How Does Natural Selection Work? Peppered Moth Example

How does natural selection work? Individuals in a population differ from one another, and some of those differences are inherited. Individuals whose traits suit their environment are more likely to survive and reproduce, so they pass those traits on to more offspring. Over many generations, helpful traits become more common in the population. The idea was set out by Charles Darwin in On the Origin of Species in 1859.

What are the steps of natural selection?

Natural selection needs four things, and it works whenever all four are present:

1. Variation: members of a species differ in their traits, such as colour, size or speed.
2. Inheritance: some of this variation is passed from parents to offspring through genes.
3. Competition: more offspring are born than can survive, so individuals compete for food, mates and safety.
4. Differential survival: individuals with traits that suit the environment survive and breed more often.

Repeat these steps for generation after generation and the population changes. That change in the frequency of inherited traits over time is evolution.

What is the peppered moth example?

The peppered moth example is the classic case of natural selection in action. The peppered moth (Biston betularia) comes in a pale, speckled form and a dark form.

Before the Industrial Revolution, birch trees in England had pale, lichen-covered bark. Pale moths blended in, and birds ate the dark moths first. In the 1800s, factory smoke killed the lichen and covered tree trunks in dark soot. Now dark moths were hidden and pale moths stood out. The dark form was first recorded in Manchester in 1848, and by 1895 about 98% of the moths there were dark.

After the Clean Air Act of 1956 reduced pollution, bark became paler again and the pale form recovered. The moths did not change on purpose. Birds simply removed the moths that were easier to see, and the survivors passed on their colour.

Does natural selection mean survival of the fittest?

Survival of the fittest is a popular phrase, but "fittest" does not mean strongest. In biology, fitness means how many offspring an individual leaves behind. A small, camouflaged moth that breeds is fitter than a large, visible one that is eaten first.

What is the difference between adaptation and evolution?

An adaptation is an inherited feature that helps an organism survive and reproduce in its environment, such as a camel's fat store in its hump or the dark wing colour of moths on sooty bark. Adaptation and evolution are linked: adaptations build up in a population through natural selection, and the resulting change in traits over generations is evolution. An individual does not adapt in its lifetime; the population does.

What are more natural selection examples?

• Antibiotic resistance: when antibiotics are used, bacteria with a resistance mutation survive and multiply, so resistant strains become common. This is why antibiotics must be taken as prescribed.
• Darwin's finches: on the Galápagos Islands, finch beak size and shape match the seeds available on each island.
• Pesticide resistance: insects with a mutation that lets them survive a pesticide breed, and the next generation is mostly resistant.

Where do new traits come from?

New variation comes from mutations, random changes in DNA, and from the shuffling of genes when sex cells form (see mitosis vs meiosis and how DNA makes a protein). Mutations are random. Selection is not random: the environment decides which variants survive.

Common mistakes

• Saying animals change because they need to. Mutations happen by chance; selection only favours the ones that already help.
• Thinking individuals evolve. Populations evolve over generations; one moth cannot change its colour.
• Thinking evolution has a goal. It has no plan. It only favours what works in the current environment.
• Confusing "fittest" with "strongest". Fitness is about leaving offspring.

Frequently asked questions

What is natural selection in simple terms?

Natural selection is the process by which organisms with inherited traits that suit their environment survive and reproduce more than others. Because they have more offspring, those traits become more common in later generations. It needs variation, inheritance, competition and differential survival.

What is an example of natural selection?

The peppered moth is the standard example. Dark moths were hidden on soot-covered trees in industrial England and survived bird predation more often than pale moths, so the dark form rose to about 98% of the population in Manchester by 1895. When pollution fell, the pale form became common again.

Is natural selection the same as evolution?

No. Natural selection is one mechanism that causes evolution. Evolution is the overall change in inherited traits in a population over generations. Other causes include mutation, gene flow and genetic drift, which is chance change in small populations.

Can natural selection create new traits?

Not directly. New traits arise from random mutations in DNA. Natural selection acts on that variation, increasing helpful versions and removing harmful ones. Over a long time, repeated selection of small changes can build complex features.

Try it yourself

Open the Natural Selection simulator. Start as the bird and press "Start hunting": click the moths you can see against the tree bark. With no soot on the bark, notice which colour you catch first. Then raise the "Soot on the bark" slider, set "Dark moths at the start" low, and press "10 generations (bird)" to watch the dark form take over. Lower the soot again and see the pale form return.