 Adaptive radiation is the
development of many species derived from a single ancestral population.
The Hawaiian silversword `ohana (family) is probably the foremost
example of adaptive radiation among plants in the world. Over the
course of millions of years, the descendants of the pioneer plant
evolved into 28 distinct species in three genera, occupying many
different habitats. |
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Scientists
believe that the entire silversword family probably descended
from a member of the sunflower family, similar to Muir's Tarweed,
from California. The barbed fruit of this tarweed may have been
carried to Hawai`i on the feathers of a bird. Since this tarweed
came from an alpine shrubland, it most likely became established
in Hawai`i in a similar kind of habitat. |
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Since
so few organisms successfully colonized Hawai`i, many diverse
habitats were available. Over time, descendants of the tarweed
slowly adapted to many of these habitats. Beneficial mutations
enabled the plant forms to change and become quite different
from the ancestor. The end result was extensive and spectacular
adaptive radiation.
Today,
plants of the silversword family occupy every terrestrial habitat
in Hawai`i--from wet forests to dry forests and from near sea
level to alpine shrublands. Although these plants are still closely
related (all species are able to hybridize, they often look extremely
different from one another.
The silversword family tree is
divided into three genera: Argyroxiphium (5 species), Wilkesia
(2 species), and Dubautia (21 species). Dubautia species are
the most diverse. All members of the silversword family are endemic
to Hawai`i; 82% are to a single island. Approximately one half
of these single island endemic species are further limited in
distribution, often growing only in one area or microclimate.
It is a very special `ohana! |
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The size and the
form of a plant are adaptations to the habitat in which it grows.
For example, large trees, shrubs and vines occur in wet areas;
small shrubs, mats  and
rosettes tend to occur in drier areas. Rainfall, sunlight, temperature,
and elevation all affect the size and form of a plant. Plants
growing in bogs are unable to use the stagnant, acidic water,
so their roots are shallow. These plants display similar adaptations
to plants growing in very dry areas. See the student
data sheet for a summary of plant adaptations to climate
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