Why do the Galápagos have animals found nowhere else?

The short answer

The Galápagos Islands are volcanoes that rose out of the Pacific about 1,000 kilometres west of mainland Ecuador, and they have never been joined to any continent. When each island appeared it was bare rock, so every plant and animal living there today descends from ancestors that crossed the open ocean: birds that flew or were blown off course, and reptiles and seeds that floated or drifted on rafts of vegetation. Only a few kinds ever made it, and many groups common on the mainland never arrived at all.

The few that did arrive found islands with plenty of open opportunities and few competitors or predators. Cut off from their mainland relatives, and often from each other on separate islands, they changed over hundreds of thousands to millions of years until they became new species. That is why the archipelago has giant tortoises, sea-going iguanas, a cormorant that cannot fly and a whole group of finches found nowhere else on Earth.

Islands born from the sea floor

The Galápagos sit above a hotspot, a place where a plume of hot material rising from deep inside the Earth melts the crust from below and builds volcanoes. The sea floor here is part of the Nazca plate, which moves slowly over the hotspot. New volcanoes form over the plume, are carried away from it, cool, erode and eventually sink beneath the waves. The youngest islands, Isabela and Fernandina in the west, are still being built: Fernandina has been the most active volcano in the archipelago since 1790. The eastern islands are older, lower and more worn down.

Volcanoes have been erupting here for at least 20 million years, and the first islands formed at least 8 million years ago, though those have since drowned. Española and San Cristóbal, the oldest islands above water today, are only a few million years old. Living things have had to colonise each new island in turn, from the islands around it or from far across the sea.

Tortoises on islands with humid highlands and plenty of low vegetation tend to be large, with domed shells and short necks.

Who could make the crossing

Reaching the Galápagos was a lottery that favoured certain animals. Giant tortoises descend from ancestors that crossed the sea from South America; their closest living relative is the Chaco tortoise of Argentina and Paraguay. Being big helped. A large tortoise can keep its head higher above the water and loses water more slowly than a small one, and fossils show that giant tortoises already lived on the mainland, so their great size probably helped them survive the voyage rather than evolving after they landed.

The ancestors of the islands' iguanas probably came the same way, floating from Central or South America, perhaps on mats of vegetation. Birds could fly, and seeds and insects could be blown on the wind or carried by birds. Three great ocean currents meet here: the cold Humboldt Current from the south, the warm Panama Current from the north and the deep, nutrient-rich Cromwell Current welling up from the west. The Humboldt Current is thought to have carried penguins from South America, and the cool water lets the Galápagos penguin survive on the equator. It is the only penguin found north of it.

So few colonists arrived that many roles were left empty. The giant tortoises, for instance, had no natural predators at all.

Evolving in isolation

Once ashore, the colonists adapted to conditions found nowhere else. The clearest case is Darwin's finches, about 18 species thought to have evolved from a single finch species that arrived more than a million years ago. The main differences between them are in the size and shape of their beaks, highly adapted to different foods. Darwin traced a near-perfect gradation from a beak bigger than a grosbeak's to one barely different from a warbler's. Biologists call this adaptive radiation, one ancestor spreading out into many species to fill many ways of making a living.

The massive beak at top left belongs to the large ground finch; the slender one at bottom right to the green warbler-finch.

The tortoises split into different forms on different islands. Where the lowlands are dry and food grows high, as on prickly pear cactus, they evolved "saddleback" shells raised at the front, with long necks and limbs that let them reach upwards. A saddleback that pulls in its head still leaves its neck exposed, a sign that the shell evolved where nothing hunted them. The species now includes 15 subspecies, 12 of which survive.

The marine iguana went further. It split from the Galápagos land iguanas about 4.5 million years ago, by one genetic estimate, and is now the only living lizard that forages in the sea, diving for the algae that make up almost all of its diet. The flightless cormorant, found only on two western islands where the cold Cromwell Current wells up, is the only cormorant known to have lost the power of flight. Its wings are about a third of the size a bird of its weight would need to fly.

Like other cormorants, its feathers are not waterproof, so it dries its wings in the sun after every dive.

Darwin's clue, and evolution still at work

HMS Beagle reached the Galápagos on 15 September 1835, and the young naturalist Charles Darwin spent five weeks there. He noticed that the mockingbirds differed from island to island, and the islands' acting governor, Nicholas Lawson, mentioned that the tortoises of different islands could easily be told apart by their shells. Back in London, the ornithologist John Gould found that 25 of the 26 land birds Darwin had collected were new forms, found nowhere else but closely allied to South American species. Darwin later wrote that it was as if "one species had been taken and modified for different ends". The islands became central to his theory of evolution by natural selection, published in On the Origin of Species in 1859.

Evolution here has not stopped. For more than 40 years the biologists Peter and Rosemary Grant have measured finches and recorded their beak sizes changing as years of heavy rain gave way to droughts. In 1981 a male finch from another island arrived on the islet of Daphne Major and bred with a local medium ground finch. Within a few generations his descendants, nicknamed "Big Bird", were breeding only among themselves, a new lineage forming in front of scientists' eyes.

Seeing it today

About 97.5 per cent of the islands' land is protected as Ecuador's Galápagos National Park, founded in 1959, and the surrounding marine reserve covers 133,000 square kilometres of ocean. Of about 500 native species of vascular plants, including 90 ferns, around 180 grow nowhere else. The giant tortoises, hunted by whalers for meat and oil and hit by introduced rats, goats and pigs, have recovered to more than 19,000 after captive-bred young were returned to their home islands, though every surviving subspecies is still classed as threatened. Marine iguanas bask in colonies on the rocky shores; the flightless cormorant lives only on Fernandina and the northern and western coasts of Isabela.