Darwin's finches

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. 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. Their behaviour differs, and they have different song melodies.

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Darwin's theory

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-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.^[1]

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. By mid March he realised that the original immigrants had somehow become altered to produce an array of new species. 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."^[2] He commented on this divergence of species in On the Origin of Species.^[3]

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.

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
  • 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."^[4] 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.^[5] 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.^[6]

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.^[7]

Real-time evolution

One of Darwin's finches (Geospiza fortis) helped prove their namesake's theory of evolution when scientists observed a shift in the species to birds with smaller beaks after competition was introduced by the arrival of another species with larger beaks. This happened over a twenty year span and was the first instance where scientists were able to document the type of evolutionary change known as character displacement from the start to completion of the process.^[8]

Molecular basis of beak evolution

In August 2006 in the Journal Nature, a group led by Harvard biologist Clif Tabin showed that the beak shapes of Darwin's finches are due to slightly different timing and spacial expressions of a gene called calmodulin. Calmodulin is used by the developing embryo to help lay down skeletal features (including the beak) and using microarray data and early embryo staining from each of the species, this groups could show how the different beak shapes were obtained. The article does not explain how adaptive radiation may have changed calmodulin expression, only that this was the way that the beak changes were reached.

Notes

References

• Darwin, Charles (1839), , vol. III, London: Henry Colburn.
• Darwin, Charles (1845), , London: John Murray.
• Darwin, Charles (1859), (1st ed.), London: John Murray,
• Darwin, Francis (1887), , in Darwin, Francis, , vol. 2, London: John Murray
• Desmond, Adrian & Moore, James (1991), , London: Michael Joseph, Penguin Group, ISBN 0-7181-3430-3
• Niles Eldredge (Spring 2006). VQR - Confessions of a Darwinist 32–53. The Virginia Quarterly Review. Retrieved on 2007-12-26.
• 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, 629–631. ISBN 1-55297-777-3. Retrieved on 2007-04-09.  It is not clear whether this placement was made by Burns and Skutch or by Perrins.
• 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.
• Frank J. Sulloway, The Beagle collections of Darwin's finches (Geospizinae) (1982), Bulletin of the British Museum (Natural History) Historical Series 43, No. 2: pp 49-94