Fauna of the Galapagos Islands
Following the writings of Charles Darwin, the different types of birds and reptiles on this isolated cluster of islands straddling the Equator 900km west of Ecuador (South America) have served as examples of evolution in action.
These 18 islands were formed as a result of volcanic action and have seen quite recent eruptions. The eastern islands are the oldest (Espanola formed 3.2 million years ago), and the western islands formed later (Fernandina, 700 thousand years ago).
Not shown on the map above: Wolf and Darwin islands - 160 km north-west of Isabela: home of the vampire finch.
Confusingly, each island has at least two names: of Spanish or English origin.
In fact Darwin was more interested in Geology than Biology
when he visited but he preserved a number of
specimens of biological material
which were examined by others when he returned to England.
Darwin noticed that different islands had different mockingbird species, but there are different species of ground finch and tree finch as well as distinctive land and marine Iguanas, and giant tortoises.
Other bird species endemic to the Galapagos and worthy of note include the Galapagos Flightless Cormorant Phalacrocorax harrisi
, the Galapagos Hawk Buteo galapagoensis
and the Galapagos Penguin Spheniscus mendiculus
- some of which are the only penguins to live in the northern hemisphere.
On the Galapagos there are also colonies of various wide-ranging seabirds known from other parts of the world: Blue-footed booby
, Nazca booby
, the Dull-colored Grassquit
These small birds are in fact not true finches but Tanagers. Their ancestors (nearest modern species the dull-coloured grassquit (Tiaris obscurus)
were presumably blown from the mainland during extreme weather conditions.
Having arrived (as a flock, or at least a pair) on an Eastern Galapagos island 2–3 million years ago, these birds searched for food such as seeds of the local island plants, and insects. They then reproduced and became established as a population on the island. Mutations occurred, which produced a variety of different inherited conditions (genotypes) within the population. Some of these variations would have made the descendants of the initial finch more successful on the island than they would have been back on the mainland. Two separate gene pools had been established, with no interbreeding between them. At this stage (1.2 million years ago), allopatric speciation
could be said to have occurred.
Within the population of colonising species there would be a range of beak sizes. Some, with (genes/alleles for) larger beaks could crack open larger seeds and cacti; others with smaller beaks confined themselves to small seeds.
As a result of adaptive radiation
, the colonising species then developed into at least 15 species, each with a slightly different ecological niche, but nevertheless competing with the other species during critical weather conditions, e.g. drought. This situation is evidently rather flexible, as it has been noted that beak sizes vary between islands, depending on competition with other species, and that different species can occasionally interbreed successfully.
Click to see/ hide finch species listed by name
Darwin commented on the basic similarity in body shape and colour in his finches and he also noted a "perfect gradation in the size of the beaks in the different species". Curiously he did not mention Galapagos finches in any edition of Origin of Species.
Since Darwin, many scientists have visited Galapagos and made observations about the finches; none more so than Peter and Rosemary Grant who for years have logged the effect of favourable and unfavourable weather conditions on population numbers and body dimensions, especially beak sizes, and recorded hybridisation between different 'species', following the offspring for several generations.
It has been shown that the regulatory gene ALX1 has a major effect on beak shape in finches, together with another gene HMGA2. This has two alleles: one is common in finches with small beaks, while the other is common in finches with large beaks. There was a major change in allele frequency after 80% of medium ground finches perished following drought in 2004-6 and only small-beaked individuals of this species survived.
Galapagos giant tortoises
These were the origin for the name of the islands in Spanish. Passing ships from the 16th century onwards would pick them up as source of meat, as they could survive for long periods of time without food. This caused a major reduction in population, as did animals introduced by Man, including rats, which were predatory on eggs and young, and goats, which ate much of the vegetation on some islands. Luckily these tortoises have a lifespan of at least 100 years, so it has been possible to rectify this somewhat by protected breeding schemes and pest removal programs.
It seems likely that tortoises reached the islands by sea, presumably washed down rivers after strong rainfall, as their shell gives then buoyancy and they can raise their heads above water to breathe. They can survive for long periods without food. The most likely ancestral species was smaller, but it is commonly observed that island populations are larger than their mainland relatives, possibly as a result of reduced predation and competition. If an individual had a mutated gene or a combination of polygenes which increased its size, or prevented it stopping growth at a particular size, that could give it a survival advantage and increase its genetic fitness.
Darwin was told that each island had tortoises with different features which could identify their origin. On islands with plenty of short vegetation the shell (carapace) shape is domed and the neck is short; on other drier islands the shell has a distinct saddle-like shape and the tortoise's neck is longer, allowing access to taller vegetation including tree cacti.
There are currently 11 varieties of Galapagos giant tortoise, each given either species or subspecies status within the genus Chelonoidis
Mitochondrial DNA analysis shows that some larger islands have several geographically distinct populations which differ significantly from one another but resemble those on other islands, suggesting there were 3 waves of colonisation. There are doubts about some populations which might have been brought by Man from other islands. This might prove to be fortuitous in re-establishing colonies of tortoises back on their presumed islands of origin, e.g Floreana.
Lonesome George - the last of his species
'Lonesome George' was the only remaining tortoise from Pinta Island (Chelonoidis abingdonii
) and he died in 2012 aged about 100, having spent 40 years in captivity with females of a closely related species (Chelonoidis becki
). He mated with them and they laid eggs but these proved to be infertile.
Historically, other giant tortoises (unrelated to Chelonoidis spp
) were known to exist on many of the western Indian Ocean islands, as well as Madagascar. The Aldabra giant tortoise (Aldabrachelys gigantea
), from the islands of the Aldabra Atoll in the Seychelles is the only remaining species today.
Anolis lizards in the Caribbean and USA
The islands of the Caribbean were produced as a result of the interaction of two or more tectonic plates, and volcanic activity along the eastern edge.
There are almost 150 species of lizards belonging to the genus Anolis
on islands in the Caribbean.
It is believed that these species evolved from (two?)
species found on mainland USA. The green or Carolina anole Anolis carolinensis
is the main one today.
The Carolina anole (Anolis carolinensis
) is an arboreal
lizard found primarily in the southeastern United States and some Caribbean islands.
Male Carolina anoles have attractive throat pouches (dewlaps) which they use for territorial displays or when courting females.
Presumably these islands were colonised by lizards from the area which is currently mainland USA or other islands, probably clinging to floating vegetation, produced as a result of violent weather conditions for which the area is famous.
Each island would have different climatic conditions and vegetation, and different predators, so the lizards would be given opportunities to feed and breed, as well as being exposed to different selective pressures.
It has been found that anole lizards vary in the colour of their skin, the length of their legs and tail and the development of toepads which are useful for gripping onto leaves, and these features could be key to their survival. Within each island there will be different types of landscape and different (layers of) vegetation so different species can find their own specific habitat and ecological niche. This is another example of adaptive radiation
Camouflage will be key to survival, but reproductive success depends on attracting a mate.
The images below show some of the variation between different Anolis
species in the dewlap - a section of skin under the throat which males use in courtship display.
It is interesting to note that several of these 'island' species have been re-introduced to the USA and these have out-competed the native green anole, forcing it to change its lifestyle and effectively evolve in response to competition and predation.
Anole lizards are undoubtedly quite photogenic, and there is a large community of afficianados who document their appearance both on the islands and in the USA.
Not quite giants ...
The St Kilda Wren (Troglodytes troglodytes hirtensis
) is a subspecies of the Common Wren found on the Scottish Isle of St Kilda - 64km west of the Outer Hebrides. It is somewhat larger than individuals of the mainland population, and more grey and less reddish-brown in coloration. Barring on the feathers is more noticeable. And it has a slightly different - and louder - song. See link below
The population on St Kilda has been estimated to be just over 200 pairs. They are undoubtedly quite geographically isolated, and must have a separate gene pool.
The St Kilda Field Mouse (Apodemus sylvaticus hirtensis
) - originally introduced (by accident?) by the Vikings, 1000 years ago - is also larger than the mainland version.
Perhaps you might want to find what happened to the St Kilda House Mouse!