Natural selection cannot act unless variation already exists. This lesson explains where variation comes from, how allele frequency describes a population genetically, and why selection pressure changes populations over generations rather than transforming individual organisms on demand.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Lock in your first instinct before we formalise the mechanism.
1. If 4% of a population carries a resistant allele before treatment, can that percentage change after a strong selection pressure is applied? Why?
2. If one organism survives a harsh environment, has that organism evolved, or has something else changed?
Write your initial answer now. We will revisit it after the allele-frequency and drift comparisons.
Write your starting answer in your book, then return later to compare it with your final explanation.
Wrong: Bacteria and viruses are the same thing.
Right: Bacteria are living cells; viruses are non-living particles that require host cells to reproduce.
Core Content
Connect this concept to the broader biology framework. Understanding how systems interact is essential for HSC success.
Mutation, recombination and gene flow as the raw material of evolution
Natural selection cannot create variation out of nothing. It can only act on differences that already exist in a population.
Mutation is the ultimate source of all new alleles because it changes the DNA sequence itself. Most mutations are neutral, some are harmful, and only occasionally does a mutation become advantageous in a specific environment. Genetic recombination does not create brand-new alleles, but it shuffles existing alleles into new combinations during meiosis and sexual reproduction. Gene flow adds another source of population-level change because migrants bring alleles in and take alleles out.
| Source of Variation | What It Does | Why It Matters |
|---|---|---|
| Mutation | Alters DNA sequence | Ultimate source of new alleles |
| Genetic recombination | Shuffles alleles during meiosis and sexual reproduction | Creates new genotype combinations |
| Gene flow | Moves alleles between populations | Can increase or reduce differences between populations |
How population genetics tracks evolutionary change
Allele frequency is the proportion of a particular allele in the gene pool of a population, often written as a decimal or percentage.
This is important because evolution at the population level can be described as a change in allele frequency over generations. If allele A improves survival or reproduction under a particular selection pressure, individuals carrying A leave more offspring. As a result, A becomes more common in the next generation. That is population evolution in measurable form.
Non-random change versus random change
Suppose 4% of a population carries a drug-resistant allele before antibiotic treatment, and 90% of susceptible individuals die before reproducing. The important conclusion is not the exact arithmetic detail first. It is that the resistant allele becomes much more common in the next generation because selection favoured it.
That makes this an example of natural selection rather than genetic drift. Natural selection is non-random with respect to fitness because the environment consistently favours certain variants. Genetic drift is different. Drift changes allele frequencies by chance, especially in small populations, without a specific adaptive reason.
| Process | Cause of Change | Pattern |
|---|---|---|
| Natural selection | Selection pressure favours some variants over others | Non-random change in allele frequency |
| Genetic drift | Chance events, especially in small populations | Random change in allele frequency |
Activities
A beetle population contains a rare dark-colour allele. A forest fire darkens the tree bark, and birds now spot pale beetles more easily. Explain how the frequency of the dark-colour allele could change over several generations.
Link the environmental change to differential survival and then to allele frequency.
Sketch the before-and-after population in your book first, then summarise the explanation here.
Two small island populations lose many individuals in a storm at random. In a separate case, a pesticide kills insects without a resistance allele. Decide which case is best explained by drift and which by natural selection, and justify each decision.
Focus on whether the change is random or linked to a fitness advantage.
Make a two-case comparison in your book, then record your final judgement here.
Once you think in allele frequencies rather than just visible traits, evolution becomes much easier to explain. Selection pressure does not magically create the useful allele. It changes how common that allele becomes in the population over time.
If your original answer focused on one organism changing, the key correction is this: the individual survives or dies, but the population evolves when allele frequencies shift across generations.
Assessment
Answer first, then read the explanation
1. What is the ultimate source of all new alleles in a population?
What is NOT the ultimate source of all new alleles in a population?
2. What does allele frequency describe?
What is NOT does allele frequency describe?
3. Why does selection pressure increase the frequency of some alleles?
4. Which statement correctly distinguishes genetic drift from natural selection?
5. Which statement best captures why populations rather than individuals evolve?
1. Explain how mutation, recombination and gene flow each contribute to variation in a population. (4 marks)
1 mark: mutation | 1 mark: recombination | 1 mark: gene flow | 1 mark: clear comparative explanation
2. Explain how a selection pressure can increase the frequency of a resistant allele in a population over generations. (3 marks)
1 mark: resistant allele advantage | 1 mark: differential reproduction | 1 mark: frequency increase over generations
3. Distinguish between genetic drift and natural selection using one example or scenario for each. (3 marks)
1 mark: drift defined/example | 1 mark: selection defined/example | 1 mark: clear distinction
Answers
SA1: Mutation contributes to variation by creating new alleles through random changes in DNA sequence. Genetic recombination contributes by reshuffling existing alleles during meiosis and sexual reproduction, producing new genotype combinations. Gene flow contributes by moving individuals and their alleles between populations, adding or removing variation from the local gene pool. Together these processes maintain the raw material on which selection can act.
SA2: If a resistant allele gives carriers an advantage under a selection pressure such as antibiotic exposure, those individuals are more likely to survive and reproduce. Because the allele is inherited, more offspring in the next generation carry it. Over multiple generations, the resistant allele becomes more common in the population gene pool.
SA3: Genetic drift is a random change in allele frequency, especially in small populations. For example, a storm might randomly kill many individuals regardless of whether they carry useful alleles. Natural selection is a non-random change in allele frequency caused by differential survival or reproduction under a selection pressure. For example, pesticide resistance becomes more common when insects carrying the resistance allele survive treatment and reproduce. The key difference is that drift is chance-based, while selection is linked to fitness advantage.
Say each answer aloud before moving to the next prompt