of Hawaii Natural history of Hawaii Madieae Plant common names Flora without expected TNC conservation status {{Hawaii-stub