The shape of the lesson
By the end of the lesson, learners can
- Overproduction, competition, survival: Choose stabilising, then press Run 10 generations and compare the yellow histogram with the grey starting population.
- Chance: drift, founders and bottlenecks: With genetic drift chosen, set Population size to 30 and press Run 60 generations: watch the five lines wander.
- Antibiotic resistance: selection you can watch: Antibiotic resistance follows the same logic as directional selection in the simulator, but in bacteria, which can divide every 20 minutes.
- Overproduction and competition lead to a struggle for existence.
- Best adapted survive, reproduce and pass on alleles: allele frequency rises.
- Stabilising keeps the mean; directional shifts it; disruptive splits it.
- Drift, founder, bottleneck: chance changes, biggest in small populations.
One picture every idea returns to
Populations have the capacity to produce many more offspring than can survive, so individuals compete for limited resources such as food, mates and territory. Because of variation, some individuals have alleles that make them better adapted to the conditions. In this struggle for existence they are more likely to survive and reproduce and pass on their alleles, so the frequency of those alleles rises in the next generation. Environmental factors act as selection pressures that may be stabilising (the mean is favoured), directional (one extreme is favoured) or disruptive (both extremes are favoured). Allele frequencies also change by chance: genetic drift, the founder effect and the bottleneck effect all have the greatest effect in small populations. Antibiotic resistance is directional selection in action.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, The struggle for existence.. Every population produces more young than the environment can support. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Overproduction, competition, survival | Choose stabilising, then press Run 10 generations and compare the yellow histogram with the grey starting population. Project tab 3 of the lab (Selection and evolution lab, types of selection) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Chance: drift, founders and bottlenecks | With genetic drift chosen, set Population size to 30 and press Run 60 generations: watch the five lines wander. Project tab 4 of the lab (Selection and evolution lab, genetic drift) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Antibiotic resistance: selection you can watch | Antibiotic resistance follows the same logic as directional selection in the simulator, but in bacteria, which can divide every 20 minutes. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 20 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Antibiotic resistance: selection you can watch, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Organisms do not change in order to survive, and an individual does not evolve. | Variation already exists; the environment selects among it, and it is the population's allele frequencies that change. |
| Drift is not caused by an allele being harmful. | A neutral or even an advantageous allele can be lost by chance in a very small population. |
| Bacteria do not become resistant because they are exposed to the antibiotic or get used to it. | The resistant mutants were already present; the antibiotic simply removes their competitors. |
Natural selection, drift and resistance: worksheet
6 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
Explain how natural selection can lead to a change in the frequency of an allele in a population.
Explain how stabilising and directional selection differ in their effect on the distribution of a continuous feature.
Outline how a population of bacteria can become resistant to an antibiotic.
Distinguish between the founder effect and the bottleneck effect.
Explain why genetic drift has a greater effect on allele frequencies in small populations than in large populations.
In a hospital, the percentage of infections caused by penicillin-resistant bacteria rose from 8% to 46% over 12 years, during which penicillin use increased. Suggest an explanation for this change.
Answer key and marking notes
- Populations produce more offspring than the environment can support, so there is competition for resources (a struggle for existence).
- There is variation; some individuals have an allele that gives a phenotype better adapted to the conditions (selective advantage).
- These individuals are more likely to survive and reproduce.
- They pass on the advantageous allele to their offspring, so its frequency increases over generations.
- Stabilising: intermediate phenotypes are favoured and both extremes selected against.
- The mean stays the same and the range / standard deviation decreases.
- Directional: one extreme is favoured, often after an environmental change, so the mean shifts in that direction over generations.
- A random mutation produces an allele that gives resistance, for example coding for an enzyme that breaks down the antibiotic.
- When the antibiotic is used, susceptible bacteria are killed; it acts as a selection pressure.
- Resistant bacteria survive and reproduce by binary fission, passing on the allele, so the proportion of resistant bacteria increases.
- Resistance genes on plasmids can be transferred to other bacteria by conjugation.
- Founder effect: a few individuals leave and start a new, isolated population.
- Bottleneck effect: a large population is drastically reduced in size, for example by a disaster, disease or hunting.
- In both, the surviving alleles are a chance sample of the original gene pool, so allele frequencies change and genetic diversity is reduced.
- Which individuals reproduce, and which alleles are passed on, is partly due to chance.
- In a small population the alleles passed on are a small sample, so chance differences form a large proportion of the gene pool.
- Alleles can be lost or fixed within a few generations; in large populations random changes tend to cancel out.
- Resistant bacteria were present at the start (8%) because of earlier random mutation.
- Increased penicillin use increased the selection pressure: susceptible bacteria were killed while resistant bacteria survived and reproduced.
- The resistance allele was passed on (and spread by plasmids), so resistant bacteria became a larger proportion, rising by 38 percentage points.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which sequence correctly summarises natural selection?
- Aoverproduction, variation, competition, survival of the best adapted, alleles passed on
- Bcompetition, mutation caused by the environment, survival, acquired features passed on
- Cvariation, all offspring survive, the best adapted reproduce more, alleles lost
- Doverproduction, identical offspring, random deaths, allele frequencies unchanged
Babies of very low or very high birth mass have a higher death rate than babies of average mass. Which type of selection is this?
- Astabilising selection
- Bdirectional selection
- Cdisruptive selection
- Dartificial selection
What is genetic drift?
- Aa random change in allele frequency from one generation to the next, greatest in small populations
- Ba change in allele frequency caused by individuals with an advantage surviving better
- Cthe movement of alleles between populations when individuals migrate
- Dthe gradual change of an allele into a new allele by repeated mutation
Hunting reduced a seal population to about 20 animals. Numbers have since recovered to over 100 000, but genetic variation is very low. This is an example of:
- Athe bottleneck effect
- Bthe founder effect
- Cstabilising selection
- Dhybrid vigour
How can a gene for antibiotic resistance spread from one species of bacterium to another?
- Aon a plasmid transferred by conjugation
- Bby the antibiotic carrying the gene between cells
- Cby the bacteria becoming used to the antibiotic
- Dby meiosis and fertilisation between the species
The graph shows the percentage of bacteria resistant to antibiotic X in a hospital. Which explanation fits the fall after year 8?
- Awithout the antibiotic, resistant bacteria have no advantage and may be outcompeted by susceptible ones
- Bthe antibiotic is no longer present to cause resistance mutations
- Cthe resistant bacteria become susceptible again once they get used to the absence of X
- Dresistance alleles are passed on only while the antibiotic is in use
In a large population, allele B gives a small advantage. In a population of 12 individuals, allele B is lost after a few generations. Which is the most likely explanation?
- Agenetic drift in the small population outweighed the weak effect of selection
- Bselection is stronger in small populations, so B must be harmful there
- Cthe bottleneck effect only removes advantageous alleles
- Dmutation changed every copy of B into another allele
A population of finches lives where only small soft seeds and large hard seeds are available. After a drought, birds with medium beaks have the lowest survival. What is the expected effect on the distribution of beak depth after many generations?
- Atwo peaks form, with fewer medium beaks: disruptive selection
- Bthe mean moves towards deeper beaks: directional selection
- Cthe range narrows around the mean: stabilising selection
- Dno change, because beak depth is controlled by many genes
Quiz key
Populations overproduce; individuals vary genetically; they compete; the best adapted are more likely to survive and reproduce; they pass on their alleles, so allele frequencies change. The environment does not cause the mutations that are needed, and acquired features are not inherited. Not all offspring survive; that is the point of the struggle for existence. Identical offspring with random deaths would give no selection at all, so allele frequencies would not change in a consistent direction.
When intermediate phenotypes survive best and both extremes are selected against, selection is stabilising: the mean stays the same and the range narrows. Directional selection would favour one extreme, such as only larger babies. Disruptive selection would favour both extremes over the average, the opposite of this pattern. Artificial selection involves humans choosing which individuals breed, which does not apply to human birth mass.
Genetic drift is a chance change in allele frequencies because which individuals reproduce and which gametes fuse is partly random; its effect is largest in small populations. Change caused by an advantage is natural selection, which is not random. Movement of alleles by migration is gene flow. Mutation produces new alleles; drift changes the frequency of alleles already present.
A drastic reduction in population size leaves a few survivors whose alleles are a chance sample of the original gene pool; when numbers recover, the lost variation does not return. This is the bottleneck effect. The founder effect involves a few individuals leaving to start a new population elsewhere. Stabilising selection narrows the range by favouring the mean, but does not involve a crash in numbers. Hybrid vigour refers to increased vigour in heterozygous offspring of two inbred lines.
Resistance genes are often on plasmids, which can be copied and passed to other bacteria, including other species, by conjugation (horizontal gene transfer). Antibiotics are not vectors and do not carry genes. Bacteria do not become resistant by getting used to the drug; resistance comes from alleles produced by mutation. Bacteria reproduce asexually by binary fission; they do not carry out meiosis or fertilisation.
While X was used, it killed susceptible bacteria and selected for resistant ones. Once use stopped, resistance no longer gives an advantage and, if it carries a cost (for example maintaining a plasmid), susceptible bacteria reproduce faster, so the proportion of resistant bacteria falls slowly. Antibiotics do not cause resistance mutations; they select among existing variants. Individual bacteria do not change their genotype by getting used to a new condition. Resistance alleles are inherited whether or not the antibiotic is present; it is their frequency that changes.
In a very small population, chance sampling of alleles each generation causes large random changes, which can override a weak selective advantage, so even an advantageous allele can be lost. Selection is not stronger in small populations, and the loss does not show that B is harmful. Bottlenecks remove alleles at random, not specifically advantageous ones. Mutation is far too rare to convert every copy of an allele in a few generations.
Both extremes (small beaks for small seeds, large beaks for hard seeds) are favoured and the intermediate is selected against, so the distribution becomes bimodal: disruptive selection. Directional selection would favour only one extreme, but small-beaked birds also survive well here. Stabilising selection would favour medium beaks, the opposite of what happens. Polygenic traits respond to selection just as well; many genes do not stop allele frequencies changing.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.