Five hundred miles to the west of Ecuador the Galápagos Islands rise, a lonely cluster of rocks, from the Pacific. They emerge from the deeps, far off the continental shelf of South America, towering above a flat seabed more than 6,500 feet down. The Galápagos are a volcanic island chain created by a “hot spot” – a plume of up welling hot magma deep within the Earth. The westernmost islands in the chain, Isabella and Fernandina, are still being enlarged by rising magma; to their east a string of extinct volcanoes straggles towards the Ecuadorean coast, carried by the slow movement of the Nazca Plate on which they stand. East of San Cristóbal the sea has worn them down enough that their summits are below the surface, and the most eroded peaks – those closest to the coast – are slowly being carried down into the mantle as the Nazca Plate slides under South America.
Their volcanic origin makes the Galápagos unusual; like Hawaii they’re an isolated group far from any continental mainland. There are 21 islands surrounded by 107 scattered islets and rocks. Significantly, apart from Fernandina the main islands – Isabela, San Salvador, Santa Cruz, San Cristóbal, Espanola and Floreana – are separated by open ocean channels between ten and 40 miles wide. It’s not impossible for land animals to cross between them but it’s very, very difficult. The westernmost islands are less than a million years old, the easternmost – San Cristóbal – less than four million.
Sometime after San Cristóbal was formed a small number of species somehow managed to reach the islands from the coast of South America. Perhaps plant seeds were washed ashore and somehow managed to gain a foothold in the volcanic soil.
Other seeds may have drifted there on the wind. Exhausted birds might have been blown there by a storm and settled gratefully on the rocky outpost. Sea lions and penguins can survive in open water long enough to make the crossing. The real mystery is how some small land animals – snakes, iguanas – tortoises and rodents – became established on the islands. Perhaps they were washed out to sea on mats of vegetation and, by chance, made an accidental landfall on the Galápagos. We’ll probably never know. What matters is that these species did reach the islands and, in most cases, spread through the group. By a few hundred thousand years ago each of the main islands had its own population of iguanas, small birds and tortoises – with every population isolated from the others by deep blue water.
It’s unknown when the first people reached the islands. The earliest European visitors were Spaniards, the crew of a ship blown off course in 1535, but remnants of broken pottery suggest that native South Americans had probably visited before then. There were no inhabitants though, and it remained that way for almost another 300 years. The Galápagos were marked on charts but, barren and far from the trade routes, nobody wanted to live there.
The remote archipelago was the haunt of pirates, mostly Englishmen, who sheltered there between raids on the Spanish treasure ships that sailed along the coast. The pirates, and later whalers, hunted giant tortoises for their meat. Rats and cats escaped from ships and colonized the islands; the pirates released goats to provide a food source, the Spanish released to eat the goats, and the whalers added pigs and cattle to the mix. Finally, in 1832, Ecuador annexed the Galápagos and set up a penal colony on Floreana. By the time HMS Beagle arrived in 1835 the ecology of the islands had already been radically altered – but enough remained to catch the attention of a naturalist as talented as Charles Darwin.
Not many species reached the Galápagos before the arrival of European sailors. We know from DNA analysis, for example, that there was one species each of tortoises and iguanas. The odds of animals like these reaching the islands are incredibly small. Once they do get there, though, it’s slightly easier for them to spread between islands. Not easy by any means, but almost inevitable given enough time. The islands were still basically separate ecosystems though. A large ocean bird like an albatross could cover the distance between them effortlessly but for a small bird even the ten miles between San Salvador and Isabela is a dangerous crossing to make in a single flight. The animals on each island were, essentially, on their own.
They were also in circumstances that put a lot of evolutionary pressure on them. Transported from their familiar habitat on the mainland to the rugged, inhospitable volcanic peaks, they weren’t well adapted to their environment. The survival of every animal on the islands was balanced on a knife edge, and minor differences could condemn part of a population to death – or send it off in a radically new direction.
Iguanas are large South American lizards. They mostly eat fruit, flowers and fresh shoots, and they would have struggled badly on the Galápagos where these tender plants are scarce. Sometimes seaweed washes up on the beaches though, and there’s plenty more in the water. Those iguanas that could clumsily swim out a few yards to graze on the kelp survived; the ones that stayed on land slowly died out. Over time natural selection favored the stronger swimmers, who could reach more kelp and return to shore safely. Now the Galápagos are home to a unique species, the marine iguana, a strong swimmer that can dive 30 feet under the ocean.
Tortoises also prefer soft, low plants, but they’re not really built for swimming. Faced with the tough vegetation of the islands larger tortoises had an advantage; they could chew harder plants, and reach higher to access more leaves. The Chaco tortoise they evolved from is usually about ten inches long and weighs three or four pounds. On the Galápagos, with no large predators able to hunt the slow-moving animals, they evolved to an immense size – often over five feet long, and weighing 600 pounds or more. Different conditions on each island meant each population of tortoises followed a slightly different evolutionary path, and British consul Robert Lawson assured Darwin that he could tell which island a tortoise came from simply by the shape of its shell. Lawson was right – each island had its own subspecies of tortoise.
And then there were Darwin’s finches. They’re not actually finches at all; they’re tanagers, but it’s an easy mistake to make because they look like finches. Small, dull brown birds, each island had several species of them and Darwin and his servant shot a few dozen. He paid little attention to them though – he was far more interested in the Galápagos mockingbirds.
These were quite similar to the ones found on the mainland but after visiting several of the islands Darwin realized that each island had its own species. Later this knowledge convinced him to have another look at the “finches”. He had already noticed that there were several varieties on each island, with beaks adapted for different food sources. Some had large, heavy beaks that could crack nuts. Others had slim ones they used to punch holes in prickly pear cacti and extract the pulp.
Others specialized in eating seeds or insects. But when he began studying how the different species were related Darwin found something unusual. He expected a large-beaked finch from, say, Floreana to be most closely related to the large-beaked finches on Isabela or Santa Cruz – but instead its closest relatives were the other finches from Floreana. At some point in the past each island had had its own population of finches, which had slowly split into several different species as the birds evolved to exploit different sources of food. And this process hadn’t happened only once; it had happened separately on each island.
The Galápagos archipelago is no tropical paradise; it’s a rugged, semi-barren place, with its newest islands still periodically devastated by volcanic eruptions and the older ones constantly swept by the ocean winds. What makes them important in Darwin’s story is that, as a natural laboratory of evolution, it’s almost perfect. The islands’ isolation meant that once a species got there it was essentially cut off from the outside world, then divided again among the islands, so each tiny population was free to evolve in its own way. That led to the differences that Darwin’s keen mind picked up on, and earned both him and the Galápagos a place in history.