No two alike
Look closely at any group of living things of one kind and you will find that no two are exactly alike. Grove snails, a single species common in gardens and hedges, carry shells that are yellow, pink or brown, plain or banded. Wild oats in one field differ in height, in how early they flower and in how well they stand a dry spell. Biologists call all the members of one species living in one place a population, and the differences between them variation.
Some variation is easy to see, such as colour or size. Much of it is hidden: how well an animal fights off a disease, how little water a plant can live on. It matters because it is the raw material for everything in this reading. If every member of a population were identical, nothing could favour one over another, and the population could never change.
What gets passed on
Not every difference counts. A blacksmith builds thick arms at the forge, but his children are not born with them. Only a difference written in the genes, the instructions in DNA that parents copy and hand to their offspring, appears again in the next generation. For a trait such as colour or height to spread through a population, it must be inherited.
Where do new inherited differences come from? Each time a cell copies its DNA, the copy now and then comes out slightly different from the original. A change of this kind in the DNA is called a mutation. Most mutations do nothing that can be noticed, some do harm, and a few help. The trouble a living thing faces does not call up the mutation that would meet it. Mutations appear whether or not they would be of use, and what becomes of each is settled afterwards.
More are born than can live
A large female cod can release several million eggs in a single season. If every one grew up and bred, the sea would be solid with cod within a few generations. It is not, because almost all of them die young: eaten, starved or crowded out.
In the ninth century al-Jāḥiẓ of Basra described one side of this in his Book of Animals. The rat, he wrote, hunts weaker creatures for its food and must itself avoid the snakes and birds of prey that hunt it. No hunter escapes being hunted in its turn. He saw in it the wisdom of the Creator. Biologists call the whole contest the struggle for existence. It is sharpest between members of one species, because they need exactly the same food and space. That contest for the same limited things is called competition.
Natural selection
In the middle of the nineteenth century, naturalists joined three plain facts into one idea. The members of a population vary. Much of that variation is inherited. And more are born than can survive.
Put them together. If some individuals carry an inherited difference that suits their surroundings, a thicker coat in a cold place or a colour that hides them, they are a little more likely to come through the struggle. Survival lets them breed, and their offspring inherit the difference. So in the next generation it is slightly more common, and more common again in the one after. This process is called natural selection. No individual changes during its own life. The population changes, because of who leaves offspring.
What “fittest” means
The phrase “survival of the fittest” was coined by the philosopher Herbert Spencer in 1864. It is easy to misread. “Fittest” sounds like the strongest, and it does not mean that.
In biology, fitness is counted in offspring. An individual’s fitness is the number of surviving young it leaves, compared with others in its population. A stag that wins every fight and fathers no young has a fitness of zero. A small, plain bird that raises more chicks than its neighbours is the fitter one.
And fitness is never fixed. It depends on the environment, the whole set of conditions an individual lives in: the climate, the food, the enemies. A thick coat is an advantage in an Arctic winter and a burden in a desert. The fittest are those that best fit the place and time they live in.
Fitted to a place
Living things fit the places they live in, often in fine detail. A cactus is the classic case. Its thick stem stores water. It has spines in place of leaves, which lose almost no water and keep grazing animals off. A waxy skin seals the water in. Every one of these features suits life in the desert.
An inherited feature that helps a living thing survive and breed where it lives is called an adaptation. A polar bear’s fat and dense fur are adaptations to cold. A camel’s ability to go for days without drinking is an adaptation to heat and thirst.
Adaptations are local. Plant the cactus in a rainforest and its slow growth leaves it shaded out by faster plants. Its features fit the desert. They are not better in general. So the surroundings matter to natural selection: among a population’s variants, they settle which fits better.
The moths that turned dark
The peppered moth rests by day on tree bark. Until the nineteenth century nearly all were pale, a match for the lichen on the bark. In 1848 a dark one was caught in Manchester. It carried a mutation for dark wings. By 1895 about 98 per cent of those around the city were dark.
No moth had changed colour. Factory smoke had killed the lichen and coated the trees with soot. On blackened bark a pale moth stood out and a dark one was hard to see, and birds ate the moths they could see. Dark moths survived to breed and passed on their dark wings.
When clean-air laws from 1956 cut the smoke, the bark lightened and the pale form became common again. In the 2000s Michael Majerus released 4,864 moths near Cambridge, where the bark is clean, and birds took a larger share of the dark ones.
That answers the question you started with: In 1848 almost every peppered moth around Manchester was pale. Less than fifty years later almost every one was dark, yet no moth had changed its colour. What happened?
Selection you can watch
Bacteria show natural selection at a speed we can watch. In good conditions a bacterium such as E. coli divides about every 20 minutes, so one day holds dozens of generations.
An antibiotic is a drug that kills bacteria. An infection may hold billions of them, and among so many a few may carry a mutation that lets them survive the drug. The antibiotic kills the rest. The survivors have the food and space to themselves. They multiply, and soon the whole population descends from them. Bacteria that a drug can no longer kill are called resistant.
This is a record of what has happened. Penicillin came into wide use in the 1940s, and resistant bacteria were found in hospital patients within the same decade. Every use of an antibiotic selects for the bacteria it fails to kill, which is why doctors try not to prescribe one without need.
The mutation comes first
Does the antibiotic cause the mutation that resists it? The guess is natural, and wrong. In 1943 Salvador Luria and Max Delbrück tested the idea on bacteria and a virus that kills them. They grew many batches and spread each on a dish holding the virus. Had the virus caused resistance, every dish should have shown about the same number of survivors. The numbers swung wildly from dish to dish. In some batches a resistance mutation had turned up long before the virus arrived, and had been copied ever since.
So the threat does not create the trait. It reveals who has it. This is the limit of natural selection: it can work only on variation that already exists. It sorts what is there and cannot supply what is missing. If no member of a population carries a variant that meets a new danger, selection has nothing to favour.
When people do the selecting
Farmers used the same principle for thousands of years before anyone named it. Wild wheat shatters when it is ripe and scatters its grain on the ground. About ten thousand years ago in the Middle East, people began to sow it. The grain they carried home, and sowed again, came mostly from the rare plants whose ears held on. Sowing after sowing, the fields filled with plants that kept their grain for the harvest.
Choosing which plants or animals may breed, so as to strengthen a trait people want, is called selective breeding. It made every dog, from the chihuahua to the Great Dane, out of the wolf. It made kale, cabbage, broccoli and cauliflower out of a single wild plant of the sea cliffs.
It works for the same three reasons as natural selection: variation, inheritance, and only some individuals breeding. The breeder takes the place of the surroundings.
What a species is
A species is a group of living things that can breed with one another and produce fertile offspring. A horse and a donkey can mate, but the mule that results is sterile. It cannot have young of its own, so horses and donkeys remain two species.
Looks are a poor guide. A chihuahua and a Great Dane differ far more in size and shape than a horse and a donkey do, yet all dogs can breed together and their pups are fertile. Thousands of years of selective breeding have produced hundreds of breeds, and every one of them belongs to a single species.
The same holds for each case in this reading. The dark and the pale peppered moths were one species throughout, and so were the bacteria before and after the antibiotic. What natural selection changed was how common one variant was within the species.
The record in the rocks
When an animal dies it usually rots or is eaten, and nothing remains. Now and then a body is buried quickly in mud or sand on the bed of a sea or a lake. Settled material of this kind is called sediment. More sediment settles on top, and over long ages the layers harden into rock. The buried remains, or the imprint they left, are preserved as fossils.
Because each layer settles on the one before, the lower a layer lies, the older it is. Ibn Sīnā reasoned this way in his Book of Healing, finished by 1027. Some mountains, he wrote, seem to have been piled up layer by layer, one formed first and the next, at a different period, on top. In the Grand Canyon the limestone at the rim is about 270 million years old, and the sandstone far below it is more than 500 million.
Most species are gone
The fossil record holds a surprise. Most of the creatures in it are not alive today. Trilobites crawled the sea floor for about 270 million years and then vanished. A species with no living member left anywhere is extinct, and biologists estimate that more than 99 per cent of all the species that have ever lived are now extinct.
Extinction is what happens when conditions change faster than a population can adapt. Selection can work only on the variation that is there, and it needs generations to do it. The passenger pigeon was once among the most numerous birds in North America, numbering in the billions. Hunting and the felling of its forests in the nineteenth century gave it no time. The last one, a female named Martha, died in a zoo in Cincinnati in 1914.
