Cobra Effect · Evolution
Nature does the selecting
How mockingbirds, finches and a letter from Wallace led to the theory of evolution.
7 cards, read aloud in 3:47, with a test and sources.
In 1835 a young naturalist spent five weeks in the Galapagos Islands.
Charles Darwin was sailing round the world on HMS Beagle, a survey ship, collecting animals, plants and rocks. The voyage lasted five years, from 1831 to 1836. In the Galapagos, far out in the Pacific, he collected birds from several of the islands. He mistook many of the finches for other kinds of bird altogether.
The mockingbirds were different on different islands.
Darwin noticed that the mockingbirds he caught on separate islands did not quite match. Islands only a short distance apart each seemed to have their own kind. He later wrote that it was comparing the mockingbirds that first thoroughly aroused his attention. Why would each island have its own mockingbird?
Back in London, the finches turned out to be something new.
In January 1837 Darwin handed his bird specimens to the ornithologist John Gould. Birds Darwin had taken for blackbirds, grosbeaks and finches were, Gould reported, all finches, an entirely new group. Gould counted thirteen new species, with beaks ranging from thick and crushing to thin and pointed. Close relatives, it seemed, had come to differ from one island to the next.
In 1837 Darwin sketched a branching tree in his notebook, and wrote above it: I think.
He had begun to suspect that species were not fixed, but slowly changed and split. His sketch showed one ancestor branching into many descendants, with some branches dying out. In September 1838 he read Thomas Malthus on how populations grow faster than their food supply. That gave him the missing piece: in a struggle to survive, small differences decide who lives to breed.
For twenty years he published nothing on the idea.
Darwin gathered evidence, bred pigeons and worked on a big book, but kept his theory out of print. Then, in June 1858, a letter arrived from Alfred Russel Wallace, who was collecting animals in the islands of South East Asia. Wallace had worked out the same idea while lying ill with fever. His essay set out almost exactly the theory Darwin had never published.
In 1858 the two men’s papers were read together, and neither of them was there.
In July 1858 Darwin’s friends arranged for his notes and Wallace’s essay to be read at the Linnean Society in London. Wallace was still far away in the Moluccas. Darwin was ill, and grieving for his baby son, who had just died. The next year Darwin published On the Origin of Species. Its argument was simple: living things vary, more are born than can survive, and the survivors pass on what helped them.
So when a thing seems designed for its life, ask what happened to all the ones that were not.
Breeders had long shaped pigeons, dogs and crops by choosing which ones to breed from. Darwin showed that fancy pigeons, from fantails to pouters, all came from the wild rock dove. Nature does the same choosing, with no plan at all, simply through who survives and breeds. Given enough time, small differences add up to new kinds of living things.
Sources
- The 1858 Darwin-Wallace paper, The Alfred Russel Wallace Website. Wallace’s letter from the islands, and how the two papers came to be read together.
- Darwin’s finches, Wikipedia. What Darwin collected in the Galapagos, what Gould made of it, and what the finches showed later.
- On the Origin of Species, Charles Darwin, 1859 (Darwin Online). The book itself, which opens with pigeons and the variation that breeders work with.
Nearby ideas
- Evolution you can watch. How two biologists measured every finch on one island and watched evolution happen.
- 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.