Natural Selection: How It Works and Why It Matters

In 1977 almost no rain fell on Daphne Major, a small volcanic island in the Galápagos. Many of the plants made few or no seeds that year, and most of the island’s medium ground finches died. Two biologists, Peter and Rosemary Grant, had been catching and measuring these birds for several years. When they measured the survivors, they found that the average beak was deeper than before. The next year the young birds had deeper beaks too. Yet no bird had changed its own beak. So how did the population change?

How does natural selection work?

The answer is natural selection: the process in which inherited differences between individuals decide who survives and has young, so that some traits become more common over generations. It does not need a plan or a goal. It needs only three conditions.

1. Individuals differ. No two members of a population are exactly alike.
2. Many differences are inherited. A trait is heritable when parents pass it on to their young, at least in part.
3. Not all young survive and reproduce. Every species produces more offspring than its surroundings can feed, so some leave many young and some leave none.

When all three hold, the result follows. Traits that help in the present conditions end up in more of the next generation. Biologists call an individual’s success in leaving offspring its fitness. Fitness always depends on the conditions. A trait that helps in one year can hurt in another.

I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection.

Darwin chose the name because of a comparison. A breeder chooses which animals may have young. In nature the conditions do this, and nobody does the choosing.

Now we can follow the finches step by step.

1. The birds differed. Before the drought, the medium ground finches on Daphne Major had beaks of different depths. The depth is the height of the beak where it meets the head. The average adult beak was about 9.2 millimetres deep, but some birds had deeper beaks than others.
2. The food changed. In the drought, the small, soft seeds ran out first. What remained were large, hard seeds, above all the woody seed cases of a plant called caltrop (Tribulus cistoides).
3. Some birds could eat, and others could not. Only birds with deep, strong beaks could crack the hard cases. Birds with small beaks starved. More than 80 percent of the population died, and the population fell from about 1,400 birds to a few hundred.
4. The survivors were not average. The adults that survived had beaks about 9.9 millimetres deep on average, about 0.7 millimetres more than before.
5. The next generation inherited the difference. Beak depth is strongly heritable in these finches. The chicks born in 1978 had beaks about 9.7 millimetres deep, so the average of the whole population had moved.

You might ask whether the birds grew deeper beaks because they needed them. They did not. Each bird kept the beak it hatched with. The drought only decided which beaks were passed on. The change happened in the population, not in any one bird.

Can anyone watch natural selection happen?

Darwin thought nobody could. In 1859 he wrote that “we see nothing of these slow changes in progress”, only their results after long ages. He imagined changes too slow for any human life.

The finches showed that he was too careful. A change of less than a millimetre in a beak sounds small. But the Grants measured it in one year, and they showed that it was inherited. Their study on Daphne Major lasted from 1973 for about forty years. It turned natural selection from a conclusion about the past into something you can measure.

The study also showed that selection can change direction. In 1982 a few large ground finches, a bigger species with huge beaks, settled on the island. They ate the large caltrop seeds far faster than the medium ground finches could. During the next long drought, in 2003 and 2004, the large seeds ran out quickly. This time the medium ground finches with smaller beaks survived better, because they could live on the small seeds that remained. In 2004 and 2005 the Grants recorded a strong shift toward smaller beaks in the same population.

So the same birds were pushed toward deeper beaks in 1977 and toward smaller beaks a generation later. Natural selection followed the conditions of each year. It did not follow a long-term plan.

What natural selection teaches you

The idea is useful far beyond biology. Wherever you find things that fit their surroundings well, you can ask three questions. What varied? What removed the rest? And what was copied into the next round?

• Medicine. When a patient takes an antibiotic, the few bacteria that happen to resist it survive and multiply. This is the drought on Daphne Major in a few days. It is why doctors do not prescribe antibiotics for colds, which are caused by viruses: the drug cannot help, but it still gives resistant bacteria an advantage.
• Farming. Spraying the same pesticide year after year removes the insects it can kill. The resistant ones remain, and their offspring take over the field.
• Your own work. Many good results come from the same three steps. You write several versions of an essay, keep the parts that work and build the next draft from them. Variation and a strict filter often beat one careful plan.
• Success stories. When you read about the habits of successful companies, remember the filter. You only hear from the ones that survived. Why Does the Universe Seem Fine-Tuned for Life? explains these selection effects, the way a filter shapes what you can observe.

Breeders have used the same logic for thousands of years, by choosing which plants and animals may reproduce. Darwin built his argument on that comparison. Engineers today use it too: computer programs called genetic algorithms produce many variants of a design, keep the best and vary them again.

The power of selection also comes from repetition. A small advantage in each generation adds up over many generations, in a way that resembles the growth described in What Is Compound Interest?

Where does the idea of natural selection go wrong?

Most mistakes about natural selection come from two misreadings. The first concerns the phrase “survival of the fittest”. The philosopher Herbert Spencer coined it in 1864, and Darwin added it to later editions of his book. Many people hear “the strongest win”. In biology, “fit” means “suited to the present conditions”, and fitness is measured in offspring. A small bird that raises many chicks is fitter than a strong bird that raises none. Even “survival” is too narrow, because surviving without reproducing passes nothing on.

The second misreading is that evolution aims at something. Selection has no foresight. It cannot prepare a species for next year, and it does not move toward a perfect form. The finches show this. Their beaks went one way in 1977 and the other way a generation later. Most traits are also compromises. The deep beaks that saved birds in 1977 became a disadvantage once a bigger competitor took the large seeds.

The strongest objection is as old as the theory. How can a process that only removes things build an organ as complex as the eye? Darwin admitted in chapter 6 that this sounds absurd. His answer was that each small step must be useful on its own. He pointed to living animals whose eyes range from a nerve coated with pigment to complex eyes with lenses.

There are real limits too. Selection cannot create new traits by itself. It only sorts variation that already exists, and that variation comes from mutations, which are random changes in the genes. Selection is also not the only force in evolution. In small populations, chance alone can make a trait more or less common. Biologists call this genetic drift.

Finally, natural selection describes how nature works. It does not tell us how people should treat each other. Around 1900, some political writers used the idea to argue that society should let the weak fail. This is often called social Darwinism. Yuval Noah Harari calls a related belief “evolutionary humanism” and links it to the Nazis, as the review “Homo Deus” by Yuval Noah Harari: Summary and Review explains. That conclusion does not follow from the biology. Nature favouring some traits says nothing about what is right.

Learn more

Two naturalists found the idea at the same time. Charles Darwin had worked on it in private for about twenty years. In June 1858 he received an essay from Alfred Russel Wallace, a naturalist who was collecting animals in the islands of what is now Indonesia. Wallace had reached the same idea on his own, on the island of Ternate. Darwin wrote to the geologist Charles Lyell that he had been “forestalled”. On 1 July 1858 the Linnean Society in London heard both men’s papers read together, while neither man was present. Darwin published On the Origin of Species the next year, in 1859.

The best next step is Jonathan Weiner’s The Beak of the Finch (Affiliate Link). It tells the story of the Grants and their finches and won the Pulitzer Prize for General Nonfiction in 1995. If you want the original argument, Darwin’s book is free online. Chapters 3 and 4 contain the core of it.