Biology Reference
In-Depth Information
In chapter 2, we looked briefly at the process of adaptive radiation as it occurred among
birds of paradise in New Guinea. Evolutionary biologists have also discovered in Hawaii
an extraordinary array of species created through adaptive radiation, such as honeycreepers,
ferns, tarweeds, and the two most prolific but least heralded radiations, that of the land snails
and the fruit flies. At least fifty-one species of Hawaiian honeycreepers, the main protagon-
ists in this chapter, evolved from a single species of primeval finch originating on the Amer-
ican mainland. Eventually some honeycreepers, such as the
'i'iwi
, evolved a long, decurved
bill to feed on nectar from species such as
Lobelia
that have appropriately curved flowers.
Other finches evolved a facility for eating fruit or for gleaning insects from bark. Still others
stayed with the ancestral finch behavior of cracking seeds. Some of these evolutionary vari-
ations never spread beyond a single island. Members of some bird species, though, managed
to make it to another island. The 'i'iwi, for example, populated several islands. Many of these
species are now quite rare, and at least twenty are extinct.
E. O. Wilson has shown how the arrival of a founding species—such as the prototype of
the honeycreeper—is typically the opening to an evolutionary play told in four acts. In act 1,
a species arrives and colonizes the entire archipelago, expanding in range and numbers. By
act 2, populations on the different islands have become distinct from the founding father and
mother, and on some islands the populations become extinct. By act 3, most of the original
birds to settle on various islands in the archipelago have gone extinct, and those that remain
have evolved into distinct species, as different as the 'akiapōlā'au is from the 'i'iwi. Finally,
in act 4, only a single population remains as an endemic species limited to a single island.
The legacy of a single, widely dispersing species colonizing many islands in an archipelago
is the generation of many new species with very narrow ranges.
This process of adaptive radiation is common on island archipelagoes because radiation
from a single ancestor and formation of new species require barriers to gene flow between
forming species, often provided by water gaps among islands. There probably would have
been no radiation of finches in either Hawaii or the Galápagos if there had been only one is-
land of a size equal to the sum of current islands. Such avian speciation did happen in New
Guinea, a single island, only because New Guinea is so large and the steep mountain chains
are sufficiently isolated from one another that they served as barriers to commingling, just as
the currents and deep water separating archipelagic islands often do.
When it comes to organisms with more limited mobility and range, it's a different story.
Even the main island of Hawaii is large enough to witness the explosive radiation of two di-
minutive and relatively weak dispersers, fruit flies and snails. Speciation in birds, and even in
land snails and fruit flies, requires geographic isolation of populations for long enough that
they accumulate sufficient genetic differences that they fail to interbreed if and when they
subsequently come into contact. One size does not fit all, however. What is a dispersal barrier
to a snail or a fruit fly is not a barrier to a bird.
