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Darwin's finches (also known as the Galápagos
Finches) are 13 or 14 different but closely related
species of
finches
Charles Darwin collected on the Galápagos Islands during the
Voyage of the Beagle. 13 reside on the Galápagos Islands and
one on the Cocos Islands.
The birds are all about the same size (10–20 cm). The
most important differences between species are in the size
and shape of their beaks, and the beaks are highly adapted
to different food sources. The birds are all brownish or
black. Their behaviour differs, and they have different song
melodies.
Although these birds were to play an important part in
the inception of Darwin's theory of evolution by natural
selection, at the time of the survey voyage of HMS Beagle Darwin had no idea of their
significance. It is often assumed that when he saw the
finches on the islands this inspired the theory, but this is
not true: Darwin believed that they were not closely
related when he encountered them; indeed he thought that
most of these birds were not finches at all (Sulloway 1982).
Following his return from the voyage, Darwin presented
the finches to the Geological Society of London at their
meeting on 4 January 1837, along with other mammal and bird
specimens he had collected. The bird specimens, including
the finches, were given to John Gould, the famous English
ornithologist, for identification. Gould set aside his
paying work and at the next meeting on 10 January reported
that birds from the Galápagos Islands which Darwin had
thought were blackbirds, "gross-bills" and finches were in
fact "a series of ground Finches which are so peculiar" as
to form "an entirely new group, containing 12 species." This
story made the newspapers. In March Darwin met Gould again,
learning that his Galápagos "wren" was another species of
finch and the mockingbirds he had labelled by island were
separate species rather than just varieties, with relatives
on the South American mainland. Darwin had not bothered to
label his finches by island, but others on the expedition
had taken more care. He now sought specimens collected by
Captain Robert FitzRoy and crewmen. From them he was able to
establish that the species were uniquely related to
individual islands, giving him the idea that somehow in this
geographical isolation these different species could have
been formed from a small number of common ancestors so that
each was modified to suit "different ends".
The term Darwin's Finches was first applied in
1936, and popularized in 1947 by David Lack. Later, Peter
and Rosemary Grant conducted extensive research in
documenting evolutionary change among the finches. Beginning
in 1973, the pair spent many years tracking thousands of
individual finches across several generations, showing how
individual species changed in response to environmental
changes. The Beak of the Finch by Jonathan Weiner is a book about the finches,
highlighting the Grants' research.
The finch species
Genus Geospiza
Large Cactus-Finch (Geospiza conirostris)
Sharp-beaked Ground-Finch (Geospiza difficilis)
Medium Ground-Finch (Geospiza fortis)
Small Ground-Finch (Geospiza fuliginosa)
Large Ground-Finch (Geospiza magnirostris)
Common Cactus-Finch (Geospiza scandens)
Genus Camarhynchus
Vegetarian Finch (Camarhynchus crassirostris syn.
Platyspiza crassirostris)
Large Tree-Finch (Camarhynchus psittacula)
Medium Tree-Finch (Camarhynchus pauper)
Small Tree-Finch (Camarhynchus parvulus)
Woodpecker Finch (Camarhynchus pallidus)
Mangrove Finch (Camarhynchus heliobates)
Genus Certhidea
Warbler Finch (Certhidea olivacea)
Genus Pinaroloxias
Cocos Island Finch (Pinaroloxias inornata)
Text from the Voyage of the
Beagle
The passage in chapter 17 in
The Voyage of the Beagle in which Darwin describes
the finches and surmises that they may have shared a common
ancestor is shown below. This was written in the months
after Gould had revealed that the birds which Darwin had
thought to be unrelated were different species of finches.
The remaining land-birds form a most singular
group of finches, related to each other in the structure
of their beaks, short tails, form of body and plumage:
there are thirteen species, which Mr. Gould has divided
into four subgroups. All these species are peculiar to
this archipelago; and so is the whole group, with the
exception of one species of the sub-group Cactornis,
lately brought from Bow Island, in the Low Archipelago.
Of Cactornis, the two species may be often seen climbing
about the flowers of the great cactus- trees; but all
the other species of this group of finches, mingled
together in flocks, feed on the dry and sterile ground
of the lower districts. The males of all, or certainly
of the greater number, are jet black; and the females
(with perhaps one or two exceptions) are brown. The most
curious fact is the perfect gradation in the size of the
beaks in the different species of Geospiza, from one as
large as that of a hawfinch to that of a chaffinch, and
(if Mr. Gould is right in including his sub-group,
Certhidea, in the main group) even to that of a warbler.
The largest beak in the genus Geospiza is shown in Fig.
1, and the smallest in Fig. 3; but instead of there
being only one intermediate species, with a beak of the
size shown in Fig. 2, there are no less than six species
with insensibly graduated beaks. The beak of the
sub-group Certhidea, is shown in Fig. 4. The beak of
Cactornis is somewhat like that of a starling, and that
of the fourth subgroup, Camarhynchus, is slightly
parrot-shaped. 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. In a like
manner it might be fancied that a bird originally a
buzzard, had been induced here to undertake the office
of the carrion-feeding Polybori of the American
continent.
"Mr. Gould" (above) refers to
John Gould, the famous English ornithologist.
Reference
Adrian Desmond and James Moore, Darwin
(London: Michael Joseph, the Penguin Group,
1991). ISBN 0-7181-3430-3