Why do living things fit where they live so well? Watch hungry birds change a moth population.

What you'll discover

• Moths in a population vary, and birds are more likely to spot and eat the ones that stand out.
• Moths that survive pass their colour on, so better-camouflaged colours become more common.
• When the background changes, the colour that is favoured changes too.

The science in plain words

Natural selection explains why living things are so well suited to their surroundings. It rests on a few simple facts. Individuals in a population vary. Some of that variation is inherited. More young are born than can survive. Individuals whose traits help them survive and breed pass those traits to more offspring, so over generations the useful traits become more common.

The peppered moth in Britain is a famous real example. Most were pale and speckled, well hidden on pale, lichen-covered tree bark. During the Industrial Revolution, soot from factories darkened the trees in many areas. Pale moths now stood out to birds, while a rare dark form was hidden. In polluted regions the dark form became the most common within a few decades. After clean air laws cut pollution in the twentieth century, the trees grew paler again and pale moths became common once more.

Notice that no moth changed its own colour. Birds simply ate the moths that were easiest to see, and the survivors' colours were passed on. Selection does not plan ahead; it only works on variation already present. Over very long periods, these small shifts can add up to large changes and even new species.

Where it fits

• India, NCERT: Natural selection goes beyond the NCERT syllabus for classes 6 to 10; it builds on Class 10 Heredity.
• United States: Grades 6 to 12
• Next Generation Science Standards: MS-LS4-4, MS-LS4-6, HS-LS4-4

Common questions

What is natural selection?

Natural selection is the process by which living things with useful inherited traits survive and reproduce more than others. Those traits are passed on, so they become more common over generations. It explains how populations become adapted to where they live.

What happened to the peppered moths?

In Britain, factory soot darkened tree trunks during the Industrial Revolution. Pale moths became easy for birds to see, while dark moths were hidden, so dark moths became common in polluted areas. When pollution fell in the twentieth century, pale moths became common again.

Do animals change on purpose to survive?

No. Individual animals cannot change their inherited traits on purpose. Instead, individuals vary, and those with helpful traits are more likely to survive and have young. Over many generations this shifts the whole population, without any plan or intention.

Related labs

• Genetic Inheritance: Two purple-flowered plants can have white-flowered babies. How?
• Predator-Prey Dynamics: Why do hare and lynx numbers boom and crash again and again?
• Population Dynamics: One plant becomes two, two become four… Can a population keep doubling for ever?

See all biology labs or every Taz science lab.

Try it yourself

The Natural Selection lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.

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