Cobra Effect · Evolution
Evolution you can watch
How two biologists measured every finch on one island and watched evolution happen.
7 cards, read aloud in 3:20, with a test and sources.
Since 1973, Peter and Rosemary Grant have studied the finches of one small island.
Daphne Major is a small, steep volcanic island in the Galapagos. Year after year the Grants caught the ground finches living there, put a ring on each one, and measured it. Over the decades they recorded thousands of birds, and knew which ones bred and which ones died. It was a place where a whole population could be followed, bird by bird.
They measured every bird’s beak with calipers.
Beak depth, from top to bottom, matters because it decides which seeds a finch can crack. Birds with deeper beaks can crack bigger, harder seeds. Beak size also runs in families: big-beaked parents tend to have big-beaked young. So any change in who survived could show up in the next generation.
In 1977 a drought struck, and most of the finches died.
The plants that make small, soft seeds dried up, and small seeds ran short. What was left were mostly large, hard seeds that only a strong beak could crack. About eighty five per cent of the birds on the island died. The survivors were mostly the ones with bigger beaks.
Their young had bigger beaks, and the change could be measured.
The survivors bred, and passed their beak size on to their young. The next generation’s beaks were a few per cent larger than those of the birds before the drought. A few per cent sounds small, but it was real, and it had happened in a single generation. This was natural selection, measured.
Then in 1983 the rains came, and the change began to run the other way.
A powerful El Nino brought about ten times more rain than the island had seen in any wet season on record. Plants with small seeds spread, and small seeds came to make up most of the food. Now smaller beaks, better suited to small seeds, had the advantage. Selection does not march in one direction. It follows whatever the conditions reward.
A stranger that arrived in 1981 went on to found a new line of finches.
He was a large male with an unusual song, unlike any finch on the island. DNA later showed he was a large cactus finch, flown in from an island more than a hundred kilometres away. He bred with a local female, and within two generations his descendants bred only with each other. The Grants followed this Big Bird line for six generations, and it behaves like a new species.
So when someone says evolution is too slow to see, ask who has been watching.
Evolution is often pictured as far too slow to watch. The Grants saw it within a few years, because they measured every bird, every year. Change that is invisible from a distance can be plain up close, with patient records. Small, steady measurements catch what a glance never will.
Sources
- The people who saw evolution, Princeton Alumni Weekly. Peter and Rosemary Grant, their decades on Daphne Major, and what the finches showed.
- Watching evolution happen in two lifetimes, Quanta Magazine. An interview with the Grants on drought, rain and the finches that changed in front of them.
- The origin of a new species of Darwin’s finches, Uppsala University. How DNA traced the Big Bird line back to a single newcomer in 1981.
Nearby ideas
- Nature does the selecting. How mockingbirds, finches and a letter from Wallace led to the theory of evolution.
- The moth that turned black. How soot turned Britain’s peppered moths black, and clean air turned them pale again.
- A fish with a neck and wrists. How a team predicted where to dig, and found a fish with a neck and wrists.
- Evolution in a flask. How twelve flasks and a giant dish let scientists watch bacteria evolve.
- The foxes that became friendly. How breeding the calmest foxes for a few decades changed their behaviour and their looks.