Darwin's finches (also known as the Galápagos finches) are a group of about fifteen species
of passerine birds.
They are well known for their remarkable diversity in beak form and function.
They are often classified as the subfamily Geospizinae or tribe Geospizini.
They belong to the tanager family and are not closely related to the true finches.
The closest known relative of the Galápagos finches is South-American Tiaris obscurus.
They were first collected by Charles Darwin on the Galápagos Islands during the second
voyage of the Beagle.
Apart from the Cocos finch, which is from Cocos Island, the others are found only on
the Galápagos Islands.
The term "Darwin's finches" was first applied by Percy Lowe in 1936, and popularised in
1947 by David Lack in his book Darwin's Finches.
Lack based his analysis on the large collection of museum specimens collected by the 1905–06
Galápagos expedition of the California Academy of Sciences, to whom Lack dedicated his 1947
book.
The birds vary in size from 10 to 20 cm and weigh between 8 and 38 grams.
The smallest are the warbler-finches and the largest is the vegetarian finch.
The most important differences between species are in the size and shape of their beaks,
which are highly adapted to different food sources.
The birds are all dull-coloured.
A long term study carried out for more than 40 years by the Princeton University researchers
Peter and Rosemary Grant has documented evolutionary changes in beak size affected by El Niño/La
Niña cycles in the Pacific.
== Darwin's theory == During the survey voyage of HMS Beagle, Darwin
was unaware of the significance of the birds of the Galápagos.
He had learned how to preserve bird specimens while at the University of Edinburgh and had
been keen on shooting, but he had no expertise in ornithology and by this stage of the voyage
concentrated mainly on geology.
In Galápagos he mostly left bird shooting to his servant Syms Covington.
Nonetheless, these birds were to play an important part in the inception of Darwin's theory of
evolution by natural selection.
On the Galápagos Islands and afterward, Darwin thought in terms of "centres of creation"
and rejected ideas concerning the transmutation of species.
From Henslow's teaching, he was interested in the geographical distribution of species,
particularly links between species on oceanic islands and on nearby continents.
On Chatham Island, he recorded that a mockingbird was similar to those he had seen in Chile,
and after finding a different one on Charles Island he carefully noted where mockingbirds
had been caught.
In contrast, he paid little attention to the finches.
When examining his specimens on the way to Tahiti, Darwin noted that all of the mockingbirds
on Charles Island were of one species, those from Albemarle of another, and those from
James and Chatham Islands of a third.
As they sailed home about nine months later, this, together with other facts, including
what he had heard about Galápagos tortoises, made him wonder about the stability of species.Following
his return from the voyage, Darwin presented the finches to the Zoological Society of London
on 4 January 1837, along with other mammal and bird specimens that 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
the birds from the Galápagos Islands that Darwin had thought were blackbirds, "gross-beaks"
and finches were actually "a series of ground Finches which are so peculiar [as to form]
an entirely new group, containing 12 species".
This story made the newspapers.
Darwin had been in Cambridge at that time.
In early March, he met Gould again and for the first time to get a full report on the
findings, including the point that his Galápagos "wren" was another closely allied species
of finch.
The mockingbirds that Darwin had labelled by island were separate species rather than
just varieties.
Gould found more species than Darwin had expected, and concluded that 25 of the 26 land birds
were new and distinct forms, found nowhere else in the world but closely allied to those
found on the South American continent.
Darwin now saw that, if the finch species were confined to individual islands, like
the mockingbirds, this would help to account for the number of species on the islands,
and he sought information from others on the expedition.
Specimens had also been collected by Captain Robert FitzRoy, FitzRoy’s steward Harry
Fuller, and Darwin's servant Covington, who had labelled them by island.
From these, Darwin tried to reconstruct the locations from where he had collected his
own specimens.
The conclusions supported his idea of the transmutation of species.
=== Text from The Voyage of the Beagle === At the time that he rewrote his diary for
publication as Journal and Remarks (later The Voyage of the Beagle), he described Gould's
findings on the number of birds, noting that "Although the species are thus peculiar to
the archipelago, yet nearly all in their general structure, habits, colour of feathers, and
even tone of voice, are strictly American".
In the first edition of The Voyage of the Beagle, Darwin said thatIt is very remarkable
that a nearly perfect gradation of structure in this one group can be traced in the form
of the beak, from one exceeding in dimensions that of the largest gros-beak, to another
differing but little from that of a warbler".
By the time the first edition was published, the development of Darwin's theory of natural
selection was in progress.
For the 1845 second edition of The Voyage (now titled Journal of Researches), Darwin
added more detail about the beaks of the birds, and two closing sentences which reflected
his changed ideas: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."
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.
=== Text from On the Origin of Species === Darwin discussed the divergence of species
of birds in the Galápagos more explicitly in his chapter on geographical distribution
in On the Origin of Species:
The most striking and important fact for us in regard to the inhabitants of islands, is
their affinity to those of the nearest mainland, without being actually the same species.
[In] the Galapagos Archipelago ... almost every product of the land and water bears
the unmistakable stamp of the American continent.
There are twenty-six land birds, and twenty-five of these are ranked by Mr. Gould as distinct
species, supposed to have been created here; yet the close affinity of most of these birds
to American species in every character, in their habits, gestures, and tones of voice,
was manifest.
... The naturalist, looking at the inhabitants of these volcanic islands in the Pacific,
distant several hundred miles from the continent, yet feels that he is standing on American
land.
Why should this be so?
Why should the species which are supposed to have been created in the Galapagos Archipelago,
and nowhere else, bear so plain a stamp of affinity to those created in America?
There is nothing in the conditions of life, in the geological nature of the islands, in
their height or climate, or in the proportions in which the several classes are associated
together, which resembles closely the conditions of the South American coast: In fact there
is a considerable dissimilarity in all these respects.
On the other hand, there is a considerable degree of resemblance in the volcanic nature
of the soil, in climate, height, and size of the islands, between the Galapagos and
Cape de Verde Archipelagos: But what an entire and absolute difference in their inhabitants!
The inhabitants of the Cape de Verde Islands are related to those of Africa, like those
of the Galapagos to America.
I believe this grand fact can receive no sort of explanation on the ordinary view of independent
creation; whereas on the view here maintained, it is obvious that the Galapagos Islands would
be likely to receive colonists, whether by occasional means of transport or by formerly
continuous land, from America; and the Cape de Verde Islands from Africa; and that such
colonists would be liable to modification — the principle of inheritance still betraying
their original birthplace.
== Polymorphism in Darwin's finches == Whereas Darwin spent just five weeks in the
Galápagos, and David Lack spent three months, Peter and Rosemary Grant and their colleagues
have made research trips to the Galápagos for about thirty years, particularly studying
Darwin's finches.
Males are dimorphic in song type: songs A and B are quite distinct.
Also, males with song A have shorter bills than B males.
This is also a clear difference.
With these beaks, males are able to feed differently on their favourite cactus, the prickly pear
Opuntia.
Those with long beaks are able to punch holes in the cactus fruit and eat the fleshy aril
pulp, which surrounds the seeds, whereas those with shorter beaks tear apart the cactus base
and eat the pulp and any insect larvae and pupae (both groups eat flowers and buds).
This dimorphism clearly maximises their feeding opportunities during the non-breeding season
when food is scarce.
If the population is panmixic, then Geospiza conirostris exhibits a balanced genetic polymorphism
and not, as originally supposed, a case of nascent sympatric speciation.
The selection maintaining the polymorphism maximises the species' niche by expanding
its feeding opportunity.
The genetics of this situation cannot be clarified in the absence of a detailed breeding program,
but two loci with linkage disequilibrium is a possibility.
Another interesting dimorphism is for the bills of young finches, which are either 'pink'
or 'yellow'.
All species of Darwin's finches exhibit this morphism, which lasts for two months.
No interpretation of this phenomenon is known.
== Taxonomy ==
=== Family ===
For some decades, taxonomists have placed these birds in the family Emberizidae along
with the New World sparrows and Old World buntings (Sulloway 1982).
However, the Sibley–Ahlquist taxonomy puts Darwin's finches with the tanagers (Monroe
and Sibley 1993), and at least one recent work follows that example (Burns and Skutch
2003).
The American Ornithologists' Union, in its North American check-list, places the Cocos
finch in the Emberizidae but with an asterisk indicating that the placement is probably
wrong (AOU 1998–2006); in its tentative South American check-list, the Galápagos
species are incertae sedis, of uncertain place (Remsen et al. 2007).
=== Species === Genus Geospiza
Española cactus finch (Geospiza conirostris) Sharp-beaked ground finch (Geospiza difficilis)
Vampire finch (Geospiza difficilis septentrionalis) Medium ground finch (Geospiza fortis)
Small ground finch (Geospiza fuliginosa) Large ground finch (Geospiza magnirostris)
Common cactus finch (Geospiza scandens) Genus Camarhynchus
Large tree finch (Camarhynchus psittacula) Medium tree finch (Camarhynchus pauper)
Small tree finch (Camarhynchus parvulus) Woodpecker finch (Camarhynchus pallidus) – sometimes
separated in Cactospiza Mangrove finch (Camarhynchus heliobates)
Genus Certhidea Green warbler-finch (Certhidea olivacea)
Grey warbler-finch (Certhidea fusca) Genus Pinaroloxias
Cocos finch (Pinaroloxias inornata) Genus Platyspiza
Vegetarian finch (Platyspiza crassirostris)In 1981, a male Española cactus finch arrived
at Daphne Major island.
Its mating with local Galapagos finches has produced a new population that can exploit
previously unexploited food due to its larger size.
They do not breed with the other species on the island, as the females do not recognize
the songs of the new males.
Genetic evidence shows that now, after two generations, it lives in a complete reproductive
isolation from the native species.
According to professor Leif Andersson of Uppsala University, a taxonomist not aware of its
history would consider it a distinct species.
== Molecular basis of beak evolution == Developmental research in 2004 found that
bone morphogenetic protein 4 (BMP4), and its differential expression during development,
resulted in variation of beak size and shape among finches.
BMP4 acts in the developing embryo to lay down skeletal features, including the beak.
The same group showed that the development of the different beak shapes in Darwin's finches
are also influenced by slightly different timing and spatial expressions of a gene called
calmodulin (CaM).
Calmodulin acts in a similar way to BMP4, affecting some of the features of beak growth.
The authors suggest that changes in the temporal and spatial expression of these two factors
are possible developmental controls of beak morphology.
In a recent study genome sequencing revealed a 240 kilobase haplotype encompassing the
ALX1 gene that encodes a transcription factor affecting craniofacial development is strongly
associated with beak shape diversity.
Moreover, these changes in the beak size have also altered vocalization in Darwin's finches.
== See also == Species flock
Adaptive radiation
== Notes