Life Science · Unit 8: Change over time
Mutations and Variation
Why is no living thing exactly like another, and are all mutations bad? You run a model beetle population where new colour variations appear by chance, then read real height data for 12-year-olds and a table of real mutations. Afterwards you can explain where variation comes from, which mutations pass to offspring, and why a mutation can help, harm or make no difference.
Where this lesson sits
- Virginia SOL LS.11 a
- Virginia SOL LS.11 c
- Practices LS.1 b, LS.1 c, LS.1 d, LS.1 e
Essential question: Why is no living thing exactly like another, and are all mutations bad?
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Life Science (grade 7) LS.11 a, LS.11 c You are here
This lesson
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Biology
Preview it at high school level
In Biology you will learn more ways a population gets its variation. Mutation makes new alleles. Gene flow brings alleles in when animals or pollen move from one population to another. Meiosis and fertilization shuffle alleles into new mixes. You will also learn that populations make more young than the environment can support, so many die before they reproduce. Individuals whose variations suit the conditions leave more young. You will measure this with real survival data, such as counts of tuskless elephants before and after years of ivory poaching.
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Biology II Biology II: Advanced Survey of Biology Topics, BIOII.5 a and b; Ecology, EC.5 c
Preview where it goes next
In Biology II: Advanced Survey of Biology Topics, you name selective pressures, such as predators, disease, climate and hunting, and connect a changing environment to changes in how common each allele is in a population. You also see why a population with more variation is more likely to have some members that survive a change. The Ecology course looks at how populations grow, what limits them, and how the way a species reproduces shapes which young survive.
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College
Watch first Why No Two Living Things Match · 2:30
Pick ten leaves from one white oak and lay them in a row. Every leaf has rounded lobes, but measure them with a ruler and you will almost surely find different lengths and shapes. Your class is the same. Everyone has two hands and one nose, yet heights and hair differ. Where do these differences come from, and do they matter?
What do you think? Decide before you read.
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Scientists disagree: A mutation can help, harm or make no difference. Scientists have found DNA changes that let some adults keep digesting milk sugar, and others that let bacteria grow with an antibiotic. They compare DNA with traits in many people and test bacteria in the lab.
What scientists think
Scientists disagree: A mutation can help, harm or make no difference. Scientists have found DNA changes that let some adults keep digesting milk sugar, and others that let bacteria grow with an antibiotic. They compare DNA with traits in many people and test bacteria in the lab.
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Scientists disagree: Only changes in sex cells, the eggs and sperm, can reach a child. Scientists have found mutated patches of skin cells in people without skin cancer, more often in sun-exposed skin. Those changes stay in the skin and are not inherited.
What scientists think
Scientists disagree: Only changes in sex cells, the eggs and sperm, can reach a child. Scientists have found mutated patches of skin cells in people without skin cancer, more often in sun-exposed skin. Those changes stay in the skin and are not inherited.
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Scientists disagree: Some mammals change their coat with the seasons, but that is their existing genes responding to the season. New genetic variations arise by chance, before they are needed. In a 1943 experiment, bacteria that could resist a virus were already there before the virus was added.
What scientists think
Scientists disagree: Some mammals change their coat with the seasons, but that is their existing genes responding to the season. New genetic variations arise by chance, before they are needed. In a 1943 experiment, bacteria that could resist a virus were already there before the virus was added.
Explore: watch new colours appear in a beetle population
In this model, 30 beetles vary in body colour and size. You set how often mutations happen, from low to high, where high stands for more radiation or chemicals. You choose whether a mutation happens in a body cell or a sex cell, then press “Next generation” and watch a bar graph of how many beetles carry each colour.
- ring: a new mutation in this generation
- small dot: a patch of body cells with a new colour
Made up for this sketch, not measured: one gene sets body colour, and a mutation turns a beetle’s colour into one of the other four at random. Low, medium and high mean about 1 in 20, 1 in 8 and 1 in 4 new beetles get a mutation. On the background shown, helpful beetles survive 9 times in 10, harmful 4 in 10, and neutral ones 7 in 10, the same chance as the starting colour, green. Which colour is helpful, harmful or neutral on each background is also made up. Survivors have young at random, with no mates. Size varies at random and does not change survival. A patch does not change survival either.
Rule to keep: mutations arise by chance, a mutation does not “know” whether it will help, only a mutation in a sex cell can pass to young, and whether a colour helps depends on where the beetle lives. This is a sketch of the logic, not a measurement.
Try this.
- Predict whether a new colour will appear in the same place each time. Set the rate to medium and run it three times.
- Predict what happens to a body-cell mutation at the next generation. Then try a sex-cell mutation and compare.
- Change the background. Predict whether a colour that was helpful stays helpful. Check the tags.
Explain: same features, slightly different
- trait
- population
- variation
- mutation
- body cells
- sex cells
- random
A trait is a feature of a living thing, such as colour, shape, body covering or height. A population is a group of the same kind of living thing in one area. All white oaks have leaves with rounded lobes. But each leaf, and each tree, differs a little. Variation is these small differences among members of a population.
Where does variation come from? Some comes from genes. Each child gets a new mix of the parents’ alleles when sex cells form and join, as you saw in Reproduction and Meiosis. Some variation comes from the environment.
Height shows both. Many genes add a little to height. Food and health during childhood matter too. So two children with very similar genes can grow to different heights. Many traits work this way: inheritance and the environment act together. You can sort traits into these kinds in Inherited or Acquired?.
Explain: what a mutation is and where it goes
A mutation is a change in the DNA sequence, the order of the bases. Recall from DNA: The Code of Life that a gene carries the code for a protein. A mutation can change that protein. It can also change how much of it a cell makes. Sometimes the protein works the same as before.
Mutations can be caused by:
- mistakes when a cell copies its DNA before dividing
- radiation, such as ultraviolet light from the Sun
- some chemicals
Mutations are random. Here, random means they happen by chance, not because a living thing needs them. A mutation does not “know” whether it will help.
Where the mutation happens matters. Body cells are all the cells of the body except eggs and sperm, such as skin, muscle and bone cells. Sex cells are eggs and sperm. A mutation in a body cell stays in that person. A mutation in a sex cell can be passed to offspring.
- skin cell with a mutation
- the mutation stays in the parent
- sex cell with a different mutation
- the child carries the sex-cell mutation in every cell
- skin cell with a mutation
- blank
- blank
- blank
Word bank: sex cell with a different mutation, the child carries the sex-cell mutation in every cell, the mutation stays in the parent
- blank
- blank
- blank
- blank
Check your labels
- skin cell with a mutation
- the mutation stays in the parent
- sex cell with a different mutation
- the child carries the sex-cell mutation in every cell
Explain: helpful, harmful or neither
Does a mutation help? That depends on the living thing and where it lives. Here is a comparison: a mutation is like a misprint in a recipe. Most misprints change little, some ruin the dish, and a rare one makes it better. The comparison breaks here: a recipe has a writer, but no one chooses a mutation.
The variation that mutations create may or may not help an individual survive and reproduce. Individuals that happen to carry a helpful variation are more likely to survive and pass it on. You will see how this changes a population in Natural Selection in Action. Over many generations, heredity keeps most traits steady. Mutations add the new variation that leads to the diversity of life.
Elaborate: read real height data and real mutations
| Percentile | Boys’ height (cm) | Girls’ height (cm) |
|---|---|---|
| 5th | 137.3 | 139.2 |
| 10th | 139.9 | 142.0 |
| 25th | 144.3 | 146.5 |
| 50th | 149.3 | 151.5 |
| 75th | 154.4 | 156.4 |
| 90th | 159.0 | 160.8 |
| 95th | 161.9 | 163.5 |
Heights are for children just over 12 years old. They come from the CDC 2000 growth charts (file statage.csv). The CDC built them mostly from U.S. surveys taken from 1963 to 1994. A 25th percentile height means about 25 percent of children in the reference were that height or shorter. These are reference values the CDC calculated from the surveys, not the heights of seven real children.
| DNA change | Living thing | What it does | Effect |
|---|---|---|---|
| Keeps adults making lactase, a milk-digesting protein | People in East Africa (study 2007) | Adults can digest milk sugar | Helpful where people drink milk |
| Change in a bacterial gene called rpoB | E. coli bacteria in a lab (2023) | Bacteria grow with the antibiotic rifampicin | Helpful to bacteria with the drug |
| Loss of a protein needed for dark colour | Cane toads in captivity, gene-edited (2025) | Albino toads competed and fed worse | Harmful |
| Change in a gene called ABCC11 | People (2006) | Dry earwax instead of wet | No survival effect shown |
| Sun-linked change in skin cells | People (1996) | Small patches of mutated skin cells | Body cells only; not inherited |
Sources: Tishkoff and colleagues (2007), Soley and colleagues (2023), Funk and colleagues (2025), Yoshiura and colleagues (2006), and Jonason and colleagues (1996). In the bacteria study, which mutant grew best changed when the food changed.
- Find the difference between the 5th and 95th percentile heights for boys. Then do the same for girls.
- Compare the 50th percentile for boys and girls. Which is taller, and by how much?
- Which rows of the mutation table show that a mutation’s effect depends on the environment?
- The heights vary from blank cm to blank cm, which shows blank.
- The blank mutation was helpful because blank, but the blank mutation was harmful because blank.
Claim, evidence, reasoning
Make a claim that answers the essential question: why is no living thing exactly like another, and are all mutations bad?
Start with: Living things vary because blank, and mutations are blank.
Start with: For boys, height ranged from blank cm at the 5th percentile to blank cm at the 95th. The table shows blank.
Start with: This variation matters because blank, and a mutation's effect depends on blank.
Compare with a model answer
Living things vary because of their genes and their environment, and mutations are not all bad. For 12-year-old boys, height runs from 137.3 cm at the 5th percentile to 161.9 cm at the 95th, a spread of 24.6 cm. The mutation table shows a helpful change (lactase in adults who drink milk), a harmful one (albino toads) and one with no survival effect shown (earwax). A limit: the height data come from U.S. surveys from 1963 to 1994, and five mutations are a tiny sample of all mutations.
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Design problem
Plan a study of how much white oak leaves vary. Do leaves from the sunny side of one tree differ in length from leaves on the shady side?
One way to do it
- Question: Are oak leaves from the sunny side of a tree longer or shorter than leaves from the shady side?
- Independent variable: the side of the tree, sunny or shady.
- Dependent variable: leaf length in centimetres, measured from where the flat part of the leaf meets the stalk to the tip.
- Constants: one tree, leaves collected on the same day, the same ruler and the same start and end points.
- Control: comparing the two sides of the same tree, so the tree’s genes are the same.
- Repeated trials: pick 20 leaves from low branches on each side, then repeat with a second tree.
- Safety: pick leaves only from branches you can reach from the ground, wash hands afterwards, and never climb the tree.
If one side’s leaves are clearly longer, the environment affects leaf length. The spread within each side shows variation even with the same genes.
Evaluate: check your thinking
No two oak leaves or students are exactly alike, because genes and the environment both shape each trait. New genetic variation starts with mutations, which arise by chance. Some help, some harm and many make little difference, and only those in sex cells reach offspring.
Science practices you used: making predictions with a model, reading and comparing data in tables, calculating a difference, planning a fair test with constants and repeated trials, and naming the limits of data.
How we measure it
Measuring height and ranking it in percentiles
Health workers measure children's height in centimetres with a fixed ruler. The CDC sorted measurements from large U.S. surveys into percentiles, which show where a height falls compared with others of the same age and sex.
Reading DNA
Scientists read the order of the bases in a stretch of DNA from many people or animals. They compare the DNA with a trait, such as earwax type, to find which DNA change goes with which trait.
Growing bacteria in the lab
Bacteria are grown in dishes or tubes with and without an antibiotic, a medicine that stops bacteria growing, and their growth is measured over time. This shows whether a DNA change helps them grow under each condition.
Finish line
Test yourself
8 questions
Practice and MCAT-style, with explained answers.
Next · Lesson 28
Natural Selection in Action
Why did nearly black peppered moths become common in smoky industrial areas, and then become rare again once the air got cleaner?
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