Variation and Allele Frequency
Grant and Grant's 2002 Science paper measured the spontaneous mutation rate in Geospiza beak morphology genes at 10⁻⁷ per base pair per generation. Without this ongoing mutation input, they calculated that standing genetic variation in the Daphne Major population would be exhausted within 200–400 generations. Mutation is the ultimate source of new alleles; L19 closes Module 6 by returning to this foundation and completing the circle: mutation → variation → selection → evolution → biotechnology.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
Know
- The main sources of variation in a population.
- What allele frequency means and how it is expressed.
- The difference between natural selection and genetic drift.
Understand
- Why mutation is the ultimate source of new alleles.
- How selection pressure changes allele frequency over generations.
- Why populations evolve but individuals do not.
Can Do
- Explain how variation enters and moves through populations.
- Interpret a simple quantitative allele-frequency scenario.
- Distinguish non-random selection from random drift.
Core Content
Mutation, recombination and gene flow as the raw material of evolution
Grant and Grant's 2002 Science paper measured that beak morphology genes in Daphne Major's Geospiza fortis population mutate at 10⁻⁷ per base pair per generation. From this rate, they calculated that without ongoing mutation, all standing genetic variation in beak depth — the very variation that drove the 17% beak-depth increase seen after the 1977 drought — would be exhausted within 200–400 generations. Mutation is not just a source of new alleles; it is the continuous low-level replenishment that makes long-term evolution possible.
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.
Mutation
Creates new alleles by changing DNA sequence. The ultimate source.
Recombination
Rearranges existing alleles into new combinations during meiosis.
Gene Flow
Moves alleles between populations through migration of individuals.
| 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 |
Genetic variation in populations arises from mutation (creates new alleles), recombination through meiosis (shuffles existing alleles into new combinations), and gene flow (moves alleles between populations by migration) — each process has a distinct role in maintaining or changing a gene pool.
Pause — copy the highlighted summary into your book before moving on.
How population genetics tracks evolutionary change
We just saw that mutation, recombination and gene flow each contribute differently to variation. That raises a question: how is change in a population measured and what drives it? This card answers it → allele frequency and population-level evolution.
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.
Natural selection can be visualised as a shift in allele frequency from one generation to the next.
Allele frequency is the proportion of a specific allele in the gene pool; evolution at population level is defined as a change in allele frequency over generations — if an allele confers a fitness advantage, carriers leave more offspring and the allele increases in frequency.
Add the highlighted point to your notes before the check below.
Non-random change versus random change
We just saw that allele frequency change defines evolution at population level. That raises a question: how does natural selection driving frequency change differ from random genetic drift? This card answers it → non-random selection vs random drift.
Suppose 4% of a population carries a drug-resistant allele before antibiotic treatment, and 90% of susceptible individuals die before reproducing. 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 |
Natural selection is non-random — the environment consistently favours certain variants so allele frequency changes for a biological reason; genetic drift is random — chance events change allele frequency without a fitness reason, and this distinction is the key exam separator.
Pause — write the highlighted distinction into your book.
Activities
Track the Advantage
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.
Selection or Drift?
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.
Sources of Variation
- Mutation creates new alleles.
- Recombination shuffles alleles; gene flow moves them between populations.
Allele Frequency
- Allele frequency is the proportion of an allele in the population gene pool.
- Population evolution = change in allele frequency over generations.
Selection Pressure
- If an allele gives an advantage, its frequency tends to increase.
- Populations evolve; individuals do not rewrite their DNA in response to need.
Selection vs Drift
- Natural selection is non-random and linked to fitness.
- Genetic drift is random and strongest in small populations.
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(4 marks) 1. Explain how mutation, recombination and gene flow each contribute to variation in a population.
AnalyseBand 3–4(3 marks) 2. Explain how a selection pressure can increase the frequency of a resistant allele in a population over generations.
EvaluateBand 4–5(3 marks) 3. Distinguish between genetic drift and natural selection using one example or scenario for each.
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Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1 — Track the Advantage
The dark-colour allele could become more common because birds now preferentially eat pale beetles that are visible against the darkened bark. Dark beetles survive and reproduce more successfully. Because the dark-colour allele is inherited, more offspring in the next generation carry it. Over several generations of this selection pressure, the allele frequency of the dark-colour allele increases in the population gene pool.
Activity 2 — Selection or Drift?
The storm example is best explained by genetic drift, because the storm randomly kills individuals regardless of whether they carry advantageous alleles. In a small population, this random mortality can cause large, unpredictable changes in allele frequency. The pesticide example is best explained by natural selection, because only insects with the resistance allele survive treatment. The selection is non-random and tied to a specific fitness advantage, causing the resistance allele to increase in frequency in a directed way.
Short Answer Model Responses
SA1 (4 marks): Mutation contributes to variation by creating new alleles through random changes in DNA sequence [1]. Genetic recombination contributes by reshuffling existing alleles during meiosis and sexual reproduction, producing new genotype combinations [1]. Gene flow contributes by moving individuals and their alleles between populations, adding or removing variation from the local gene pool [1]. Together these processes maintain the raw material on which selection can act [1].
SA2 (3 marks): 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 [1]. Because the allele is inherited, more offspring in the next generation carry it [1]. Over multiple generations, the resistant allele becomes more common in the population gene pool [1].
SA3 (3 marks): 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 [1]. 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 [1]. The key difference is that drift is chance-based, while selection is linked to fitness advantage [1].
Mutation = new alleles
Mutation changes DNA and creates new alleles — it is the ultimate source. Recombination only shuffles existing ones.
Allele frequency = evolution measure
Evolution is a change in allele frequencies across a population over generations, not a change in one individual.
Selection = non-random
Selection consistently favours certain alleles because of fitness. Drift is random and strongest in small populations.
Most common exam trap
Saying an individual evolved. Individuals survive or die — the population evolves when allele frequencies shift across generations.
Rapid-fire questions on variation sources, allele frequency, selection pressure and genetic drift. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
Return to Grant and Grant's 2002 Science measurement of the spontaneous mutation rate in Geospiza beak genes at 10⁻⁷ per base pair per generation, and their calculation that standing variation would be exhausted within 200–400 generations without it. You should now be able to explain that this mutation rate is the long-term engine that replenishes the variation pool: without it, selection could only draw down existing allele diversity, and the gene pool would eventually converge on a single uniform genotype. Mutation is the ultimate source of the new alleles that keep evolution, inheritance, and biotechnology possible.
Module 6 closes here: mutation creates new alleles → variation enters the gene pool → natural selection (M3) changes allele frequencies → populations evolve → biotechnology manipulates those same processes deliberately. Every lesson in this module traces back to that first mutation.