Finches and Tortoises
on the Galapagos Islands
Derivation of the name Galapagos is from an old Spanish word for saddle, in reference to the unusual shells of some of the tortoises.
The Galapagos supports peculiar birds, reptiles and plants.
The Galapagos Islands lie on the equator nearly 1000 kilometres from the coast of Ecuador. This volcanic archipelago consist of 13 major islands, and numerous other smaller islands and rocky outcrops. Craters, lava and at times active volcanic activity are clear evidence of their geological origin as seamounts. The majority of plant and animal species on the Galapagos are endemic. The islands are from 700 000 to 5 million years old.
During five weeks in the Galapagos, Darwin visited four of the islands: Chatham (Isla San Cristobal), Charles (Santa Maria), Albemarle (Isabela) and James (Santiago).
He encountered land iguanas, marine iguanas and giant tortoises. He also collected mockingbirds and finches, which were very tame.
The local governor pointed out that each island had its own recognisable kind of land tortoise.
Finches
The 13 species of finches on the Galapagos Islands are related members of an endemic adaptive radiation that have evolved from an ancestor that dispersed from South America.
The 13 species of Galapagos finches are small, brown, relatively non-descript birds. It is thought that they evolved via a four-stage process(Lack, 1947)
The stages are colonisation, isolation and speciation, reinvasion, and specialisation. The general lack of other land birds has enabled the finches to evolve into forms that would not have survived on the mainland. This is due to the presence of competitors on the mainland already occupying these niches.
Most of the finch species inhabit several islands.
It took a little while to correctly establish the taxonomy of the group. The ornithologist John Gould (1804-1881) originally placed the finches in a single genus with three subgenera. These were later elevated to the status of genera.
The archipelago is a little world within itself, or rather a satellite attached to America, whence it has derived a few stray colonists, and has received the general character of its indigenous productions.(Charles Darwin, Journal of Researches, 2nd edition, 1845, p. 377)
.. 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 sub-groups. All these species are peculiar to this archipelago..(Charles Darwin, Journal of Researches, 2nd edition, 1845, p. 379)
The most curious fact is the perfect gradation in the size of the beaks in the different species of Geospiza..
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.(Charles Darwin, Journal of Researches, 2nd edition, 1845, p. 380)
There is a distinction in diet, the finches became specialised to obtain particular types of food.
one kind of small bird managed to reach an oceanic archipelago and diversify, in the absence of competitors, into a wide range of niches and feeding types.Stephen J Gould (1985) p.355
This type of speciation on small islands has been termed evolutionary release.
Ground finches granivores (seed eaters)
- Large beak, Large Ground Finch - Geospiza magnirostris
- Medium beak, Medium Ground Finch - Geospiza fortis
- Small beak, Small Ground Finch - Geospiza fuliginosa
- Pointed beak, Sharp-beaked Ground Finch - Geospiza difficilis
Cactus eaters
- Large Cactus, Ground Finch - Geospiza conirostris
- Cactus Finch - Geospiza scandens
Mangrove eaters
- Mangrove Finch - Cactospiza heliobates
Arboreal finches
- Herbivore, Vegetarian Finch - Platyspiza crassirostris
- Large insects, Large Tree Finch - Camarhynchus psittacula
- Medium insects, Medium Tree Finch - Camarhynchus pauper
- Small insects, Small Tree Finch - Camarhynchus parvulus
Warbler-like species (slender beak)
- Warbler Finch - Certhidea
Species that uses cactus spines to obtain insects
- The Woodpecker Finch - Cactospiza
Cocos Island
- Cocos Finch - Pinaroloxias inornata
Tortoises
Chelonoidis [Geochelone] nigra subspecies occur on seven of the islands.
The tortoises evolved from a common ancestor that reached the Galapagos by overwater dispersal from South America.
The vice-governor, a Mr. Nicholas Lawson, informed Darwin that 'the tortoises differed from the different islands, and that he could with certainty tell from which island any one was brought'.
This is probably not entirely true, but it did illustrate that the shells of the tortoises varied on different islands.
Darwin only observed live tortoises with dome-shaped carapaces (on James and Chatham Islands). Domed shell tortoises have short necks, Saddleback shell tortoises have long necks.