The shape of the lesson
By the end of the lesson, learners can
- Reading relationships in DNA: Compare the five 20-base sequences.
- Barriers to gene flow: Choose allopatric and press Run 2000 generations: a river forms and the two halves change colour separately.
- Evolution: gene pools change over generations, forming new species from old.
- Fewer DNA differences = more recent common ancestor.
- Allopatric: geographical barrier. Sympatric: ecological or behavioural separation.
- New species: can no longer interbreed to produce fertile offspring.
One picture every idea returns to
The theory of evolution states that new species form from pre-existing species over time, as a result of changes to gene pools from generation to generation, driven by mutation, natural selection and genetic drift. DNA sequence data give evidence of relationships: mutations accumulate at a roughly steady rate, so the fewer the differences between the base sequences of two species, the more recently they shared a common ancestor. A species is a group of organisms that can interbreed to produce fertile offspring. Speciation needs genetic isolation, so that there is no gene flow between two populations. In allopatric speciation a geographical barrier separates them; in sympatric speciation they live in the same area but are separated ecologically (different habitats or food) or behaviourally (different courtship or breeding times). Different mutations, selection pressures and drift then change each gene pool until the populations can no longer interbreed.
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, How one species becomes two.. Evolution is change in the gene pool of a population over many generations. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Reading relationships in DNA | Compare the five 20-base sequences. Project tab 7 of the lab (Selection and evolution lab, DNA and relationships) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Barriers to gene flow | Choose allopatric and press Run 2000 generations: a river forms and the two halves change colour separately. Project tab 8 of the lab (Selection and evolution lab, speciation) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 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, 18 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 Barriers to gene flow, 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.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
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 |
|---|---|
| Humans did not evolve from chimpanzees. | The tree shows that humans and chimpanzees share a common ancestor more recently than either does with gorillas; both lineages have changed since the split. |
| Separation alone does not make a new species. | The populations must become reproductively isolated: unable to interbreed to produce fertile offspring. Two populations separated briefly that can still interbreed are still one species. |
Evolution and speciation: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Outline the theory of evolution.
A section of a gene was sequenced in four species of bird. The numbers of base differences were: A-B 3, A-C 14, A-D 15, B-C 13, B-D 14, C-D 4. Use the data to suggest the evolutionary relationships between the four species and explain your reasoning.
Explain how allopatric speciation can occur.
Explain how sympatric speciation can occur without a geographical barrier.
Suggest why comparing DNA sequences can give more reliable evidence of evolutionary relationships than comparing the appearance of organisms.
Two populations of fish in a lake look different and rarely mate in the wild, but in an aquarium they mate and produce fertile offspring. Discuss whether they should be classified as separate species.
Answer key and marking notes
- New species are formed from pre-existing species over (long periods of) time.
- This happens through changes in the gene pool / allele frequencies of populations from generation to generation (by mutation, natural selection and genetic drift).
- A and B differ by only 3 bases, so they share a recent common ancestor.
- C and D differ by only 4 bases, so they also share a recent common ancestor.
- The A/B pair differs from the C/D pair by 13 to 15 bases, so the two pairs diverged from a common ancestor much longer ago.
- Mutations accumulate at a roughly constant rate, so the number of differences reflects the time since divergence.
- A population is split by a geographical barrier such as a river, mountain range or sea.
- There is no gene flow between the two populations.
- Different mutations occur, and different selection pressures (and genetic drift) act on each, so allele frequencies / gene pools change differently.
- Eventually the populations cannot interbreed to produce fertile offspring even if they meet: they are reproductively isolated, separate species.
- Individuals in the same area become separated ecologically (for example using different host plants, habitats or food) or behaviourally (different courtship or breeding times).
- So they mate mainly within their own group, reducing gene flow between groups.
- Different selection pressures and mutations change each gene pool until the groups can no longer interbreed to produce fertile offspring.
- Similar appearance can evolve separately in unrelated organisms living in similar environments (convergent evolution), whereas DNA differences reflect shared ancestry.
- DNA data are quantitative and objective: the number of base differences can be counted and related to time since divergence.
- They produce fertile offspring when mated, which suggests they are still one species (not reproductively isolated).
- However, in the wild there is little gene flow between them because of behavioural or ecological separation, so they may be in the process of sympatric speciation.
- Further divergence of their gene pools could lead to full reproductive isolation; DNA comparison could show how different the gene pools are.
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 statement best outlines the theory of evolution?
- Anew species form from pre-existing species over time as gene pools change from generation to generation
- Bindividual organisms change during their lifetime to become new species
- Cspecies are fixed, but the number of individuals in each species changes over time
- Dnew species form when organisms choose to adapt to a new environment
The figure shows the same section of a gene in four species. Which species is most closely related to species W?
- Aspecies X, which differs from W at 1 base
- Bspecies Y, which differs from W at 2 bases
- Cspecies Z, which differs from W at 1 base
- Dspecies Y, which differs from W at 6 bases
Why can the number of differences in DNA base sequence between two species indicate how long ago they shared a common ancestor?
- Amutations accumulate in each lineage at a roughly constant rate after the species diverge
- Bspecies that look similar always have identical DNA sequences
- CDNA sequences change only when a new species forms
- Dthe environment changes DNA bases to match the species' habitat
What is essential for two populations of a species to evolve into separate species?
- Athey are genetically isolated so that there is no gene flow between them
- Bthey live on different continents
- Cthey become different in size
- Dthey produce more offspring than can survive
Two populations of salamanders on opposite sides of a newly formed canyon gradually become unable to interbreed. This is an example of:
- Aallopatric speciation
- Bsympatric speciation
- Cthe founder effect
- Ddisruptive selection
Apple maggot flies lay eggs on hawthorn or apple trees in the same area, and mate on the tree where they hatched. Apples fruit earlier than hawthorns. Why might the two groups become separate species?
- Aecological and temporal separation reduce gene flow, so each group's gene pool changes independently
- Bthe two tree species form a geographical barrier between the groups
- Cflies on apples acquire new features from feeding and pass them to their offspring
- Dthe two groups are already separate species because they eat different food
Cytochrome c differs between humans and rhesus monkeys by 1 amino acid, between humans and dogs by 11, and between rhesus monkeys and dogs by 10. Which conclusion is valid?
- Ahumans and rhesus monkeys share a more recent common ancestor than either does with dogs
- Bhumans evolved from rhesus monkeys
- Cdogs are more closely related to humans than to rhesus monkeys
- Dthe three species diverged from each other at the same time
Two populations of a bird separated by a mountain range for 50 000 years meet again. Their hybrids are healthy but all sterile. What does this show?
- Athe populations are now separate species, because they cannot produce fertile offspring
- Bthe populations are still one species, because they can interbreed
- Cthe mountain range has made the birds' DNA identical
- Dspeciation has been reversed because the populations now share a habitat
Quiz key
Evolution is the change in the gene pools of populations over many generations, leading to new species arising from existing ones. Individuals do not evolve during their lifetime. Changes in population size alone are not evolution; the allele frequencies must change. Organisms do not choose to adapt; variation arises by mutation and is selected.
X differs from W only at position 6 (A to G), so X and W share the most recent common ancestor. Y differs from W at 3 positions (2, 6 and 8), not 2, and so is less closely related. Z differs from W at 7 positions, not 1. Y does not differ at 6 bases; that count confuses Y with Z.
After two lineages separate, mutations build up independently in each at a roughly steady rate, so the more differences there are, the longer ago they diverged (a molecular clock). Similar-looking species can still differ in DNA, and unrelated species can look alike. DNA changes continually by mutation, not only at speciation. The environment does not direct which bases change; mutations are random and then selected.
Speciation requires genetic isolation: without gene flow, mutation, selection and drift change each gene pool separately until the populations cannot interbreed. Different continents is one form of geographical isolation, but sympatric speciation shows separation in space is not essential. A size difference alone does not stop gene flow. Overproduction drives natural selection but does not, by itself, divide a population into two species.
A geographical barrier (the canyon) separating populations that then diverge is allopatric speciation. Sympatric speciation happens without geographical separation. The founder effect concerns a few individuals starting a new population with a chance sample of alleles, not a barrier splitting a population. Disruptive selection favours extremes within one population and can contribute to sympatric speciation, but the key cause here is geographical isolation.
Flies mate on their host tree, and the trees fruit at different times, so flies from the two groups rarely meet to mate: ecological and behavioural (timing) separation reduce gene flow within the same area, which can lead to sympatric speciation. The trees grow in the same area, so there is no geographical barrier. Features acquired by feeding are not inherited. Eating different food does not by itself make them separate species; they are separate species only when they can no longer interbreed to produce fertile offspring.
Humans and rhesus monkeys differ by only 1 amino acid, so they diverged more recently from each other than either did from the dog lineage (10 or 11 differences). Living species do not evolve from each other; they share common ancestors. Dogs differ from humans by 11 but from monkeys by 10, so they are not closer to humans. Very different numbers of differences mean the divergences happened at different times.
A species is a group that can interbreed to produce fertile offspring. Sterile hybrids show the populations are reproductively isolated, so they are separate species formed by allopatric speciation. Being able to mate is not enough if the offspring are infertile. Isolation lets the DNA of the populations diverge, not become identical. Sharing a habitat again does not reverse speciation, because the gene pools can no longer mix.
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.