Life Science · Unit 8: Change over time

Natural Selection in Action

Why do some traits become more common in a population over time? You run a model of birds hunting pale and dark moths on clean and sooty bark, then read real counts of dark peppered moths near Cambridge, England. Afterwards you can explain natural selection, adaptation and artificial selection, and use moth data to support a claim.

Where this lesson sits

  • Virginia SOL LS.11 a
  • Practices LS.1 c, LS.1 d, LS.1 e

Essential question: Why do some traits become more common over time?

  1. Life Science (grade 7) LS.11 a You are here

    This lesson

  2. Biology

    Preview it at high school level

    In Biology you will name the three things natural selection needs: individuals that differ, differences that are inherited, and differences that change how many offspring survive. You will read real beak measurements from finches that lived or died in a drought. You will learn that selection is not the only way a population changes. Genetic drift is change by chance, and gene flow is change when individuals move between groups. You will also learn how one species can split into two when groups stop breeding with each other.

  3. Biology II Biology II: Advanced Survey of Biology Topics, BIOII.5 c-f; Genetics, GEN.6; Ecology, EC.6 a and c

    Preview where it goes next

    Virginia's Biology II courses look at all the forces that change a population's genes: selection, drift, gene flow, mutation and non-random mating. They use the Hardy-Weinberg model, a population with none of these forces, as a baseline. They study how small populations lose variation, how new species form, and how ecologists tell changes caused by selection from changes caused by chance.

  4. College

Watch first Why Some Traits Become Common · 2:39

Supports

Supports change how much help you get, not what you learn.

Peppered moths rest on trees by day, on trunks and branches. Some are pale and speckled and some are nearly black, and birds are more likely to find the moths that stand out.

In smoky industrial areas, soot darkened the trees, and dark moths became common. Later the air got cleaner, and dark moths became rare again, in Britain and in parts of the United States. No moth changed its colour. So what changed?

What do you think? Decide before you read.

  1. A moth can evolve a new colour during its life.

    Scientists disagree: A moth keeps the colour it was born with. Scientists count how common each colour is in a population, year after year. What changes is the share of moths of each colour, as some die and others survive and have young.

    What scientists think

    Scientists disagree: A moth keeps the colour it was born with. Scientists count how common each colour is in a population, year after year. What changes is the share of moths of each colour, as some die and others survive and have young.

  2. Moths change their colour on purpose because they need to hide.

    Scientists disagree: Colour is inherited, and a moth cannot choose it. Scientists found that birds removed a larger share of the colour that showed up more on the bark, so the other colour became more common.

    What scientists think

    Scientists disagree: Colour is inherited, and a moth cannot choose it. Scientists found that birds removed a larger share of the colour that showed up more on the bark, so the other colour became more common.

  3. Natural selection means only the strongest animals survive.

    Scientists disagree: In peppered moths, colour mattered more than strength. Scientists placed pale and dark model moths on trees and checked which ones birds ate. The model that matched the bark survived better. What counts is which traits suit the environment and help an animal have young.

    What scientists think

    Scientists disagree: In peppered moths, colour mattered more than strength. Scientists placed pale and dark model moths on trees and checked which ones birds ate. The model that matched the bark survived better. What counts is which traits suit the environment and help an animal have young.

Explore: be the birds that hunt moths

In this model, pale and dark moths rest on bark. You choose the bark: dark and sooty, or clean and pale with lichen. You set how sharp-eyed the birds are and how many generations to run. Eaten moths have no young, and survivors have young of their own colour. A line graph shows the percent of dark moths in each generation. You can switch the bark in the middle of a run, the way cleaner air did.

Birds and moths: natural selection in a sketch⚙ original · interactive
1. Bark (you can switch it during a run)
2. How sharp-eyed the birds are
3. Generations to run

dark moth pale moth × crossed out = eaten by a bird

simulated (random luck) expected (the rule, no luck)

Generation
0
Dark moths, simulated (young of this generation)
5 of 100
Dark moths, expected (young of this generation)
about 5 percent

In this sketch (made up, not measurements): 100 moths in every generation, and 5 of them dark at the start. Colour is inherited, and young are the same colour as their parent, except that 1 baby moth in 100 is born the other colour by chance (the lesson says new variations appear by chance; it gives no rate, and it does not say this happens both ways). Birds are the only cause of death, and no moths arrive or leave. Chance of being eaten, for a moth that stands out / a moth that matches the bark: less sharp-eyed 20 / 5 percent, sharp-eyed 40 / 10 percent, very sharp-eyed 60 / 15 percent. The graph holds up to 90 generations.

Rule to keep: birds find more of the moths that stand out from the bark, so the colour that matches the bark becomes more common over generations. No moth changes its colour.

Try this.

  1. Predict the line on sooty bark if the population starts with only a few dark moths. Run 30 generations and check.
  2. Partway through the run, switch to clean bark. Predict the shape of the line before you press play.
  3. Make the birds less sharp-eyed. Predict whether the change will be faster or slower, then test it.

Explain: how natural selection works

  • population
  • environment
  • generation
  • trait frequency
  • natural selection
  • adaptation
  • artificial selection
  • evolution

A population is a group of the same kind of living thing in one area. The peppered moths in one wood are a population. An environment is everything around a living thing, such as bark, air, food and predators. A generation is one step from parents to their offspring.

Members of a population are not all the same. Some moths are pale and some are dark, and the colour is inherited. New variations first appear by chance, as you saw in that lesson.

Natural selection is the process in which individuals with traits that suit their environment are more likely to survive and reproduce. Here is how it works in moths.

  1. Moths in the population vary in colour.
  2. Birds spot more of the moths that stand out from the bark.
  3. Moths that are not eaten live to reproduce.
  4. Their offspring inherit their colour.
  5. Over generations, the colour that matched the bark becomes more common.

Trait frequency is how common a trait is in a population, often given as a percent. Natural selection shifts trait frequency. It does not change any one moth. As an environment changes, some organisms survive and reproduce. Others may move to a new place or die.

AI-generated illustration: four panels of the same dark sooty bark. In the first, pale and dark peppered moths rest in equal numbers. In the second, a bird takes a pale moth that stands out. In the third, the surviving moths, mostly dark, have small young of their own colour. In the fourth, the next generation has more dark moths than pale. Four numbered pointers mark one step in each panel.
  1. moths vary in colour
  2. birds spot moths that stand out
  3. survivors reproduce
  4. the next generation
  1. moths vary in colour
  2. birds spot moths that stand out

Word bank: survivors reproduce, the next generation

Check your labels
  1. moths vary in colour
  2. birds spot moths that stand out
  3. survivors reproduce
  4. the next generation
Natural selection on sooty bark, step by step. Name each numbered step, and notice that no moth ever changes its colour.

You may hear “survival of the fittest.” In biology, fit means well suited to the environment and able to leave offspring. It does not mean strongest. A dark moth on sooty bark is fit because birds miss it more often.

Key idea: Individuals with traits that suit their environment survive and reproduce more often, so those traits become more common over generations.

Explain: adaptations, breeders and evolution

An adaptation is a structure, function or behaviour that comes from natural selection and helps an organism survive and reproduce. A dark colour on sooty bark was an adaptation. On clean, pale bark, the same colour was not. Whether a trait helps depends on the environment.

People can shift traits too. Artificial selection is when people choose which plants or animals breed, so the traits they want become more common. Dog breeds, from tiny Chihuahuas to huge Great Danes, all came from breeding one kind of animal. Corn came from a wild grass called teosinte, after farmers kept seeds from the plants they liked best. The idea is the same as natural selection: some individuals breed and others do not. The difference is who decides.

AI-generated illustration: a slim wild teosinte grass with a small seed head on the left and a corn plant with a large ear on the right, joined by three gold arrows with hands choosing kernels beneath them. An inset shows a tiny Chihuahua beside a huge Great Dane.
Artificial selection: people choose which plants or animals breed. Over many generations, a wild grass became corn, and one kind of animal became many dog breeds.
Natural selectionArtificial selection
What decides who breedsThe environment, such as predators or weatherPeople
ExampleDark moths on sooty barkDog breeds, corn
Time neededMany generationsMany generations

Evolution is a change in the inherited traits of a population over many generations. Natural selection drives evolution. If no individuals have traits that suit a changed environment, the population may die out, as you will see in a later lesson. Scientists also have fossils and DNA as evidence for evolution, which that lesson explores.

Key idea: Adaptations come from natural selection, people use the same idea in artificial selection, and natural selection drives evolution.

Elaborate: read the moth counts

Each year near Cambridge, England, moths were caught in light traps and the dark ones counted.

YearDark moths (percent of moths caught)
200112
200210
20036
20047
20054
20062
20071

Data from Cook, Grant, Saccheri and Mallet (2012), Table 2: wild peppered moths in light-trap samples at Madingley, near Cambridge, England, 2001 to 2007. The authors read these values from a graph by Michael Majerus, so each one is rounded.

From 2002 to 2007, Majerus released pale and dark moths nearby and checked which ones were gone. Birds removed a larger share of dark moths than of pale moths.

AI-generated illustration: on the left, at night, pale and dark moths fly toward the lamp of a moth trap, with an inset of caught moths sorted by colour. On the right, in morning light, pale and dark moths rest on a tree's branches and trunk, one resting spot is empty, a bird eats a dark moth, and a person with a clipboard checks the tree.
Two ways the Madingley moths were studied: light traps counted each colour form at night, and released moths were checked hours after sunrise to see which ones birds had taken.
  1. Describe the trend from 2001 to 2007. In which year does the count break the trend?
  2. By how many percentage points did the dark form drop from 2001 to 2007? (Subtract the 2007 percent from the 2001 percent.)
  3. Find the mean percent of dark moths for the seven years.
  • The percent of dark moths went from in 2001 to in 2007.
  • This happened because birds , so moths had more young.

Claim, evidence, reasoning

Use these data to make a claim that answers the essential question for the Madingley moths: why do some traits become more common over time?

Start with: The data show that the form of the peppered moth became common over time.

Start with: In 2001, percent of moths caught were dark. By 2007, only percent were dark.

Start with: On clean bark, birds spot moths more easily, so moths survive and pass on their colour to .

Compare with a model answer

The dark form of the peppered moth became less common at Madingley. In 2001, about 12 percent of moths caught were dark, and by 2007 only about 1 percent were. On clean bark, birds spot dark moths more easily, and in a nearby experiment birds removed a larger share of dark moths. Pale moths survived and passed on their colour. One limit: this is one place over seven years, and the values were read from a graph, so they are rounded.

Design problem

Plan a classroom model to test this question: does the colour of the background change which paper moths “predators” find first?

One way to do it

Cut 40 paper moths, each about 2 centimetres wide: 20 white and 20 black. The independent variable is the background: a sheet of white paper or a sheet of black paper, each 50 centimetres by 50 centimetres. The dependent variable is how many moths of each colour a classmate picks up.

Keep these the same: moth size and shape, the number of each colour, a picking time of 10 seconds, and a distance of 1 metre to the sheet. The picker looks away while you spread the moths. Run the white sheet and the black sheet as two cases to compare. Repeat with five different classmates and find the mean for each colour on each background. If fewer black moths are picked on black paper, and fewer white moths on white paper, background matching helps the moths survive. Safety: use safety scissors, and do not use anything to eat.

Evaluate: check your thinking

No moth changed colour. Dark moths first appeared as a rare, inherited form. Where soot darkened the bark, birds were more likely to find pale moths, so dark moths had more young and became common. When the air got cleaner, birds found the dark moths more easily, and the pale form became common again.

Science practices you used: planning a fair test with a comparison and repeated trials, reading a data table and finding a mean, using a model and naming its limits, and making a claim from evidence.

How we measure it

Light-trap surveys

A bright lamp draws night-flying moths into a trap, and researchers count each colour form. Counts from year to year show whether a form is becoming more or less common.

Release-and-watch predation experiments

Researchers place moths of each colour on natural resting spots, then check hours later which ones are missing and watch for birds eating them. They compare the share of each colour that disappears.

Model prey

Paper moths with a small edible body are pinned on tree trunks and checked over two days. Because the models differ only in colour, any difference in how many are eaten points to camouflage.

Finish line

Test yourself

8 questions

Practice and MCAT-style, with explained answers.

Next · Lesson 29

Evidence for Evolution

How can a scallop shell millions of years old, a bat wing and a protein in your cells all tell the same story?

Continue →

← Lesson 27 · Mutations and Variation

Michael Majerus

Michael Majerus

Recorded where wild peppered moths rest and ran a six-year experiment near Cambridge, England, that counted which pale and dark moths birds removed.

Michael Majerus studied moths near Cambridge, England. His last experiment tested whether birds take pale and dark peppered moths at different rates. While setting up the experiment he searched for wild moths and wrote down where each one was resting. Just over half were on branches, and many were on tree trunks. From 2002 to 2007, he released 4,864 moths, a few at a time, onto natural resting places. A few hours after sunrise he checked which moths were gone, and he sometimes saw birds eating them. He shared his results at a meeting in Sweden in 2007. He died in 2009 before he could publish them. In 2012 four colleagues analysed the results he had shared and published them, so his careful counting still adds to what we know.

✓ verbatim Cook, Grant, Saccheri and Mallet, Selective bird predation on the peppered moth: the last experiment of Michael Majerus, Biology Letters 8:609-612 (2012).
Test yourself
Question 1 of 8 Score 0 / 0

Which statement describes natural selection?
Sources & Integrity

Every structure, slide, simulation, and clip on this page is listed below with the source it came from. None of the 3D structures is drawn by hand or by AI: each one loads by accession ID from RCSB PDB or PubChem. This panel is generated from the lesson's own data file, so what it lists is what the page loads.

Simulations

  • Moths and birds · Original interactive — Biochemistrypedia (Deppmann Lab).

Scientist profiles & quotes

  • Michael Majerus — Cook, Grant, Saccheri and Mallet, Selective bird predation on the peppered moth: the last experiment of Michael Majerus, Biology Letters 8:609-612 (2012).

Quotes are verbatim from the cited source. Profile text is original to this site and was checked against the sources cited.

Animated explainer

Why Some Traits Become Common · narration: an AI narrator voice (Gemini, Charon) reading narration written from this lesson's text · animation: AI-generated in code (Manim) by Claude, a schematic with no drawn molecular structures, plots computed from their equations · captions from the narration script.

Images & figures

3 illustrations in this lesson are AI-generated: drawn by an image model from a written spec, then checked by a separate AI judge against the spec and the lesson text. None shows a chemical structure or data.

  • four panels of the same dark sooty bark. In the first, pale and dark peppered moths rest in equal numbers. In the second, a bird takes a pale moth that stands out. In the third, the surviving moths, mostly dark, have small young of their own colour. In the fourth, the next generation has more dark moths than pale. Four numbered pointers mark one step in each panel.
  • a slim wild teosinte grass with a small seed head on the left and a corn plant with a large ear on the right, joined by three gold arrows with hands choosing kernels beneath them. An inset shows a tiny Chihuahua beside a huge Great Dane.
  • on the left, at night, pale and dark moths fly toward the lamp of a moth trap, with an inset of caught moths sorted by colour. On the right, in morning light, pale and dark moths rest on a tree's branches and trunk, one resting spot is empty, a bird eats a dark moth, and a person with a clipboard checks the tree.

Every 3D structure is rendered live from an accession ID, and every plot is drawn from its equation in the browser.

integrity Machines check the mechanics. The build confirms that every local file exists, that every question's answer key points at a real choice, and that any AI-generated scientific figure names the person who checked it; a weekly job re-resolves every accession ID against its database. Questions are original; MCAT-style items are written for this site and are not real AAMC questions. Banned sources (the Tymoczko textbook, publisher decks, copyrighted figures) are not used.

Life Science (grade 7)

Members of a population vary, and some of those differences are inherited. In a given environment, individuals with some traits are more likely to survive and reproduce. They pass those traits to their offspring, so the traits become more common over generations. This is natural selection. Others may move away or die. Traits shaped this way are adaptations. People can shift traits too, by choosing which plants or animals breed. Peppered moths show natural selection: dark moths became common where soot darkened trees and rare again after the air got cleaner.
Biology

In Biology you will name the three things natural selection needs: individuals that differ, differences that are inherited, and differences that change how many offspring survive. You will read real beak measurements from finches that lived or died in a drought. You will learn that selection is not the only way a population changes. Genetic drift is change by chance, and gene flow is change when individuals move between groups. You will also learn how one species can split into two when groups stop breeding with each other.
Biology II

Virginia's Biology II courses look at all the forces that change a population's genes: selection, drift, gene flow, mutation and non-random mating. They use the Hardy-Weinberg model, a population with none of these forces, as a baseline. They study how small populations lose variation, how new species form, and how ecologists tell changes caused by selection from changes caused by chance.
College

A college course treats natural selection as a hypothesis to test. You show that a trait varies and is inherited, measure survival or reproduction, and name the agent of selection, such as bird predators. The peppered moth is a classic test case, now backed by predation experiments and bird-vision models. College courses also measure the strength of selection with numbers, compare species concepts and reproductive barriers, and design studies that separate selection from genetic drift and gene flow.
Moths made of paper and pastry

In June 2017, Olivia Walton pinned 500 model moths onto tree trunks covered in lichen (a crusty, pale grey-green growth) in southern England. Each had paper wings and a small edible body. Half looked like pale moths and half like dark moths. Birds ate some of them. After 48 hours, the estimated chance of surviving was 0.64 for pale models and 0.43 for dark ones. Photos of real moths and lichen bark, analysed for how birds see colour, showed pale moths matched that bark better.

— Walton and Stevens, Avian vision models and field experiments determine the survival value of peppered moth camouflage, Communications Biology 1:118 (2018).
Where do peppered moths really rest?

Many textbook pictures show peppered moths sitting on tree trunks. But where do wild moths rest when no one has placed them? Michael Majerus searched for wild moths near Cambridge, England. Of 135 resting moths he found, 70 were on branches, 48 were on trunks and 17 were under or among twigs. Moths do rest on trunks, but more of the ones he found rested on branches.

— Cook, Grant, Saccheri and Mallet, Biology Letters 8:609-612 (2012), Table 1.

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