Genetic Variation and Mutations
In 2019, MRSA golden staph killed over 100,000 people worldwide, all because a single DNA base substitution gave bacteria resistance to an antibiotic invented in 1959.
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Q1 · What is a mutation? Write down anything you already know about what causes mutations and what effects they can have.
Think about whether all mutations are harmful, and what role they might play in evolution.
Q2 · A population of insects lives in an area where pesticide use is increasing. How might mutations help some insects survive?
Consider what would happen if a random mutation made an insect resistant to the pesticide.
● Know
- That genetic variation is caused by differences in DNA sequences
- That a mutation is a change to the DNA sequence
- The three main types of mutation: substitution, insertion and deletion
- That mutations can be harmful, beneficial or neutral
● Understand
- How mutations create genetic diversity within populations
- Why genetic variation is essential for natural selection
- The difference between harmful, beneficial and neutral mutations
● Can do
- Identify substitution, insertion and deletion mutations from DNA sequences
- Explain how mutations lead to genetic variation in a population
- Evaluate whether a mutation is likely to be harmful, beneficial or neutral
A hospital patient takes a full course of antibiotics; 99.9% of bacteria die, but a handful survive and multiply. That survivor population carries a changed DNA sequence. A mutation is any permanent change in the DNA sequence. Mutations arise from copying errors during DNA replication, from exposure to radiation or certain chemicals (mutagens), or from viral insertion into the genome. The simplest type is a substitution, where one base is swapped for another. Insertions and deletions add or remove bases, which can shift the reading frame and alter every amino acid downstream, often with severe consequences.
Most mutations are neutral: they either occur in non-coding DNA or do not change the amino acid sequence (thanks to the redundancy of the genetic code). Some mutations are harmful, disrupting essential proteins and causing diseases like cystic fibrosis or cancer. A rare few are beneficial, giving the organism an advantage in its environment. All genetic variation, the raw material of evolution, ultimately comes from mutation.
The CCR5-delta32 mutation is a 32-base deletion in a human gene. People with two copies of this mutation are highly resistant to HIV infection because the virus cannot enter their cells. This mutation was probably neutral or mildly harmful when it first appeared, but it became beneficial when HIV emerged. Evolution often repurposes existing mutations in new environments.
Australian research: Scientists at the Menzies School of Health Research in Darwin study how genetic mutations affect susceptibility to tropical diseases like melioidosis. Understanding which mutations protect against infection helps researchers develop new treatments and vaccines tailored to northern Australian populations.
Many students think all mutations are harmful or cause visible deformities. In reality, the vast majority of mutations have no effect at all. Your body acquires thousands of mutations every day, and most are repaired or are silent. The idea that mutation equals bad is a caricature from science fiction, not biology.
Match each mutation type to its effect on the DNA sequence.
Mutations create new alleles, but most genetic variation in sexually reproducing species comes from reshuffling existing alleles. Three mechanisms do this. Crossing over during meiosis I swaps segments between homologous chromosomes, creating chromosomes with allele combinations never seen before. Independent assortment randomises which maternal and paternal chromosomes end up in each gamete. Finally, random fertilisation means any sperm can fuse with any egg, adding another layer of chance.
Together, these three processes ensure that every human gamete is genetically unique (except in identical twins). The number of possible human genetic combinations is astronomically large, far greater than the number of people who have ever lived. This variation is essential because it provides the raw material on which natural selection acts.
Consider a couple having two children. Each child receives a random half of Mum's genes and a random half of Dad's genes, with crossing over remixing the chromosomes first. The chance that the two children are genetically identical (without being twins) is effectively zero. Even full siblings share only about 50% of their variable DNA on average.
Australian agriculture: Wheat breeders at CSIRO exploit genetic recombination to combine desirable traits from different wheat varieties. By crossing plants and selecting offspring with the best combinations of disease resistance and yield, they accelerate what would take centuries of natural selection.
Genetic variation is the foundation of evolution. Without differences between individuals, every member of a population would respond identically to environmental challenges, and if that response were inadequate, the entire population could go extinct. Variation spreads risk. In a genetically diverse population, some individuals may tolerate heat better, some may resist disease better, and some may find new food sources more easily.
When the environment changes, individuals with advantageous traits tend to survive and reproduce more successfully. Over generations, those advantageous alleles become more common in the population. This is natural selection, and it cannot occur without pre-existing variation. Mutation and recombination continuously replenish the variation pool, ensuring that populations have the genetic resources needed to adapt.
The peppered moth in England is a classic example. Before the Industrial Revolution, light-coloured moths were common because they camouflaged against pale tree bark. Dark-coloured moths were rare. When pollution darkened the bark, the dark moths had better camouflage and survived more often. The variation already existed; the environment simply changed which variant was favoured.
Australian conservation: Researchers at the Australian Museum study genetic diversity in koala populations. Low diversity makes koalas vulnerable to disease and climate change. Conservation biologists use genetic data to design breeding programs and habitat corridors that maintain healthy variation across fragmented landscapes.
Students sometimes think variation appears because organisms 'need' it. This is backwards. Variation arises randomly through mutation and recombination; the environment then selects which variants survive. Organisms do not generate variation on purpose in response to a challenge. Random mutation, non-random selection, that is the core of Darwinian evolution.
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 in Australian Hospitals
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.
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 and have no noticeable effect. Some are harmful, and a small number are beneficial. Natural selection acts on this variation.
Wrong: 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.
Right: Mutations are random changes in DNA. Most are neutral, some harmful and a few beneficial. Natural selection determines which variations persist in a population.
Wrong: "Evolution happens because organisms need to adapt, so they develop mutations."
Right: Mutations occur randomly. Natural selection acts on existing variation, the environment does not direct or cause specific mutations.
Wrong: 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.
Right: Evolution is not directed by need. Mutations happen randomly, and natural selection favours those that happen to be advantageous in a given environment.
Identify the Mutation
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
Evaluate Mutation Effects
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.
Copy Into Your Book
▼Genetic Variation
- Genetic variation = differences in DNA sequences
- Sources: sexual reproduction, mutations, gene flow
- Variation provides raw material for natural selection
- More diversity = greater ability to adapt
Types of Mutation
- Substitution = one base replaced by another
- Insertion = extra base(s) added
- Deletion = base(s) removed
- All three change the DNA sequence permanently
Mutation Effects
- Harmful = reduces survival or reproduction
- Beneficial = increases survival or reproduction
- Neutral = no effect on survival or reproduction
- Most mutations are neutral
Mutations and Evolution
- Mutations are the ultimate source of genetic variation
- Natural selection acts on variation
- Advantageous mutations spread in populations
- Random mutation + non-random selection = evolution
At the start of this lesson you were confronted with the case of Golden Staph, a bacterium that gained antibiotic resistance through a single mutation, going from easily treated to potentially deadly. That story was meant to show you how powerful even one change to a DNA sequence can be.
Now that you understand what mutations are, what causes them, and how some can be neutral, harmful or even beneficial, revisit the Golden Staph example. Can you now explain in molecular terms exactly how that resistance arose and why it spread through the population?
Q1. Explain the difference between substitution, insertion and deletion mutations. 4 MARKS
Q2. Describe a situation where a mutation could be beneficial, harmful and neutral. Give one example of each. 4 MARKS
Q3. 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
Revisit Your Initial Thinking
Go back to your Think First responses at the top of the lesson.
- Did you correctly identify that differences between siblings come from genetic variation, including mutations?
- Did you recognise that mutations can sometimes be beneficial, not just harmful?
- Write one sentence explaining why genetic diversity is like an insurance policy for a species.
Model answers (click to reveal)
Comprehensive Answers
▼Activity 1, Identify the Mutation
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.
Activity 2, Evaluate Mutation Effects
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].
Multiple Choice
1. BA mutation is a change in the DNA sequence. It is not a repair process, cell division or replication method.
2. ASubstitution involves replacing one base with another. Insertion adds bases; deletion removes bases.
3. CA mutation that does not change protein function is most likely neutral. Harmful and beneficial mutations affect function.
4. DMutations create genetic variation, which provides the raw material for natural selection to act upon.
5. BSelective breeding for one trait (size) can reduce genetic variation for other traits (disease resistance), making the herd vulnerable.
Short Answer Model Answers
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].