Darwin, Wallace and the Theory of Natural Selection

A letter that changed everything

In June 1858, Charles Darwin opened a letter from a young naturalist working on islands in Southeast Asia. Inside was a short paper explaining how species change: individuals vary, more are born than can survive, and those best suited to their surroundings leave more offspring. It was almost exactly the theory Darwin had been working on, mostly in secret, for about 20 years.

The letter was from Alfred Russel Wallace. Their ideas were presented together to the Linnean Society of London on 1 July 1858. The next year Darwin published On the Origin of Species. In this lesson you will learn the theory they shared, which is the foundation of all modern biology.

Darwin's voyage

Charles Darwin (1809 to 1882) sailed around the world on HMS Beagle from 1831 to 1836 as a naturalist. He collected fossils, plants and animals, and he noticed patterns:

• Fossils of giant extinct mammals in South America (such as giant ground sloths and armadillo-like glyptodonts) resembled the smaller sloths and armadillos living there today.
• Species changed gradually from place to place along the South American coast.
• On the Galápagos Islands, about 1000 km off the coast of Ecuador, species were similar to those on the mainland but different from them, and often different from island to island. Tortoises, mockingbirds and the small birds now called Darwin's finches varied between islands.

Back in England, Darwin also studied artificial selection: how pigeon breeders produced very different breeds by choosing which birds to mate. If people could change a species this much in a few centuries, what could nature do in millions of years? Then he read Malthus (1838) and saw the missing piece: the struggle for existence would select individuals, just as breeders do.

The theory of natural selection

Darwin and Wallace's reasoning can be laid out as a series of observations and inferences.

1. Overproduction. Every species produces more offspring than can survive (Malthus). A maple tree drops thousands of seeds; only a few become trees.
2. Variation. Individuals in a population differ in their traits.
3. Inheritance. Many of these differences are inherited: offspring tend to resemble their parents. (Darwin did not know how inheritance worked. Today we know it is through genes and alleles in DNA.)
4. Struggle for existence. Resources are limited and there are predators, diseases and harsh weather, so many individuals die before reproducing.
5. Differential survival and reproduction. Individuals with traits better suited to the environment are more likely to survive and reproduce. This is natural selection.
6. Change over generations. The helpful inherited traits become more common in the population, generation after generation. Over long periods, these changes can produce new species.

Darwin called the result descent with modification.

The giraffe again, Darwin's way

Compare this with Lamarck's explanation from the last lesson. Lamarck said individual giraffes changed by stretching. Darwin said individuals were born different, and the environment selected among them. Nobody's neck has to grow during its life: the population changes because some individuals reproduce more than others.

Evidence in action: the peppered moth

The peppered moth lives in Britain (and a related form lives in North America). Before the Industrial Revolution, most peppered moths were light with dark speckles, which hid them on pale, lichen-covered tree bark. A dark (melanic) form appeared in Manchester in the mid-1800s. As coal smoke killed the lichens and blackened tree trunks, dark moths became common in industrial areas; by the late 1800s, almost all peppered moths near some cities were dark. This is called industrial melanism.

Experiments by the British biologist Bernard Kettlewell in the 1950s, and a larger later study by Michael Majerus, showed that birds eat more of the moths that stand out against the bark. After clean-air laws reduced pollution, the light form became common again. The population's colour changed in both directions as the environment changed: natural selection in action.

What "theory" means in science

In everyday speech, theory can mean a hunch. In science, a theory is a well-tested explanation that ties together a huge range of observations and has survived repeated testing. The theory of evolution by natural selection is as well supported as the cell theory or the theory that matter is made of atoms. Since Darwin, it has been confirmed by fossils, comparative anatomy, DNA and direct observation, as you will see in the next two lessons.

Scientists still debate details: how fast evolution happens, and how important each mechanism is. That is normal in science, and it does not mean the core theory is in doubt.