No two people on Earth have identical DNA — except identical twins. The differences between our genetic sequences are the raw material of evolution. In this lesson, you will explore where genetic variation comes from, how mutations change DNA, and why this matters for the survival of species.
Think about the people in your classroom. No two of you look exactly alike — even siblings have differences in height, eye colour, hair texture and many other traits.
Now answer: Where do you think these differences come from? If DNA contains instructions for building a body, what could cause two people with the same parents to have different instructions? And can these changes ever be helpful?
Every cell in your body contains a copy of your genome — approximately 3 billion base pairs of DNA. If you compared your DNA letter by letter with anyone else in your class, you would find millions of differences. These differences are called genetic variation.
Genetic variation arises from several sources:
At the molecular level, genetic variation simply means that the DNA sequences of different individuals are not identical. Some variations affect visible traits (like eye colour or height), while others have no obvious effect. But all variation matters because it provides the raw material that natural selection acts upon.
A mutation is any permanent change to the DNA sequence of an organism. Mutations can happen spontaneously during DNA replication, or they can be caused by environmental factors called mutagens — including UV radiation, X-rays and certain chemicals.
At Stage 5, you need to know three types of gene mutation:
Substitution mutations affect only a single codon (a three-base sequence), while insertions and deletions can shift the entire reading frame of the gene. However, at Stage 5 you do not need to classify mutations as missense, nonsense or frameshift — you only need to identify the three basic types and understand their effects.
When a mutation changes a DNA sequence, it may change the protein that the gene codes for. The effect on the organism depends on how important that protein is and how much the mutation alters its function.
Harmful mutations reduce an organism's fitness — its ability to survive and reproduce. For example, a mutation in the gene for haemoglobin can cause sickle cell disease, producing misshapen red blood cells that cannot carry oxygen efficiently. Harmful mutations are often removed from populations over time because affected individuals are less likely to pass them on.
Beneficial mutations increase fitness. For example, a mutation that makes bacteria resistant to an antibiotic allows those bacteria to survive when antibiotics are present. In plants, a mutation that improves drought tolerance can help individuals survive in arid conditions. Beneficial mutations tend to spread through populations over generations because carriers leave more offspring.
Neutral mutations have no effect on fitness. Many mutations occur in non-coding regions of DNA or change an amino acid without affecting the protein's function. Neutral mutations can accumulate in a population and still serve as genetic markers for tracing ancestry.
The cane toad invasion of Australia provides a dramatic example of beneficial mutation in action. Since their introduction to Queensland in 1935, cane toads have spread across northern Australia. Researchers at the University of Sydney have found that toads at the invasion front have longer legs — a trait controlled by genetic variation that allows them to travel faster and colonise new areas. This is not a single mutation but a combination of genetic differences that natural selection has favoured in the expanding population. Australian scientists use this case study to teach how genetic variation and selection interact in real time.
Antibiotic resistance is one of the greatest health challenges facing Australia. When bacteria are exposed to antibiotics, most die — but a few may carry a mutation that makes them resistant. These survivors reproduce, passing the resistance mutation to their offspring. Within days, a resistant population can emerge. Australian hospitals now track resistant strains such as MRSA (methicillin-resistant Staphylococcus aureus) through genomic sequencing. Understanding mutation is essential for developing new treatments and prescribing antibiotics responsibly. The Australian Commission on Safety and Quality in Health Care runs national campaigns to reduce unnecessary antibiotic use and slow the spread of resistance.
Without genetic variation, natural selection would have nothing to work with. If every individual in a population had identical DNA, the population could not adapt to changing environments. Mutations are the ultimate source of all genetic variation — they create the new alleles that natural selection can favour or eliminate.
Consider a population of wallabies living in a forest. A mutation arises in one individual that gives its fur slightly better camouflage against a new predator. If this mutation is heritable, the camouflaged wallabies are more likely to survive and reproduce. Over many generations, the frequency of the camouflage allele increases in the population. This is evolution by natural selection in action — and it begins with a mutation.
Populations with higher genetic diversity are generally more resilient. They contain a wider range of alleles, making it more likely that some individuals will possess traits that help them survive environmental change — whether that change is a new disease, a shift in climate or the arrival of a new competitor.
Tasmanian devil facial tumour disease (DFTD) is a contagious cancer that has devastated wild devil populations since 1996. The cancer spreads when devils bite each other during mating or feeding. Researchers at the University of Tasmania discovered that some devils carry natural genetic variations in genes related to immune function — mutations that help them resist the cancer. These resistant individuals are now being bred in conservation programs to restore genetic diversity to wild populations. This remarkable Australian story shows how genetic variation, generated by mutation, can be the difference between extinction and survival.
Wrong: "Mutations are always harmful and cause diseases like cancer."
Right: Most mutations are neutral. Some are harmful, and a small number are beneficial. Even harmful mutations can persist if they also provide some advantage — for example, the sickle cell mutation protects against malaria in heterozygotes.
Wrong: "Evolution happens because organisms need to adapt, so they develop mutations."
Right: Mutations occur randomly — they do not happen because an organism "needs" them. Natural selection then acts on the variation that already exists. The environment does not direct mutations; it selects among them.
1 Original: A T G C C G A T A
Mutated: A T G CT G A T A
2 Original: T A C G G C T A
Mutated: T A C G G C T T A
3 Original: G C T A A T G C
Mutated: G C T A _ T G C
1 A mutation in a gene produces a protein that works slightly better than the original. Is this mutation harmful, beneficial or neutral? Explain your reasoning.
2 A population of fish lives in a lake. A mutation arises that makes some fish able to tolerate slightly warmer water. If the lake temperature increases over the next 50 years, explain how this mutation could lead to evolutionary change in the population.
3 Explain why a small, isolated population is more vulnerable to extinction than a large population with high genetic diversity. Use the concepts of mutation and natural selection in your answer.
1. What is a mutation?
2. Which type of mutation involves replacing one base with another?
3. A mutation changes a single DNA base but the protein function stays the same. What type is this most likely?
4. Why are mutations important for evolution?
5. A farmer breeds only the largest cattle. Over time, the herd becomes less resistant to disease. What does this illustrate?
6. Explain the difference between substitution, insertion and deletion mutations. 4 MARKS
7. Describe a situation where a mutation could be beneficial, harmful and neutral. Give one example of each. 4 MARKS
8. Explain why populations with higher genetic variation are more likely to survive environmental change. Use the concept of natural selection in your answer. 4 MARKS
Go back to your Think First responses at the top of the lesson.
1. Substitution [1 mark]. The third base from the left in the codon CCG has changed from C to T (CCG → CTG) [1 mark]. Only one base has been replaced.
2. Insertion [1 mark]. An extra T has been inserted after the C in the sequence [1 mark]. This shifts all subsequent bases along by one position.
3. Deletion [1 mark]. The second A in the sequence has been removed [1 mark]. All bases after the deletion point shift left by one position.
1. This is a beneficial mutation [1 mark]. A protein that works better than the original is likely to improve the organism's function or fitness [1 mark]. The organism may survive and reproduce more successfully, passing the beneficial allele to offspring [1 mark].
2. Fish with the warm-water tolerance mutation are more likely to survive as temperatures rise [1 mark]. They will reproduce and pass the beneficial allele to their offspring [1 mark]. Over generations, the frequency of the tolerance allele increases in the population [1 mark]. This is evolution by natural selection acting on genetic variation created by mutation [1 mark].
3. A small isolated population has limited genetic diversity because there are fewer individuals and fewer mutations [1 mark]. If the environment changes, there may be no alleles that confer an advantage [1 mark]. In a large diverse population, some individuals are more likely to carry beneficial alleles that help them survive [1 mark]. Natural selection can then act on this variation, allowing the population to adapt rather than go extinct [1 mark].
1. B — A mutation is a change in the DNA sequence. It is not a repair process, cell division or replication method.
2. A — Substitution involves replacing one base with another. Insertion adds bases; deletion removes bases.
3. C — A mutation that does not change protein function is most likely neutral. Harmful and beneficial mutations affect function.
4. D — Mutations create genetic variation, which provides the raw material for natural selection to act upon.
5. B — Selective breeding for one trait (size) can reduce genetic variation for other traits (disease resistance), making the herd vulnerable.
Q6 (4 marks): A substitution mutation occurs when one base in the DNA sequence is replaced by a different base [1 mark]. An insertion occurs when one or more extra bases are added into the DNA sequence [1 mark]. A deletion occurs when one or more bases are removed from the DNA sequence [1 mark]. All three types permanently change the DNA sequence and can create new alleles [1 mark].
Q7 (4 marks): Beneficial: A mutation in bacteria that makes them resistant to an antibiotic allows them to survive antibiotic treatment [1 mark]. Harmful: A mutation in the haemoglobin gene causes sickle cell disease, producing misshapen red blood cells that cannot carry oxygen efficiently [1 mark]. Neutral: A mutation in a non-coding region of DNA that does not affect any protein or trait [1 mark]. Accept any valid examples with clear reasoning [1 mark].
Q8 (4 marks): Populations with higher genetic variation contain a wider range of alleles [1 mark]. When the environment changes, some of these alleles may confer an advantage that helps individuals survive and reproduce [1 mark]. Natural selection favours these advantageous traits, increasing their frequency in the population over time [1 mark]. Populations with low genetic variation have fewer options for adaptation, making them more vulnerable to extinction when conditions change [1 mark].
Climb platforms using your knowledge of genetic variation, mutations and natural selection. Pool: Lesson 5.
Tick when you have finished all activities and checked your answers.