The shape of the lesson
By the end of the lesson, learners can
- Species and scientific names: Cross each pair of animals in the simulator, then try to breed their offspring.
- Following a dichotomous key: Look at the mystery organism (drag to turn it), answer each question with (a) or (b) and follow the key until it names the organism.
- Extended: classification and DNA: Extended content.
- Species: can reproduce to produce fertile offspring.
- Binomial name: Genus species, capital then small letter, in italics.
- Key statements: a pair of opposites about one visible, fixed feature; n organisms need n − 1 steps.
- Extended: more similar DNA base sequences mean a more recent common ancestor.
One picture every idea returns to
Organisms are classified into groups by the features that they share. A species is a group of organisms that can reproduce to produce fertile offspring. The binomial system is an internationally agreed system in which the scientific name has two parts: the genus then the species, as in Panthera leo. A dichotomous key identifies an organism through pairs of statements about features you can see. Extended: classification aims to reflect evolutionary relationships, and organisms that share a more recent ancestor have more similar DNA base sequences.
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, Two names, two choices, one answer.. Organisms are sorted into groups by the features they share. 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: Species and scientific names | Cross each pair of animals in the simulator, then try to breed their offspring. Project tab 2 of the lab (Classification lab, species) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Following a dichotomous key | Look at the mystery organism (drag to turn it), answer each question with (a) or (b) and follow the key until it names the organism. Project tab 3 of the lab (Classification lab, dichotomous key) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Extended: classification and DNA | Extended content. Project tab 8 of the lab (Classification lab, DNA) and let a student drive it while the class predicts. | 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, 16 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 Extended: classification and DNA, 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 |
|---|---|
| A species is not defined by appearance. | Animals that look alike can be separate species, and animals that look very different, like dog breeds, can be one species. The test is whether they produce fertile offspring. Offspring alone is not enough: a mule is offspring, but it is sterile. |
| Base every statement on a feature you can see and that does not change. “Is large”, “is brown” or “lives in a pond” lose marks: size and colour vary between individuals and with age, and habitat is not a feature of the organism. | Return to Following a dichotomous key: ask a pair to state the correct version and show it on the lab before moving on. |
Classification and dichotomous keys: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
The domestic cat is Felis catus and the lion is Panthera leo. (a) State the genus of the lion. (b) Explain why the binomial system is useful to scientists.
A horse and a donkey can mate to produce a mule. Mules are almost always sterile. Explain whether horses and donkeys belong to the same species.
Four arthropods have these features. A: three pairs of legs, wings. B: four pairs of legs, no antennae. C: five pairs of legs, two pairs of antennae. D: three pairs of legs, no wings. Construct a dichotomous key to identify A, B, C and D.
A student's key contains the statements “1 a: is large” and “1 b: is small”. Explain why these are poor statements and suggest a better kind of statement.
Extended. Part of the same gene was compared in four mammals. The percentage of bases that are the same as in humans is: chimpanzee 90%, gorilla 85%, mouse 70%. Explain what these results show about the evolutionary relationships of these animals.
Extended. Whales look like fish, but they are classified as mammals. Explain how DNA evidence supports this classification.
Answer key and marking notes
- (a) Panthera.
- (b) It is internationally agreed, so every scientist uses the same name for a species, whatever the common name in their own language.
- A species is a group of organisms that can reproduce to produce fertile offspring.
- The mule is offspring, but it is sterile, so it cannot reproduce.
- So horses and donkeys are different species (they are in the same genus, Equus).
- 1 a: three pairs of legs .... go to 2. 1 b: more than three pairs of legs .... go to 3.
- 2 a: has wings .... A. 2 b: has no wings .... D.
- 3 a: has antennae .... C. 3 b: has no antennae .... B.
- Three pairs of statements for four organisms; each pair is about one visible feature.
- Size varies between individuals of the same species and with age, and large or small depends on opinion, so an organism might fit either.
- Use a pair of opposite statements about a clear visible feature, for example “has wings” / “has no wings”.
- The chimpanzee sequence is the most similar to the human sequence, so humans and chimpanzees share the most recent common ancestor.
- The gorilla is the next most closely related, and the mouse is the most distantly related.
- Mutations build up over time, so species that separated longer ago have more differences in their base sequences.
- Classification aims to reflect evolutionary relationships.
- The base sequences in whale DNA are more similar to those of other mammals than to those of fish, showing that whales share a more recent common ancestor with mammals.
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.
The grey wolf is Canis lupus and the domestic dog is Canis familiaris. What do these scientific names show?
- AThey are in the same genus but are different species
- BThey are the same species but are in different genera
- CThey are the same species, because both names start with Canis
- DThey are not closely related, because their second names differ
Use the key to identify an animal that has no shell and whose body is flat like a ribbon and is not divided into segments.
- Aflatworm
- Broundworm
- Cearthworm
- Dmussel
A student constructs a dichotomous key to identify six different species of tree. What is the smallest number of pairs of statements the key needs?
- A5
- B6
- C3
- D12
Why do scientists use the binomial system instead of common names for organisms?
- AIt is agreed internationally, so each species has the same name everywhere
- BIt describes the colour and size of the organism in its name
- CIt names the habitat first and then the type of organism
- DIt gives every organism in the same genus the same name
Two populations of frogs look almost identical. Which observation shows that they belong to two different species?
- AWhen they mate, all of their offspring are infertile
- BThey live in two ponds on opposite sides of a valley
- COne population has slightly darker skin than the other
- DWhen they mate, they produce a large number of tadpoles
The table shows the number of base differences in the same 200-base section of a gene in four species, P, Q, R and S. Which two species share the most recent common ancestor?
- AP and Q
- BP and S
- CR and S
- DQ and R
What do modern classification systems aim to reflect?
- AThe evolutionary relationships between organisms
- BThe habitats in which organisms are found
- CThe order in which organisms were first discovered
- DThe size and diet of different organisms
Species X and Y separated from a common ancestor 5 million years ago. Species X and Z separated from a common ancestor 20 million years ago. Which prediction about their DNA is correct?
- AThe base sequences of X and Y are more similar than those of X and Z
- BThe base sequences of X and Z are more similar than those of X and Y
- CThe base sequences of X, Y and Z are all equally similar
- DThe base sequences of X and Y are exactly the same
Quiz key
The first part of a binomial name is the genus and the second is the species, so a shared first name (Canis) with different second names means same genus, different species. Different genera would need different first names. Sharing a genus does not make them one species, and being in the same genus means they are closely related, not distantly related.
No shell: 1b, go to 3. Not segmented: 3b, go to 4. Flat like a ribbon: 4a, flatworm. Roundworm is the other choice at step 4 (round, thread-like body). Earthworm is reached by choosing 3a, but this animal is not segmented. Mussel needs a shell, so it is reached through 1a.
Each pair of statements separates one group into two, so starting from one group of six, it takes 6 − 1 = 5 pairs to leave each tree on its own. 6 counts one pair per tree, 3 halves the number of trees as if each pair named two trees at once, and 12 counts single statements rather than pairs.
The binomial system is internationally agreed, so a species has one scientific name whatever the common name in a given language or region. The name shows genus and species, not colour, size or habitat, and species in the same genus share only the first part of the name, not the whole name.
A species is a group of organisms that can reproduce to produce fertile offspring, so infertile offspring show two species. Living in different places does not tell you whether they could interbreed. Small differences in colour occur within one species. Producing offspring is not enough: the offspring must be fertile.
Mutations build up over time, so the pair with the fewest base differences (P and Q, 6) separated most recently and shares the most recent ancestor. P and S have the most differences (38), so choosing them confuses differences with similarities. R and S (35) and Q and R (21) both have many more differences than P and Q.
Classification aims to reflect evolutionary relationships, so organisms in the same group share a common ancestor. Habitat, diet and size can be shared by unrelated organisms (a shark and a dolphin share a habitat and diet), and the order of discovery says nothing about how organisms are related.
X and Y have been separate for only 5 million years, so fewer mutations have built up and their sequences are more similar. X and Z have had 20 million years to build up differences, so they are less similar, not more. The sequences cannot all be equally similar when the times differ, and even closely related species are not identical, because mutations still build up over 5 million years.
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.