Galapagos: Evolution
A river of lava cuts a tortoise population in two. A finch near a town stops evolving. A pink iguana turns up on a remote volcano, unknown to science. The islands that gave Charles Darwin his greatest idea are still showing scientists how new species begin and how people change the story.
How Can a River of Lava Start a New Species?
The lava split their home in two. A flow 100 metres wide, razor sharp and almost impossible to cross, cut straight through the tortoises' territory. Many died trying to get over it. David Attenborough explains why it matters: two groups that were once one now live apart, and if their two sides are different enough, they may one day become two species.
That he could tell which island a tortoise came from by its shell. The vice-governor's remark stuck in the young naturalist's mind. Why should tortoises on islands so close together all be slightly different? The question helped lead him to the idea of evolution by natural selection.
Put Your Instincts to the Test
Think about what would happen to animals cut off on different islands, each with its own food and weather. Pick an answer for each question, then see if your instincts were right.
Those with a saddle-shaped shell and a long neck. A tortoise with a high front to its shell could stretch up further. Those tortoises ate, survived and had young, so over thousands of generations the shell's front peak became more and more exaggerated. On wetter islands with plenty of low plants, a rounded dome works better.
They can't read each other's signals, so they don't breed. Each island's species does its press-ups with its own number, speed and height; David Attenborough calls it each species' own language. Even if two kinds meet, the wrong press-ups mean no mates, so the species stay apart.
The two forms are merging back into one. Away from people this finch is splitting into a small-beaked kind and a big-beaked kind, because each suits different seeds. The biologist Andrew Hendry found that near towns, where any beak can eat human food, the in-between birds survive too and the two kinds are blending again. People have pushed evolution into reverse.
Understanding the Science
Tap each card to see the forces that turned a few castaways into one of the richest collections of unique species on Earth.
Key Concepts
Natural Selection
Tap to learn moreLiving things vary, and some of those differences are passed from parents to young. When food is short or danger is near, the ones whose features suit the conditions are more likely to survive and breed. Over many generations those helpful features become common. Charles Darwin called this natural selection, and the Galapagos tortoises and finches gave him the evidence.
Isolation Makes New Species
Tap to learn moreIf one population is split into two that can't meet, each half changes in its own way. Sea separates the islands; on Wolf volcano a lava flow now separates two groups of tortoises. Given enough time and different enough conditions, the two groups can become so different that they're separate species. Most of the islands' unique animals began like this.
Drifting Islands
Tap to learn moreThe islands sit on a plate of the Earth's crust that drifts slowly south-east. David Attenborough puts it at about 3 centimetres a year (roughly as fast as fingernails grow) and shows it with a rock he sat on in 1978 that has moved about a metre since. A hotspot under the plate builds a volcano, the plate carries it away, and a new volcano grows behind it. That's why the islands are all different ages.
One Ancestor, Many Species
Tap to learn moreDarwin's finches all descend from one kind of bird that reached the islands long ago. As they spread to different islands and different foods, they split into many species with different beaks and habits. The woodpecker finch uses a cactus spine as a tool to winkle grubs out of holes; the vampire finch drinks blood from seabirds. One ancestor filling many ways of life is called adaptive radiation.
Behaviour Can Keep Species Apart
Tap to learn moreNew species don't only differ in how they look. Every island's lava lizard has its own pattern of press-ups for claiming territory and attracting a mate. A lizard that does the wrong number, speed or height of press-ups simply isn't recognised. Even when two kinds meet, this language barrier stops them breeding, so each stays separate.
Small Differences in Small Places
Tap to learn moreGalapagos land snails have split into more species than any other animal on the islands, often several on one small island. X-rays of their shells show why: each fits its own patch of damp or dry. Snails from wet places have wide shell mouths and rounder shells; on dry ground a small mouth loses less water, which gives a long, cone-shaped shell.
No Big Predators
Tap to learn moreThe Galapagos has no jaguars or other big hunting mammals. They'd have died of thirst within days on a raft, while tough reptiles survived the crossing. With little to fear, Galapagos animals are famously tame and spend less time hiding and more time feeding and breeding. Fast breeding means more young each generation for natural selection to act on, so evolution runs quickly.
People Change Evolution Too
Tap to learn moreFrom 1535 people brought new dangers: whalers took more than 100,000 tortoises for meat; ships left rats that ate their eggs. Today seeds hitch rides on cars, and European blackberry now lines the roads in thickets metres deep. People can also shift evolution itself, as with the town finches whose two beak forms are merging. They can protect too: tortoise breeding programmes have brought several species back from the brink.
Try It: Beaks and Seeds
Run the seasons and watch a flock of finches change. Try a drought, a very wet year, years of mixed seeds and life near a town. The graph shows every beak in the flock.
In the drought of 1977 on the little island of Daphne Major, Peter and Rosemary Grant watched most of the medium ground finches die. The survivors had deeper beaks that could crack the hard seeds left, and their young were born with deeper beaks too, about 4 per cent deeper on average. After the very wet El Niño of 1983, small soft seeds took over and the advantage swung to smaller beaks. This island speeds things up: each season here stands for a year.
Wildlife of the Galapagos: Evolution
Meet the animals that show evolution at work, from the tortoises that puzzled Charles Darwin to a pink iguana that hid from science for millions of years.
Giant Tortoise
Tap to learn moreOnce there were around 15 kinds, all descended from one founder; each island had its own. On wet islands they have high, domed shells for pushing through plants; on dry Española, saddle-shaped shells let them reach tall cactus. They can survive months without food or water, which is exactly why sailors took them away by the thousand.
Lonesome George
Tap to learn moreThe Pinta tortoise was thought to be extinct until 1972, when one male turned up in the island's undergrowth. Named Lonesome George, he was moved to a protected enclosure on Santa Cruz and became a worldwide symbol of what can be lost. In the documentary he's an old tortoise living out his days in safety.
Woodpecker Finch
Tap to learn moreThere are no woodpeckers in the Galapagos, so this little finch does their job a different way. It snaps off a cactus spine or a twig and holds it in its beak to winkle beetle grubs out of their burrows in branches. Very few birds anywhere use tools like this.
Vampire Finch
Tap to learn moreOn a couple of remote islands, a kind of ground finch has learned to peck at the skin of nesting boobies and drink their blood. It's a startling habit for a small seed-eater and a sign of how far one ancestor's descendants have spread into new ways of living.
Lava Lizard
Tap to learn moreEach island has its own species of these small, quick lizards. They're everywhere on the rocks. Males guard territories and court females with press-ups, and every species has its own way of doing them. Scientists even use a robot lizard to test which signals each species answers.
Galapagos Racer
Tap to learn moreA slim snake and one of the islands' very few land predators. On Fernandina, racers hunt young marine iguanas around the edges of the colonies, using both venom and constriction. Even so they're small, and it can take more than one snake to overpower a victim.
Galapagos Hawk
Tap to learn moreThe islands' top bird of prey, yet still too small to tackle a healthy adult marine iguana. It waits for one that's weak. David Attenborough says there are only about 150 pairs, limited by the number of good nesting places, so they hardly dent the iguana population.
Pink Iguana
Tap to learn moreIt lives only high on Wolf volcano, one of the least explored places in the islands. Park rangers first noticed it in 1986 and it was named as a species in 2009. Its genes show it split from the yellow land iguanas more than five million years ago. Nobody knows why it's pink.
Apply Your Knowledge
Each of these animals shows natural selection at work. Can you match them to the feature that helps them survive?
Match the Animal to Its Survival Feature
Click an item to select it, then click the matching description to place it.
Real-World Challenge
European blackberry is spreading along the Galapagos roads, rats still threaten tortoise eggs on some islands, and visitor numbers keep growing. Imagine you're running the Galapagos National Park with money for only one big project this year. Which would you choose and why? Think about which unique species would gain most and what might happen to the others while you wait.
Protecting the Galapagos Today
David Attenborough reminds us that people can destroy, but they can also conserve. Here are some of the big challenges for the islands and how people are meeting them.
Response: On Pinzón, rats brought by ships ate nearly every tortoise egg for more than a century, so the only young tortoises were ones raised in captivity and taken back. In 2012 rats were cleared from the island. In 2014 rangers found wild-hatched baby tortoises there for the first time in over 100 years.
Response: It took wildlife millions of years to reach the islands, but seeds and insects can now arrive in hours on planes and boats. Bags, food and cargo coming to the Galapagos are inspected to stop new invaders getting in, and rangers work to clear plants such as blackberry that are already spreading.
Response: About 30,000 people live on the islands, and many more visit every year. Visitors may only go ashore at chosen sites with a trained guide, and in 2024 Ecuador doubled the park entry fee for most foreign visitors to help pay for protecting the islands.
What Has Changed Since This Documentary Aired
This documentary was first shown in 2013. Here's what has happened since.
Updated: Lonesome George died in June 2012, the last known pure Pinta tortoise. The story didn't quite end there: on Wolf volcano scientists have found tortoises with Pinta ancestors in their family tree, probably descended from tortoises that sailors moved or dumped long ago, which raises hopes that something of the Pinta tortoise could be brought back.
Updated: Yes. In 2019 rangers found a female giant tortoise on Fernandina, where the last known tortoise had been collected in 1906. In 2022 DNA tests confirmed she belongs to the Fernandina species, long thought extinct. Searches for more are going on.
Updated: In 2017 scientists described a new line of finches on Daphne Major that began when a large cactus finch from another island flew in and bred with local medium ground finches in 1981. Within two generations its descendants, nicknamed Big Bird, were breeding only with each other, with their own song and beak size: a new kind of finch forming in front of researchers' eyes.
Test Your Understanding
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Reflection
Charles Darwin spent only five weeks in the Galapagos, yet what he noticed there changed how we understand all life. Think about the living things around your home or school. Can you spot two that are closely related but different, perhaps two kinds of bird or two kinds of tree? What might have made them different?
Episode Discussion
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