The shape of the lesson
By the end of the lesson, learners can
- Breeding a better wheat: The sim shows a field of 24 wheat plants with ears of grain of different sizes.
- Crops and domesticated animals: The same method is used for any feature people want, as long as it is inherited.
- Natural versus artificial selection: Extended.
- Step 1: select individuals with the desirable feature.
- Step 2: cross them to produce the next generation.
- Step 3: select offspring showing the feature; repeat over many generations.
- Extended: natural selection is by the environment for survival; artificial selection is by humans for usefulness, and usually faster.
One picture every idea returns to
In selective breeding (artificial selection) humans select individuals with desirable features, cross them to produce the next generation, and select the offspring showing the desirable features. Repeating this over many generations improves crop plants and domesticated animals. Extended: in natural selection the environment decides which individuals reproduce; in artificial selection humans decide.
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, Humans do the choosing.. Modern wheat, dairy cows and pedigree dogs look nothing like their wild ancestors. 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: Breeding a better wheat | The sim shows a field of 24 wheat plants with ears of grain of different sizes. Project tab 7 of the lab (Selective breeding of wheat) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Crops and domesticated animals | The same method is used for any feature people want, as long as it is inherited. | Pairs |
| 21 to 30 min | Teach and check: Natural versus artificial selection | Extended. | Connect-Extend-Challenge |
| 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, 21 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 Natural versus artificial selection, 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.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
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 |
|---|---|
| Selective breeding only works on inherited variation. | A plant that is large only because it grew in richer soil will not pass that on, which is why the sim's yield climbs gradually rather than in one leap. |
| Artificial selection can make an organism less able to survive in the wild. | Very high-yield crops may need fertiliser and pesticides, and some dog breeds have breathing problems. |
Selective breeding: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how selective breeding is carried out.
A plant breeder wants a variety of tomato plant that is resistant to a fungal disease. Outline how she could produce it by selective breeding.
The table shows the mean milk yield of cows in a herd that was selectively bred.
| Generation | 0 | 2 | 4 | 6 |
|---|---|---|---|---|
| Mean yield / litres per day | 12 | 16 | 19 | 21 |
(a) Calculate the percentage increase in mean yield from generation 0 to generation 6. (b) Describe the pattern. (c) Suggest why the increase is smaller in later generations.
A farmer feeds his best cow extra food so that it gives more milk. Explain why breeding from this cow because of its extra milk may not improve the milk yield of its offspring.
Extended. Outline four differences between natural selection and artificial selection.
Extended. Suggest one disadvantage of selectively breeding crop plants for a single feature, such as high yield, over many generations.
Answer key and marking notes
- Humans select individuals with the desirable feature.
- These individuals are crossed (bred together) to produce the next generation.
- Offspring showing the desirable feature are selected and bred, and this is repeated over many generations.
- Grow many tomato plants and expose them to the fungus (or find plants that are not infected).
- Select the plants that show resistance to the disease.
- Cross these plants and grow the seeds; expose the offspring to the fungus again.
- Select the most resistant offspring to breed, and repeat for many generations.
- (a) (21 − 12) ÷ 12 × 100 = 75%.
- (b) The yield increases every generation, but by less each time (4, then 3, then 2 litres per day over two generations).
- (c) Most cows now carry the alleles for high yield, so there is less genetic variation left to select from.
- Yield may also be limited by the environment, such as the amount of food the cows can eat.
- The extra milk is caused by the environment (more food), not by its genes.
- Only inherited (genetic) variation is passed on to offspring.
- So the offspring may not inherit any alleles for a high milk yield.
- Natural selection: the environment selects; artificial selection: humans select.
- Natural: features that help survival and reproduction; artificial: features useful to humans.
- Natural selection is usually slower; artificial selection is usually faster.
- Natural selection leads to adaptation to the environment; artificial selection produces new varieties and breeds, which may be less able to survive in the wild.
- Breeding only from a few selected plants reduces genetic variation in the crop.
- If a new disease or a change in climate occurs, there may be no plants with alleles for resistance.
- So the whole crop could be lost (or the crop may depend on fertiliser and pesticides).
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.
What is the first step in selectively breeding a variety of rice with a higher yield?
- ASelect rice plants that already give a high yield
- BCross any two rice plants and collect the seeds
- CExpose rice seeds to radiation to cause mutations
- DGive the rice plants extra fertiliser
A breeder wants hens that lay more eggs. Which plan is selective breeding?
- ABreed from the hens laying most eggs and a cockerel whose mother laid many; repeat with the best daughters
- BFeed all the hens extra protein so they lay more eggs, then breed from them
- CBreed from hens chosen at random and keep every chick that hatches
- DBreed from the hens laying fewest eggs so the flock lays more next year
Why must selective breeding usually be carried out over many generations?
- AEach generation shows only a small improvement, and offspring still vary
- BThe desired feature only appears after the parents grow old
- CCrossing two plants always produces identical offspring
- DHumans need time to change the genes of each individual
The chart shows the mean ear length of maize over five generations of selective breeding. Which statement describes the results?
- AEar length increased each generation, but by smaller amounts later
- BEar length increased by the same amount every generation
- CEar length increased quickly at first, then fell after generation 3
- DEar length did not change until generation 3, then increased
Use the chart of mean ear length. Calculate the percentage increase from generation 0 to generation 5.
- A45%
- B31%
- C2.7%
- D145%
Extended: suggest why the increase in ear length on the chart becomes smaller in the later generations.
- AMost plants now carry the useful alleles, so there is less variation to select from
- BThe plants have used up all their mutations by generation 5
- CSelective breeding only works for the first two generations
- DThe farmer stopped selecting plants after generation 3
Extended: which row correctly compares natural and artificial selection?
- ANatural: environment selects features aiding survival. Artificial: humans select features they want.
- BNatural: humans select features they want. Artificial: environment selects features aiding survival.
- CNatural: needs no variation. Artificial: needs genetic variation.
- DNatural: usually faster. Artificial: usually slower.
Extended: modern broiler chickens grow so fast that some cannot walk properly. What does this show about artificial selection?
- AIt selects features useful to humans that may harm the organism
- BIt always makes organisms better adapted to their environment
- CIt works in the same way as natural selection by predators
- DIt changes the chickens' genes during their own lifetime
Quiz key
Breeders first select individuals that show the desirable feature, high yield, and then cross them. Crossing random plants skips the selection step. Radiation is not part of selective breeding as described in the syllabus. Fertiliser raises yield through the environment, but that is not inherited.
Choosing the best layers and a cockerel from a high-laying line, crossing them and selecting the best daughters, then repeating, follows all three steps. Extra protein changes egg laying through the environment, which is not inherited. Random parents and keeping every chick involve no selection. Breeding from the poorest layers would select the wrong feature.
Each round of selection improves the population only a little, and the offspring still vary, so the selection must be repeated for many generations. The age of the parents is not the reason. Crossing produces varied, not identical, offspring. In selective breeding humans choose parents; they do not change the genes of individuals.
The increases are 0.8, 0.7, 0.6, 0.4 and 0.2 cm: always rising, but by less each generation. The increases are not equal. The bars never fall. Ear length rose from the very first generation, not only from generation 3.
(8.7 − 6.0) ÷ 6.0 × 100 = 2.7 ÷ 6.0 × 100 = 45%. Dividing by the final value (2.7 ÷ 8.7) gives 31%. 2.7 is the increase in cm, not a percentage. 145% is the final length as a percentage of the starting length, not the increase.
Repeatedly breeding from the best plants makes the useful alleles common, so there is less genetic variation left to select, and the environment also limits growth. Organisms do not have a store of mutations to use up. Selection keeps working while variation remains, not just for two generations. The chart still shows increases after generation 3, and nothing suggests selection stopped.
In natural selection the environment selects features that help survival and reproduction; in artificial selection humans select features useful to them. The second row swaps the two. Both processes need genetic variation. Artificial selection is usually faster, because only chosen individuals breed, so the last row is the wrong way round.
Breeders selected fast growth because it is profitable, even though it can harm the chickens' legs, so artificial selection favours features useful to humans, not necessarily to the organism. Natural selection, not artificial, makes organisms better adapted to their environment. Predators are not involved here. Selection changes allele frequencies in a population over generations, not the genes of an individual during its life.
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.