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Argyroxiphium sandwicense
Thank you to newsflash66@UBC Botanical Garden Forums for sharing this image via the Botany Photo of the Day Submissions forum. It''s handy to have a species where I don''t have to dig to find an interesting story, since this is a member of a group of plants that represent a classic case-study in botany about adaptive radiation, hybridization and evolution.
The origin of this silversword species and approximately thirty to fifty other close relatives in Hawaii, collectively known as “the silversword alliance”, dates back to roughly five or six million years ago, when diaspores (an agent of dispersal, e.g., a seed) of an ancestral Californian tarweed species established on one of the Hawaiian islands (begging the question of how the diaspores arrived there, but that is a one for the biogeographers). Over the next five or six million years, the descendants of those few first-to-establish individual plants evolved into the thirty (fifty) present-day species with a diversity of forms and preferred environments. This rapid burst of evolution is termed adaptive radiation. The modern day species of the silversword alliance include members of the genera Argyroxiphium, Wilkesia and Dubautia, and these species occur in the habits of cushions, mat-forming perennials, shrubs, trees and climbing woody vines. Species in the alliance can be found in dry sites (less than 25cm of precipitation per year) to the purported “wettest spot on Earth”, where precipitation can exceed 1000cm in a year. Furthermore, they are found at almost all elevations in the Hawaiian islands, from 75m above to 3750m (see ecological and physical adaptations of the silversword alliance for more).
The question may have arisen in your mind as to how it is known that these plants evolved from an ancestral California tarweed. There are several ways to verify this. The strategy in most cases today would be to determine the relationships of the species by comparing the number of changes in certain molecules. This was done in this case (and it indeed bolstered these facts), but a more traditional method was also used. Artificial hybridization experiments between modern-day Californian tarweed species and members of the silversword alliance were undertaken, with the idea being that if the species could successfully cross, more evidence would be added to the pile (and again, yes, this is the case). More than incidentally, the hybridization experiments also help make inferences as to how this rapid evolution took place, suggesting mechanisms such as autopolyploidy and allopolyploidy (i.e., chromosome doubling within a cross of a single species or a cross between two species).
I''ve already referenced this site a few times, but here''s the main page: read Adaptive Radiation and Hybridization in the Hawaiian Silversword Alliance by Dr. Gerald Carr for more on the science behind the silversword alliance, including this page specifically on today''s species, Argyroxiphium sandwicense subsp. sandwicense, or Mauna Kea silversword. To read more about the genus, visit Wikipedia''s page on Argyroxiphium.