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Darwin's Finches

Darwin's finches (also known as the Galápagos Finches or as Geospizinae) are 13 or 14 separate combinatory species of Passerine birds (related to American Emberizidae or Tanagers rather than European finches) related to a group that Charles Darwin collected on the Galápagos Islands during the voyage of the Beagle. Thirteen reside on the Galápagos Islands and one on Cocos Island.

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.

Contents
1 Darwin's theory
2 Taxonomy
2.1 Family
2.2 Species
3 Text from the Voyage of the Beagle
4 Molecular basis of beak evolution
5 Notes
6 References
7 External links

Darwin's theory
Although these birds were to play an important part in the inception of Darwin's theory of evolution by natural selection, during the survey voyage of HMS Beagle Darwin had no idea of their significance. He had learnt 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 and mostly left bird shooting to his servant Syms Covington.[1]

On the Galápagos Islands and afterwards, Darwin thought in terms of "centres of creation" and rejected ideas of transmutation of species.[2] From Henslow's teaching he was interested in 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, but paid little attention to the finches. When writing up his notes on the way to Tahiti Darwin was astonished to find that all the mockingbirds caught on Charles Island were of one species, those from Albemarle of another, and those from James and Chatham Islands of a third species. As they sailed home about nine months later this, together with other facts including what he'd heard about Galápagos tortoises, made him wonder about the stability of species.[3][4]

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-beaks" 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.[5][6]

Darwin had been in Cambridge at that time. In early March he met Gould again and for the first time got a full report on the findings, including the point that his Galápagos "wren" was another closely allied species of finch. The mockingbirds Darwin had labelled by island were separate species rather than just varieties. Gould found more species than Darwin had anticipated, 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.[6] 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.[7] From these, Darwin tried to reconstruct the locations where he had collected his own specimens. The conclusions led shortly afterwards to his conversion to the idea of transmutation of species.[6]

At that time he was rewriting his diary for publication as Journal and Remarks (later republished as The Voyage of the Beagle) and 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 particular, "A group of finches, of which Mr. Gould considers there are thirteen species; and these he has distributed into four new sub-genera. These birds are the most singular of any in the archipelago.... All the species, excepting two, feed in flocks on the ground, and have very similar habits. It 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."[8] In 1839 Darwin conceived of his theory of natural selection, and he added more detail to the second edition, published as Journal of Researches in 1845, with an illustration showing the beaks of four finches and the closing remark that "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."[9]

He discussed this 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 unmistakeable 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."[10]

The term Darwin's Finches was first applied by Percy Lowe in 1936, and popularised in 1961 by David Lack with the second edition of his book Darwin's Finches.[11] Others in modern life term them Grant's Finches, for their research on the finches distribution and variation.
 

Taxonomy
Family

For some decades taxonomists have placed these birds in the family Emberizidae 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 Island 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
Large 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
Darwin's Large Ground-finch, Geospiza magnirostris magnirostris - possibly extinct (1957?)
Common Cactus-finch, Geospiza scandens
Genus Camarhynchus
Vegetarian Finch, Camarhynchus crassirostris - sometimes separated in Platyspiza
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
Warbler Finch, Certhidea olivacea
Genus Pinaroloxias
Cocos Island Finch, Pinaroloxias inornata

 Text from the Voyage of the Beagle
In the first edition of The Voyage of the Beagle Darwin simply described the finches without mentioning his thoughts at that time, merely stating that "It 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."[12] The book was written in the months after John Gould had revealed that the birds which Darwin had thought to be unrelated were different species of finches.[13] The proofs were finished later that year, but the book did not go into print until 1839. By the time of the second edition in 1845 Darwin had brought together his theory, and he cautiously added two closing sentences hinting at his ideas.[14]

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.[15]

Molecular basis of beak evolution
In 2004 a group led by Harvard biologist Dr.Cliff Tabin proposed that the bone morphogenetic protein 4 (BMP4) and its differential expression during development was responsible for the variation in beaks' size and shape among finches. BMP4 acts in the developing embryo to lay down skeletal features (including the beak). They supported their in situ hybridization data by functional experiments in the model organism Gallus gallus, getting the hypothesized transformations. The work was published in Science.[16] As Published in Nature in August 2006, the same group showed that the different beak shapes of Darwin's finches develop also due to slightly different timing and spatial expression of a gene called calmodulin (CaM).[17] Calmodulin acts in a similar way to BMP4, affecting some of the features of beak growth. Using microarray data and early embryo staining from each of the species, this group could show how the different beak shapes were obtained. Additionally, they got similar but partial transformations of the developing beak by providing elevated calmodulin in chick (G.gallus) embryos. The authors propose changes in the temporal and spatial expression of these two factors as possible ways of obtaining the beak diversity observed but do not explain how this may have happened.

Additionally, boldinudin occurs in part of the beaks to stimulate action of the CMP1, which is similar to HOX4 in homeotic development, and causes sustained expression and epigenetic methylation; including histone modification; due to the party-theory of population genetics. Furthermore, double-helical formations in the CMP1 is influenced by a methyl regulator called tertaforin-melanin-cumusforus. Microarray data combined with RFLP agrees that beak evolution occurs due to this homeotic methylation change. A 1999 study was particularly influenced by the information on CAM2, which differs from both HOX4 and BMP4.
 

Notes
1^ Steinheimer 2004, pp. 301–303
2^ Keynes 2000, p. xix.
   Eldredge 2006
3^ Gordon Chancellor; Randal Keynes (October 2006). "Darwin's field notes on the Galapagos: 'A little world within itself'". Darwin Online.
4^ Eldredge 2006
5^ Desmond & Moore 1991, p. 209
6^ a b c Sulloway 1982, pp. 57–58
7^ Sulloway 2006
8^ Darwin 1839, pp. 461–462
9^ Darwin 1845, pp. 379–380
10^ Darwin 1859, pp. 397–398.
11^ Steinheimer 2004, p. 300
Lack 1940
12^ Darwin 1839, p. 462.
13^ Desmond & Moore 1991, p. 248
14^ Darwin 1887
15^ Darwin 1845, p. 380.
16^ Abzhanov, Arhat; Meredith Protas, B. Rosemary Grant, Peter R. Grant, Clifford J. Tabin (September 3, 2004). "Bmp4 and Morphological Variation of Beaks in Darwin's Finches". Science (USA: AAAS) 305 (5689): 1462–1465. doi:10.1126/science.1098095. ISSN 0036-8075. OCLC 1644869. PMID 15353802. http://www.sciencemag.org/cgi/content/abstract/sci;305/5689/1462. Retrieved on 8 March 2008.
17^ Abzhanov, Arhat; Winston P. Kuo, Christine Hartmann, B. Rosemary Grant, Peter R. Grant and Clifford J. Tabin (August 3, 2006). "The calmodulin pathway and evolution of elongated beak morphology in Darwin's finches". Nature (UK: Nature Publishing Group) 442: 563–567. doi:10.1038/nature04843. ISSN 0028-0836. OCLC 1586310. PMID 16885984. http://www.nature.com/nature/journal/v442/n7102/abs/nature04843.html. Retrieved on 8 March 2008.

References
Darwin, Charles (1839), Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle between the years 1826 and 1836, describing their examination of the southern shores of South America, and the Beagle's circumnavigation of the globe. Journal and remarks. 1832-1836, III, London: Henry Colburn .
Darwin, Charles (1845), Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. 2d edition, London: John Murray .
Darwin, Charles (1859), On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1st ed.), London: John Murray, http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=1
Darwin, Francis (1887), "Chapter 1, The Foundations of the 'Origin of Species'", in Darwin, Francis, The life and letters of Charles Darwin, including an autobiographical chapter, 2, London: John Murray
Desmond, Adrian; Moore, James (1991), Darwin, London: Michael Joseph, Penguin Group, ISBN 0-7181-3430-3, OCLC 185764721
Eldredge, Niles (2006), "Confessions of a Darwinist", The Virginia Quarterly Review (Spring 2006): 32–53, http://www.vqronline.org/articles/2006/spring/eldredge-confessions-darwinist/, retrieved on 4 November 2008
Lack, David (07 September 1940), "Evolution of the Galapagos Finches", Nature (146): 324–327, doi:10.1038/146324a0, http://www.nature.com/nature/journal/v146/n3697/abs/146324a0.html, retrieved on 9 December 2008
"Check-list of North American Birds". American Ornithologists' Union (1998–2006). Retrieved on 2007-04-09.
Kevin J. Burns and Alexander F. Skutch (2003). "Tanagers and Tanager-Finches". in Christopher Perrins, ed.. The Firefly Encyclopedia of Birds. Firefly Books. pp. 629–631. ISBN 1-55297-777-3. http://www.amazon.com/Firefly-Encyclopedia-Birds-Christopher-Perrins/dp/1552977773/ref=si3_rdr_bb_product/103-5704731-0011011. Retrieved on 2007-04-09.  It is not clear whether this placement was made by Burns and Skutch or by Perrins.
Keynes, Richard (2000), Charles Darwin’s zoology notes & specimen lists from H.M.S. Beagle., Cambridge University Press, http://darwin-online.org.uk/content/frameset?itemID=F1840&viewtype=text&pageseq=1, retrieved on 8 December 2008
Burt L. Monroe and Charles G. Sibley, A World Checklist of Birds. New Haven, Conn.: Yale University Press (1993). ISBN 0-300-07083-7. Accessed 2007-04-09. Monroe and Sibley consider the tanagers to be a tribe (Thraupini) of a big family Fringillidae rather than a family of their own (Thraupidae).
J. V. Remsen, Jr., C. D. Cadena, A. Jaramillo, M. Nores, J. F. Pacheco, M. B. Robbins, T. S. Schulenberg, F. G. Stiles, D. F. Stotz, and K. J. Zimmer. [Version 2007-04-05.] A classification of the bird species of South America. American Ornithologists' Union. Accessed 2007-04-09.
Steinheimer, F. D. (2004), "Charles Darwin's bird collection and ornithological knowledge during the voyage of H.M.S. Beagle, 1831-1836", Journal of Ornithology (145(4)): 300–320, doi:10.1007/s10336-004-0043-8, http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=A161&pageseq=1, retrieved on 8 December 2008
Sulloway, Frank J. (1982), "The Beagle collections of Darwin’s finches (Geospizinae)", Bulletin of the British Museum (Natural History), Historical Series (43, No. 2): 49–94, http://darwin-online.org.uk/content/frameset?itemID=A86&viewtype=image&pageseq=1, retrieved on 8 December 2008
Sulloway, Frank J. (Spring 1982), "Darwin and His Finches: The Evolution of a Legend", Journal of the History of Biology 15 (1): 1–53, http://www.sulloway.org/Finches.pdf, retrieved on 9 December 2008
Sulloway, Frank J. (2006), "Why Darwin Rejected Intelligent Design", in Brockman, John, Intelligent Thought: Science versus the Intelligent Design Movement, New York: Vintage, pp. 107–126

External links
 Wikimedia Commons has media related to: Geospizinae
Sulloway, F.J. (1982): "Darwin and his finches: the evolution of a legend". J. Hist. Biol. 15: p.1–53
Different bills and song melodies
Genetics and the Origin of Birds Species, Grant and Grant in PNAS
Sato et al Phylogeny of Darwin's finches as revealed by mtDNA sequences in PNAS
Sato A, Tichy H, O'hUigin C, Grant PR, Grant BR, Klein J (March 2001). "On the origin of Darwin's finches". Mol. Biol. Evol. 18 (3): 299–311. PMID 11230531. http://mbe.oxfordjournals.org/cgi/content/full/18/3/299.
Galápagos Online. Darwin's Finches.
Galapagos Online. List of birds of the Galapagos Islands.
Darwin's Finches Evolve Before Scientists' Eyes: new developments reported 13 July 2006
Fink F.A.Q. Darwin's finches inspired the naming of the Fink project, a collaborative initiative for porting open source software to the Darwin platform to enable its use and evolution in the Apple Mac OS X environment. "Fink" is the German name for "finch."
Aug 2006 Nature Article that shows how modulation of a certain gene during development can account for the differences seen in beak shape.
Speciation Kimball's Biology Pages
Retrieved from "http://en.wikipedia.org/wiki/Darwin%27s_finches"
Categories: Charles Darwin | History of evolutionary biology | Evolution by taxon | Geospizini | Tribes of birds | Endemic birds of the Galápagos Islands
Hidden categories: All articles with unsourced statements | Articles with unsourced statements since December 2008



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How Darwin's finches got their beaks
A gene's-eye view of evolution
By William J. Cromie

Harvard News Office

Darwin's finches are the emblems of evolution. The birds he saw on the Galapagos Islands during his famous voyage around the world in 1831-1836 changed his thinking about the origin of new species and, eventually, that of the world's biologists.

Darwin wondered about the changes in shape of bird beaks from island to island. So-called cactus finches boast longer, more pointed beaks than their relatives the ground finches. Beaks of warbler finches are thinner and more pointed than both. These adaptations make them more fit to survive on available food.

Researchers at Harvard Medical School have taken the story one step further. Using modern genetic analyses, they found a molecule that regulates genes involved in shaping the beaks of Darwin finches. "Calmodulin is a protein that binds and activates certain enzymes, which triggers a signal that eventually turns specific genes on or off," explains Arkhat Abzhanov, an evolutionary biologist at Harvard. These signals alter the behavior of cells responsible for beak sculpturing.

Members of the research team received permission to collect finch eggs from the Galapagos National Park, a group of rocky islands in the Pacific Ocean, about 600 miles west of Ecuador. Female finches lay clutches of four to five eggs, one per day. To avoid disruption and abandonment of the nests, the researchers took only the third eggs laid.

In the Department of Genetics at Harvard Medical School, 26 bird embryos were examined, using gene chips that reveal which genes are most active in the heads of the developing finches. This activity was then matched with the size and shapes of adult beaks.

The investigation soon focused on calmodulin as the switch that can turn on genes involved in increasing beak length. This protein had never before been implicated in the development of the skulls and faces of any birds.

"We found that calmodulin was indeed expressed at detectably higher levels in cactus finches compared to ground finches, and thus associated with their longer beaks," says Clifford Tabin, professor of genetics. "This higher level is both biologically relevant and functionally important for shaping of elongated beaks, which are used in a specialized manner to probe cactus flowers and fruit for pollen, nectar, and seeds." The same surge of calmodulin was not found in more blunt-beaked ground finches.

A beak at evolution
When Charles Darwin first saw the Galapagos Islands he described them as 10 islands "situated under the equator." He noted that they originated as volcanoes and were pockmarked with craters. "Some of the craters, surmounting the larger islands, are of immense size, and they rise to a height of between three and four thousand feet."

Noting differences in the feeding habits of the finches, Darwin wrote that cactus finches "may often be seen climbing about the flowers of the great cactus trees." Seeing the diversity of beaks and other structures in the closely related finches, he wrote in his notebook, "one might really fancy that one species had been taken and modified for different ends."

Darwin elaborated on this idea when he published his intellectual bombshell, the "Origin of Species," some 25 years later in 1859. He speculated that birds, resembling starlings, came to the Galapagos Islands by wind. Evolution took over and different groups developed different diets. When, he wrote, "an immigrant first settled on one of the islands, ... it would undoubtedly be exposed to different conditions in the different islands (where) it would have to compete with a different set of organisms. ... Then, natural selection would probably favor different varieties in the different islands."

In other words, beaks changed as the birds developed different tastes for fruits, seeds, or insects picked from the ground or cacti. Long, pointed beaks made some of them more fit for picking seeds out of cactus fruits. Shorter, stouter beaks served best for eating seeds found on the ground. Eventually, the immigrants evolved into 14 separate species, each with its own song, food preferences, and beak shapes. Warbler finches, for example, catch insects in beaks that are sharper and more slender than those of cactus eaters.

For the future, Abzhanov notes, "there remain seven or eight other unique-beaked Darwin finches to explore. These birds serve as an ideal starting point [for studying the role of calmodulin], because they are very closely related yet very diverse in shape and structure.

"We also expect calmodulin to be important in other groups of long-beaked birds. However, this is not going to be the whole story for birds such as storks and ibises. Increasing calmodulin activity leads to a modest 10-14 percent increase in beak length, which matches well with the length differences between cactus and ground finches but additional mechanisms might be required for even longer beaks."

Abzhanov, Tabin, and their colleagues at Harvard, Princeton, and the Institute of Molecular Pathology in Vienna, Austria, published the result of their finch research in the Aug. 3 issue of the journal Nature.

Asked about the possibility of calmodulin in the heads of humans, Abzhanov answers, "At this point we don't know whether mammals in general or humans in particular employ calmodulin during development of their skulls and faces. It is, however, very likely as calmodulin appears to be involved in very basic craniofacial developmental processes. We do know it is expressed at the right time and in the right place in the development of mice embryos. We will certainly pursue its role(s) during both mouse and chicken development."


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The warbler finch (top) boasts a thin, sharp beak best suited for spearing insects. Ground finches' shorter, more robust beaks (center) are adapted for eating seeds found on the ground. Those of cactus finches (bottom) are shaped for getting seeds from cacti. (Harvard Medical School and Margaret Bowman)



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