Darwin's finches

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iDarwin's Finches

Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Passeriformes
Family: Emberizidae
Genera

Geospiza
Camarhynchus
Certhidea
Pinaroloxias

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 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.

Contents

[edit] The finches and 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. 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.

[edit] 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)
      • Darwin's Large Ground Finch (Geospiza magnirostris magnirostris) - possibly extinct (1957?)
    • 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

[edit] 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.

[edit] References

  • Adrian Desmond and James Moore, Darwin (London: Michael Joseph, the Penguin Group, (1991). ISBN 0-7181-3430-3

[edit] External links

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