Darwin's finches essay.docx - Wikispaces by liuqingyan


									BS1013 ESSAY:

                Adaptive radiation of Darwin’s Finches

«Both in space and time, we seem to be brought somewhat near to that great
fact, that mystery of mysteries, the first appearance of new beings on this
earth». (Campbell, Reece, Biology 8th ed. p.487) The changes of evolution take place
in all parts of biology of animals and any change must result in a better
adaptation of the living animal. Darwin’s finches consist one of the stronger
example of that successful adaptation to the environment.

Picture 1: Galapagos Islands


They were first collected by Charles Darwin during his voyage on the Beagle
ship, in a group of volcanic islands, Galapagos Islands. While these islands are
located west of south America, they had been colonized by organisms which
they gave rise to new species. Some of the finches are unique to individual
islands whereas some others live on more neighboring islands. Thirteen of
these species reside in Galapagos Islands and just a single one on Cocos Island.
However, finches are highly adapted to the specific environment. As
numerous of their habits and characters have evolved by the descent with
modification in order to achieve a better matching with their home islands.
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Like all birds, finches are vertebrate animals and were evolved by parallel
evolution i.e. they have evolved independently, having no common ancestor
later than an early reptilian stage. Furthermore, the brownish or black body
with size between 10 – 20 cm, the short tail and short rounded wings are
some general features that relate the species with each other. But a finch’s
most obvious adaptation is its beak which ranges from delicate and thin to
stout and large. «Only the variety of their beaks and the number of their
species excite attention-small finch-like beaks, huge finch-like beaks, parrot-
like beaks, straight wood-boring beaks, decurved flower-probing beaks,
slender warbler-like beaks, are species which look very different and species
which look closely similar»(D.Lack, 1936,Darwin’s finches,p.11). This is due to the
different food sources and more usually characterize a different genera;
Geospiza, Camarhynchus, Certhidea, Pinaroloxias.

Picture 2: Darwin’s Finches are divided according their food source.


The heavy finch-like beaks characterize large, medium and small ground-
finches of Geospiza magnirostis, G.fortis and G.fuliginosa (species of subgenus
Geospiza).These species habit in coastal zone and feed mainly by seeds
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especially during the breeding season, but also eat flowers, buds, fruits, young
leaves and large caterpillars. Sometimes spiders and small insects may be part
of their diet. On the other hand, the ground-finch G.difficilis lives in hunid
zone and has a sharp beak. Geospiza scandens and Geospiza conirostris
species have long decurved beak and chiefly feed on opuntia (cactus feeding).

The three species of Camarhynchus genera; C.psittacula, C.pauper, C.parvulus
have thick, short and somewhat decurved beak for a diet consist of insects.
While the birds are searching for their meals in trees, in agile movements,
«they examine the twigs, bark and leaf clusters, and also excavate shallow
holes in soft wood» (D.Lack, 1936, Darwin’s finches, p.58). Finches may feed on
ground and include nectar, buds, young leaves and large caterpillars (and
grain) in their food source. C.pallidus and C.heliobates prefer to dig insects out
of wood, by the using of twigs, thus they have stout and straight beak. The
vegetarian one (finch), C.crassirostris feed primarily on leaves, buds and fruits.

As for Certhidea olivacea species, their diet consists of small insects which can
be found in leaves, twigs, as well as, in ground or air. This type of finch has a
slender beak and generally has similar habits and appearance with a warbler.

In contrast with other subgenus, the genus Pinaroloxias refers to the Cocos
finche. This finch is feed mainly with insects both from ground and trees and
its beak looks like Certhidea , except it is a little longer and somewhat
decurved. The main characteristic of the finch is the unusually long, grooved
and bifid tongue.

It must be mentioned that according to Dr.Cliff Tabin biologist «the bone
morphogenetic protein 4 (BMP4) and its differential expression during
development is responsible for the variation in beaks' size and shape among
finches» (http://en.wikipedia.org/wiki/Darwin%27s_finches). In addition, the
modification of the beak affects the singing of finches. Males use singing as an
attractive agent of the opposite sex especially during the breeding season.

The speciation of Darwin’s finches was contribute by main three factors; the
ecological opportunity, the geological isolation and evolutionary change. The
ecological opportunity refers to the absence of predators which accelerated
the adaptive radiation of finches, and permit evolution in certain directions
which would otherwise be impossible. Apart from the complete freedom from
enemies, for a long time these areas were without food competitors. But
«event at the present time there are few other passerine birds in the
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Galapagos, and these are predominantly insectivorous species» (D.Lack, 1936,
Darwin’s finches, p.118), without any access on other finch diets. Although the
speciation has become more successful by the variety of islands, which has
permitted the migration of finches between them, the great distances have
been a disadvantage for interbreeding. However, geological isolation is
illuminated by the single species of Darwin’s niches in Cocos Island.
Furthermore, the evolutionary change can be occurred through the
combination of natural selection, genetic drift (chance events that change
allele frequencies) and founder effect i.e. «loss of genetic variation that occurs
when a new population is established by a very small number of individuals
from a larger population» (Campbell, Reece, Biology 8th ed. p.468).

 To conclude, the archipelago is a little world within itself. Charles Darwin
pointed out that by «seeing this gradation and diversity of structure in one
small, intimately related group of birds, one might really fancy that from an
original paucity of birds in this archipelago, one species had been taken and
modified for different ends» (D.Lack, 1936, Darwin’s finches, p.10). Finches are
undoubtedly adaptive by the action of natural selection in order to be
favorable spread through the population.


                           Kingdom: Animalia
                            Phylum: Chordata
                                Class: Aves
                          Order: Passeriformes
                       Family: EmberizidaeGeospiza



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Essay material:

    D.Lack, 1936, Darwin’s finches
    Campbell, Reece, Biology 8th ed.
    http://en.wikipedia.org/wiki/Darwin%27s_finches
    http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciat


    Pic1: http://www.iciclesoftware.com/galapagos/gifs/mapgalap2.gif
    Pic2: http://www.math.ucla.edu/~tao/finch.jpg

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