Evolution by Natural Selection: a full term of science

Ten ready-to-teach lessons for Year 10 Biological sciences. Print this pack and the term is prepared: every lesson comes with a step-by-step plan, the questions to ask, a student worksheet, cut-out cards, a class sheet or exit tickets, an assessment kit and worked answers.

Start here: five minutes to Monday

1. Skim the term at a glance on the next page.
2. Print the lesson you need. Each lesson is three A4 sheets: plan, worksheet, and cards, a class sheet or tickets.
3. Gather the few items under “You need” on the plan. Most come from the classroom or a supermarket.
4. Open the free interactive unit on your board or projector. Every plan tells you which picture to show and when.
5. Teach straight from the plan. Timings, talk prompts, misconceptions and worked answers are all on the one page.

For any Year 10 science teacher

The pack suits a biology specialist and a teacher working outside their own subject alike: each plan explains the idea in plain words, lists the misconceptions students bring, and gives worked answers. The big idea of the term: the individuals of a population vary, and much of that variation is inherited. When something in the environment means some variants survive and reproduce more than others, their traits become more common in the next generation. Over many generations this natural selection changes populations and, over very long times, gives rise to new species. Fossils, the structure of bodies and DNA are separate lines of evidence, and they agree.

Two ways to run each lesson

Every lesson works as one 60-minute block, or as two 30-minute sessions; the split point is marked in every plan. Look ahead: Lesson 1 needs a bag of dried lima beans, Lesson 2 needs paper dots in four colours from a hole punch and a sheet of green cloth, and Lesson 7 needs coloured pencils.

Safety and care in one look

The dried beans in Lesson 1 are for measuring only and are never eaten. No bacteria are grown in Lesson 5: the bacteria are cards. When antibiotics come up, the message is general: take them only when they are prescribed, exactly as the doctor says. The term uses beans, moths, finches, rabbits and toads as its examples rather than students’ own families.

The term at a glance

One lesson a week for a term. Each lesson stands on the ones before it, so run them in order where you can.

How the sequence builds

Lessons 1 and 2 meet the two ingredients of natural selection: variation, and a pressure that lets some variants survive better. Lesson 3 puts the steps in order, and Lessons 4 and 5 follow selection in real populations, moths and bacteria. Lessons 6 to 8 weigh three separate lines of evidence for evolution: fossils, the structure of bodies, and DNA. Lesson 9 joins them into one branching tree of life and shows how one species can become two, and Lesson 10 brings it together.

Curriculum links (Australian Curriculum V9)

The whole term teaches the Science Understanding descriptor AC9S10U02 quoted on the cover. The lessons also work these Science Inquiry and Human Endeavour descriptors:

Assessment in this pack

• Every plan ends with “Answers and look-fors”: what meeting the idea sounds like in a Year 10 voice, with working.
• The assessment sheet near the front has a class observation checklist and a three-level rubric.
• Lesson 10 is the summative pair: the cane toad task, plus the “Show what we know” check.

Materials for the whole term

One gathering session covers all ten lessons, and most of it is already in the classroom. Two things are worth doing ahead: buy a bag of dried lima beans for Lesson 1, and punch the paper dots for Lesson 2 (or ask a keen class to punch them in a spare five minutes).

The one-trip list

• From the supermarket: a bag of dried lima beans.
• From the classroom: 30 cm rulers, a hole punch, coloured paper in four colours, scissors, glue sticks and coloured pencils.
• From the art room or a fabric shop: a sheet of plain green cloth, or green card, for each group.

Safety and care in one look

• The dried beans are for measuring only and are never eaten.
• No bacteria are grown: Lesson 5 uses cards. The message about antibiotics stays general: only when prescribed, exactly as the doctor says.
• The term’s examples are beans, moths, finches, rabbits and toads, not students’ own families.

Assessment without extra work

The term assesses itself. Every lesson plan ends with worked answers and look-fors, and Lesson 10 is the summative pair: the cane toad task, in which students explain why toads at the front of their spread across northern Australia have longer legs, and the final check. This sheet is the place to note what you see along the way.

The three levels

Class observation checklist

A tick a lesson is plenty; the Lesson 10 task and check fill the gaps.

Word wall cards

Cut out the cards and build the wall as the words arrive: population, variation and inherited trait in Lesson 1; selection pressure and camouflage in Lesson 2; natural selection, adaptation and evolution in Lesson 3; frequency in Lesson 4; mutation and resistance in Lesson 5; fossil, law of superposition and transitional fossil in Lesson 6; common ancestor and the three kinds of structure in Lesson 7; DNA sequence in Lesson 8; species, speciation and evolutionary tree in Lesson 9.

Dear families

This term in science, Year 10 studies evolution by natural selection: how the members of a population differ, how the environment lets some of them survive and reproduce more than others, how that changes a population over many generations, and the evidence from fossils, bodies and DNA that living things share common ancestors.

Along the way your student will measure forty beans, hunt paper “prey” on green cloth, graph the rise and fall of a dark moth in England, treat a population of card bacteria with an antibiotic, colour the bones of a bat wing and a whale flipper, and explain why the cane toads at the front of their spread across northern Australia have longer legs.

Try this at home

• Look at the bananas, apples or potatoes in the kitchen. How much do they vary in size? Which differences might be inherited, and which come from how they were grown?
• Spot camouflage on a walk: an insect on bark, a lizard on a rock, a bird in leaf litter. What would make it easier to see?
• Ask about a pet breed: how did people select for its looks, and how is that like natural selection?
• Watch a nature documentary together and listen for the words variation, adaptation and evolution.

What to ask your scientist

• Why does natural selection need variation to work?
• Why do antibiotics stop working against some bacteria?
• What do a bat wing and a human arm have in common, and why?
• How can a fossil tell you how old it is?

A small note on care: the beans in the first lesson are for measuring only and are never eaten, and no bacteria are grown; the bacteria in the antibiotics lesson are cards. When antibiotics come up, the message is general: only when prescribed, exactly as the doctor says.

Warm regards,

The Year 10 science team

Printed from the free seegongsik Evolution by Natural Selection teaching pack · seegongsik.com/au/y10/biological/AC9S10U02/pack

Variation

No two members of a population are the same. Some of the differences are inherited, carried in genes from the parents; some come from the environment, such as how much water or light a plant had; most traits are shaped by both. Natural selection can only work on the inherited part, because only that is passed on. Students measure dried beans of one kind, tally the lengths of an example class of forty, and draw the spread: most beans sit near the middle and fewer at either end, the pattern of continuous variation.

We are learning to

• measure and record variation in a population,
• describe a spread of data with the mean, the range and the most common value,
• tell inherited variation from variation caused by the environment.

Success criteria

• I can tally a set of measurements, draw a column graph and describe its shape.
• I can explain why natural selection acts only on variation that is inherited.

You need

• for each group: about 40 dried lima beans in a cup, never eaten; a 30 cm ruler for each pair,
• the worksheet (next sheet), one per student, and the class results sheet (third sheet), on the board or a bench.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the beans, the board and the measuring in Session A, and Parts B to D in Session B.

Watch for these ideas

• “All differences are inherited.” A bean from a well-watered plant can be bigger with the same genes; only inherited differences are passed on.
• “Natural selection creates the differences.” It acts on variation that is already there; new alleles arise from mutations.
• “Members of a species are basically identical.” The beans show otherwise, and so does every population that has been measured.

Make it easier, make it bigger

• Easier: tally the example class together on the board before students draw the graph.
• Bigger: plan a fair test to find out whether the largest beans grow plants with larger beans. What would you keep the same?

Answers and look-fors

• Part B: 18 mm 1, 19 mm 2, 20 mm 4, 21 mm 6, 22 mm 8, 23 mm 7, 24 mm 5, 25 mm 3, 26 mm 3, 27 mm 1; the counts add to 40.
• Part C: a hump with its peak at 22 mm, falling away on both sides.
• Part D: 1. Mean 22.5 mm, range 9 mm (18 to 27 mm), most common 22 mm. 2. Most beans are close to the middle, with fewer at each end. 3. Inherited: alleles from the parent plants for large seeds; not inherited: more water, light or nutrients while it grew, or its place in the pod. 4. Only inherited traits pass to the offspring, so a difference caused by growing conditions cannot build up over generations. 5. Continuous: the length can take any value in a range; a discontinuous trait falls into separate groups, such as blood group or the purple or white flowers of peas.
• Look for students who say “most beans are about” a value and give the spread, rather than one bean’s length.

Variation

A. Our twenty beans (mm)

Measure each bean along its longest side, to the nearest millimetre. The beans are for measuring only: never eat them.

B. An example class of forty beans

Tally the lengths, then write how many beans have each length.

C. The column graph

D. Questions

Class results: bean lengths

Each pair adds a tally mark for every bean it measured. When everyone has added theirs, total each row.

Lima beans vary a lot between bags: add rows if yours run outside 14 to 29 mm. Compare the shape of the class graph with the example class.

Hidden or eaten

Variation alone changes nothing; something in the environment has to make some variants more likely to survive and reproduce. That is a selection pressure. In this model, paper dots in four colours are the prey, a sheet of green cloth is their habitat, and a student is the predator who hunts by sight. Each survivor has one offspring of its own colour. Over three generations the colours that blend in become more common, though no dot ever changes colour. Students then check their own result against an example class and graph it.

We are learning to

• explain what a selection pressure is, and name one,
• model natural selection, and say how the model is like the real thing and how it is not,
• track and graph how common a trait is over generations.

Success criteria

• I can explain why the green dots became more common, using the words variation and selection pressure.
• I can name one way the model is like natural selection and one way it is not.

You need

• for each group of four: 10 paper dots in each of green, brown, yellow and red to start, about 30 spares of each colour, a sheet of plain green cloth or green card about A3, and a timer,
• the worksheet (next sheet), one per student, and the class results sheet (third sheet), on the board or a bench.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the dots, the board and the hunt in Session A, and Parts C and D in Session B.

Watch for these ideas

• “The dots changed colour to survive.” No dot changed; the mix of colours in the population did.
• “The predator chose which dots to keep.” The predator takes the dots it sees; the survivors are the ones it missed.
• “Fittest means strongest.” Fit means suited to this environment: on red cloth, red would be the fittest.

Make it easier, make it bigger

• Easier: run the first generation together at the front, with the class counting the survivors aloud.
• Bigger: rerun the hunt on a sheet of newspaper. Predict which colours will do best, then test the prediction.

Answers and look-fors

• Part B: results vary; most groups see green, and often brown, rise while yellow and red fall.
• Part C: green is 25% in generation 1, 40% in generation 2, 65% in generation 3, 85% in generation 4; the graph rises steeply.
• Part D: 1. Green: it is hard to see on green cloth, so fewer green dots are taken and more survive to breed, and their offspring are green. 2. The predator, hunting by sight. 3. Like: the dots vary, the variation is inherited (offspring are the parent’s colour), and the predator removes the easy-to-see ones. Not like: real prey move and hide, real offspring numbers vary, and real predators learn. 4. Red would become more common, because it would now be the hard one to see.
• Look for students who describe the population changing (“more of the dots were green”) rather than the dots.

Hidden or eaten

A. Predict

B. The hunt

C. An example class

Each row is the 40 dots at the start of a generation. In each round the predator took 20: round 1, 2, 4, 6, 8; round 2, 3, 5, 8, 4; round 3, 9, 11, 0, 0 (green, brown, yellow, red).

D. Questions

Class results: generation 4

Each group adds the number of dots of each colour at the start of generation 4. Total each column and work out each colour’s share.

Every group started with 10 of each colour, a quarter each. Which colours gained, and which were lost from the population?

Natural selection, step by step

Charles Darwin and Alfred Russel Wallace reached the same idea separately, and their papers were read together in London in 1858; Darwin’s book On the Origin of Species followed in 1859. Every case of natural selection runs through the same four steps: the individuals vary in an inherited trait; something in the environment, a selection pressure, makes some variants more likely to survive; those survive and reproduce more, passing the trait on; and over generations the trait becomes more common. Students rebuild three real stories from twelve cards, then fix three statements that make the most common mistake: saying an individual changes because it needs to.

We are learning to

• describe the four steps of natural selection in order,
• apply the steps to a new example,
• explain why populations change but individuals do not.

Success criteria

• I can sort a story of natural selection into its four steps.
• I can correct a statement that says a living thing changed because it needed to.

You need

• the selection cards (third sheet), one set per pair, with scissors and glue sticks,
• the worksheet (next sheet), one per student.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the giraffe, the board and the card stories in Session A, and Parts B to D in Session B.

Watch for these ideas

• “Living things change because they need to.” The useful variant was already there; the environment let it survive and breed more.
• “An individual evolves.” Individuals survive or do not; it is the population whose make-up changes over generations.
• “Evolution is always far too slow to see.” Rabbits, finches, insects and bacteria have been seen to change within years or decades.

Make it easier, make it bigger

• Easier: give pairs the four step names as headings to lay the cards under, one story at a time.
• Bigger: write a fourth story as four cards, for example weeds and a weedkiller, and swap it with another pair to rebuild.

Answers and look-fors

• Part A: rabbits and a virus L, J, B, I; finches in a drought G, C, F, D; mosquitoes and a spray E, K, A, H (in step order).
• Part B: 1. Some rabbits already had bodies that could fight the virus. They survived and passed the ability on in their genes. 2. Birds that already had deeper beaks survived the drought and bred, and their young inherited deeper beaks. 3. The spray killed the mosquitoes without the resistance gene; the few that had it survived and multiplied.
• Part C: Peppered moths vary: some are pale and some dark, and the colour is inherited. Soot darkens the bark, so birds find the pale moths more easily. The dark moths survive and breed more, and their young are dark. Over many generations most of the moths become dark.
• Part D: 1. When two scientists working apart reach the same explanation from different evidence, it is less likely to be one person’s mistake, and it gave both their work more weight. 2. Variation: without differences, there is nothing for the environment to select between.

Natural selection, step by step

A. Rebuild the three stories

Sort the twelve cards into three stories of four, and put each story in order. Write the card letters.

B. Fix the statement

Each statement says a living thing changed because it needed to. Rewrite it the way natural selection works.

C. The peppered moth

D. Questions

Three stories of natural selection

Cut out the twelve cards. Sort them into three stories, then put each story in order: variation, selection pressure, survive and reproduce, change over generations.

Teacher note: the answer letters are in the plan. Every story has one card for each of the four steps.

The peppered moth

Peppered moths rest on tree trunks by day, and birds hunt them by sight. Most were pale and speckled until the dark form was recorded near Manchester in 1848. Coal smoke blackened the trunks and killed the pale lichens on them, and by the end of the 1800s nearly all the peppered moths around Manchester were dark. In the 1950s Bernard Kettlewell released marked moths in a polluted wood and a clean one, and recaptured more of the camouflaged form in each. After clean-air laws in the 1950s the soot fell, the lichens came back, and the dark form became rare again. Students graph example survey data of that fall and explain both the rise and the fall.

We are learning to

• graph and describe a change in a population over time,
• explain a rise and a fall in a trait with natural selection,
• judge what an experiment would need to show to support an explanation.

Success criteria

• I can describe the trend in the survey, and say when the change was fastest.
• I can explain why the dark form rose and then fell, without saying a moth changed colour.

You need

• the worksheet (next sheet), one per student, with a ruler and a pencil,
• the exit tickets (third sheet), cut into four.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the moths, the board and the graph in Session A, and Parts C and D and the ticket in Session B.

Watch for these ideas

• “The moths turned dark to hide.” Each moth keeps its colour for life; the dark ones were eaten less, so more dark moths were born.
• “The soot caused the dark form.” The dark form comes from a mutation that can appear anywhere; soot changed which form birds missed.
• “Evolution only goes one way.” When the bark changed back, so did the population: selection follows the environment.

Make it easier, make it bigger

• Easier: plot the first three points together on the board, then let students finish the graph.
• Bigger: list three things that could make a release-and-recapture experiment unfair, and how to fix each one.

Answers and look-fors

• Part B: the dark share falls at every survey, from 94% in 1960 to 9% in 2000; slowly at first, fastest between 1990 and 1995 (down 23 percentage points), then slower again.
• Part C: the rise: moths varied in colour, inherited; soot darkened the bark, so birds found the pale ones more easily; dark moths survived and bred more, passing on the dark colour; over many generations most moths were dark. The fall: the same steps with the pressure reversed, as clean bark made the dark moths easy to see.
• Part D: more dark moths recaptured in the polluted wood and more pale ones in the clean wood, because birds ate more of the moths that stood out. The experiment supports the explanation only if both woods were treated the same way: the same release method, time and number of moths.
• Exit ticket: 1. Soot darkened the bark, so dark moths were eaten less and bred more. 2. Clean air let the bark lighten, so the pale moths were eaten less. 3. No.
• Bigger: releasing moths by day onto trunks, at higher numbers than in the wild, or in only one wood of each kind; fix by matching natural numbers and places and repeating in several woods.

The peppered moth

A team surveyed the peppered moths in one woodland every five years after clean-air laws came in. These are example numbers, in the pattern scientists recorded in England.

A. Graph the survey

B. Describe

C. Explain

D. An experiment

Exit tickets

Resistance

Bacteria reproduce by dividing, in good conditions as often as every 20 minutes, so a population of bacteria can change within days. In a large population a few bacteria carry alleles, often from a mutation, that let them survive an antibiotic. The antibiotic kills the others, the resistant ones multiply, and soon the antibiotic no longer works. This is natural selection, and it happens faster the more often antibiotics are used. Students treat a colony of twenty card bacteria, double the survivors, and work out the mean resistance before and after. No bacteria are grown in this lesson.

We are learning to

• explain antibiotic resistance as natural selection,
• use a mean to describe a change in a population,
• explain why antibiotics should be used only when they are needed.

Success criteria

• I can explain, step by step, why a second course of the antibiotic killed none of the colony.
• I can work out the mean resistance before and after the first course.

You need

• the bacteria cards (third sheet), one set per pair, with scissors,
• the worksheet (next sheet), one per student; calculators.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the doubling, the board and the treatment in Session A, and Parts B and C in Session B.

Watch for these ideas

• “Bacteria get used to the antibiotic.” No bacterium adapts during its life; the resistant ones were already there and survived.
• “People become resistant to antibiotics.” It is the bacteria that become resistant, not the person taking the medicine.
• “Antibiotics cure colds.” Colds are caused by viruses, which antibiotics do not kill.

Make it easier, make it bigger

• Easier: sort the cards by level first, into five piles, before removing the ones the antibiotic kills.
• Bigger: doctors sometimes use two different antibiotics together. Why does that make resistance much less likely to spread?

Answers and look-fors

• Part A, the counts at levels 1 to 5: at the start 6, 6, 4, 3, 1 (20 in all); killed 6, 6, 0, 0, 0 (12); survivors 0, 0, 4, 3, 1 (8); next generation 0, 0, 8, 6, 2 (16); killed by a second course 0.
• Part B: mean at the start 2.35; mean of the next generation 3.63.
• Part C: 1. Every bacterium left had resistance above 2, inherited from a survivor. 2. From variation already in the colony, which arises from mutations (and bacteria can pass resistance genes to each other). 3. The bacteria vary in resistance; the antibiotic is the selection pressure; the resistant ones survive and divide; the colony becomes mostly resistant. 4. A cold is a virus, so the antibiotic does nothing for it, but it still selects for resistant bacteria in the body.
• Bigger: a bacterium would need resistance to both at once, which is far rarer than resistance to one.
• Look for answers that keep the message general: antibiotics only when prescribed, exactly as the doctor says.

Resistance

Each card is one bacterium in a colony, with its resistance level from 1 to 5. A course of the antibiotic kills every bacterium with resistance 2 or less. Each survivor then divides into two, with the same resistance.

A. Treat the colony

B. The mean resistance

Mean = (the sum of every bacterium’s level) ÷ (the number of bacteria).

C. Questions

A colony of twenty bacteria

Cut out the twenty cards. Each bacterium’s resistance level is the number of dots. Nothing is grown: these are the only bacteria in the lesson.

Teacher note: the counts and means are in the plan.

Evidence: fossils

A fossil is the preserved remains or trace of a living thing. In undisturbed sedimentary rock the lower layers are older, the law of superposition, so the order of fossils in the layers records the order in which living things appeared. The oldest fossil-bearing rocks hold only simple living things; fish appear before four-legged animals, and both before mammals and flowering plants. Transitional fossils, such as a feathered animal with teeth and a long bony tail, or early whales with small back legs, have features of an older group and a younger one. Australia holds three famous windows on this record: the Ediacara Hills, Gogo and Riversleigh. Radioactive dating of the rock gives the ages in years.

We are learning to

• use the law of superposition to put rock layers in order of age,
• explain how the order of fossils supports the theory of evolution,
• describe the limits of the fossil record.

Success criteria

• I can order the five layers from oldest to youngest and say how I know.
• I can explain why there are gaps in the fossil record, and why they do not count against evolution.

You need

• the worksheet (next sheet), one per student,
• the exit tickets (third sheet), cut into four; a real fossil or a photo of one, if the school has one.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the papers, the board and Part A in Session A, and Parts B and C and the ticket in Session B.

Watch for these ideas

• “Every living thing becomes a fossil.” Most decay or are eaten; fossils form only when remains are buried quickly, often under water.
• “Gaps in the record disprove evolution.” Gaps are expected because fossils are rare, and every find so far has fitted the order.
• “A fossil’s age is just its depth.” Depth gives the order of the layers; radioactive dating of the rock gives the age in years.

Make it easier, make it bigger

• Easier: give the rule “lowest is oldest” and order the first two layers together.
• Bigger: no fossil of a mammal has ever been found in rock as old as the Ediacara Hills. Explain why that matters to the theory.

Answers and look-fors

• Part A: oldest to youngest C, D, A, E, B; fish appear before four-legged animals, because their layer is lower.
• Part B: Ediacara Hills at about 550 million years ago, Gogo at about 380 million years ago, Riversleigh at about 25 million years ago; Riversleigh’s fossils run on towards today.
• Part C: 1. A fossil with features of an older group and a younger one, such as a feathered animal with teeth and a long bony tail (reptile and bird), or a whale with small back legs. 2. Most remains decay or are eaten; soft bodies rarely fossilise; rock is worn away or heated; and much of the record is still unfound. 3. Simpler living things are in the oldest layers and new groups appear in order, each in younger rock than the group it shares features with, as descent with change predicts.
• Exit ticket: 1. The bottom layer. 2. Remains usually decay or are eaten before they are buried. 3. For example Gogo, about 380 million years.
• Bigger: the theory predicts mammals appeared long after the first animals; a mammal in rock that old would contradict it, and none has been found.

Evidence: fossils

A. Read the rocks

A cliff shows five layers of sedimentary rock that have not been disturbed. The fossils found in each are listed.

B. Australian windows on the past

Mark and label each site on the timeline.

C. Questions

Exit tickets

Evidence: bodies

Comparing bodies is a second line of evidence. Homologous structures share the same plan because they were inherited from a common ancestor, even when they now do very different jobs: a human arm, a whale flipper, a bat wing and a bird wing all have one upper bone, two lower bones, small wrist bones and digits. Analogous structures do the same job but are built in completely different ways, like the wings of a bird and of a butterfly, because they arose separately. Vestigial structures, such as the small hip bones of a whale, are reduced and no longer do the job they did in an ancestor. Students colour the matching bones of four limbs and sort twelve examples into the three kinds.

We are learning to

• identify homologous structures in different species,
• tell homologous structures from analogous ones,
• explain what vestigial structures tell us about ancestors.

Success criteria

• I can colour the same bones in four different limbs and explain why they match.
• I can sort examples into homologous, analogous and vestigial, and justify one of each.

You need

• coloured pencils in three colours; the structure cards (third sheet), one set per pair, with scissors,
• the worksheet (next sheet), one per student.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the arms, the board and the colouring in Session A, and Parts C and D in Session B.

Watch for these ideas

• “Parts that look alike are homologous.” A bird wing and a butterfly wing look alike but are built differently: analogous.
• “Homologous parts do the same job.” They share a plan, often for very different jobs: lifting, swimming, flying.
• “Vestigial means useless.” It means the part no longer does its ancestral job; the human tailbone still anchors muscles.

Make it easier, make it bigger

• Easier: colour the human arm together first, naming each group of bones, before students colour the other three.
• Bigger: a bat wing and a bird wing are homologous as front limbs but analogous as wings. Explain how both can be true.

Answers and look-fors

• Part A: in every limb, one upper bone, two lower bones, then small wrist bones and digits; the bird has three digits, short and partly joined.
• Part B: human arm, grasping, lifting and throwing; whale flipper, steering while swimming; bat wing, flying, on skin stretched between very long fingers; bird wing, flying, on feathers attached along the arm and hand.
• Part C: homologous A, D, F, I, L; analogous B, E, G, K; vestigial C, H, J.
• Part D: 1. Limbs built from scratch for such different jobs would not need the same bones in the same order; inheriting one plan from a shared ancestor explains it. 2. Analogous: both fly, but a bird wing is bone and feathers and an insect wing is a thin sheet of the insect’s outer covering, with no bones; they arose separately. 3. For example, the whale’s hip bones, which supported back legs in its land-living ancestors.
• Bigger: both wings are built on the front limb inherited from a four-limbed ancestor (homologous), but flight evolved separately in bats and birds, in different ways (analogous).

Evidence: bodies

A. Colour the bones

Use one colour for the upper bone, one for the two lower bones, and one for the wrist and digits. Fill in the key.

B. Four jobs

C. Sort the structure cards

D. Questions

Homologous, analogous or vestigial?

Cut out the three headings and the twelve cards. Sort each card under the kind of structure it describes.

Teacher note: the answer letters are in the plan. The three piles are different sizes.

Evidence: DNA

DNA is a third line of evidence, and the newest. After two lines split from a common ancestor, mutations build up in each line separately, so the longer ago the split, the more their DNA differs. Human and chimpanzee DNA are about 99% the same; human and mouse DNA differ far more. Students compare a short stretch of DNA in five species, count the differences from the human sequence, order the species and build a simple tree. The stretch is made up to show the method, but it follows the pattern real comparisons find, and that pattern agrees with the evidence from bodies and fossils.

We are learning to

• compare DNA sequences and count the differences,
• use the differences to judge how closely species are related,
• explain why separate lines of evidence that agree make a theory stronger.

Success criteria

• I can count the differences between two sequences and order species by them.
• I can explain why fewer differences means a more recent common ancestor.

You need

• the worksheet (next sheet), one per student, with a highlighter,
• the exit tickets (third sheet), cut into four.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the recipe, the board and Part A in Session A, and Parts B and C and the ticket in Session B.

Watch for these ideas

• “Humans evolved from chimpanzees.” Humans and chimpanzees share a common ancestor; neither came from the other.
• “99% the same means a chimpanzee is 99% human.” It means 99% of the compared bases match; a few changes in genes that control growth make large differences.
• “Fewer differences means more advanced.” The differences measure the time since a shared ancestor, not progress.

Make it easier, make it bigger

• Easier: find the chimpanzee’s one difference together, base by base, before students go on.
• Bigger: DNA comparisons show that the closest living relatives of whales are hippopotamuses. How would you check a claim like that with today’s method?

Answers and look-fors

• Part A: differences from the human sequence: chimpanzee 1, gorilla 2, mouse 6, chicken 10.
• Part B: most to least closely related to humans: chimpanzee, gorilla, mouse, chicken. The tree: human and chimpanzee at the first join, then gorilla, mouse and chicken in turn.
• Part C: 1. Mutations build up separately in each line after it splits, so less time since the split means fewer differences. 2. Yes: apes are the most like humans in body plan, mice are mammals like us, and birds split off before the mammals did, as the fossils show. 3. Each line could be wrong in its own way; when fossils, bodies and DNA, gathered by different methods, give the same tree, the explanation is far more likely to be right.
• Exit ticket: 1. The chimpanzee. 2. Mutations. 3. No: both come from a common ancestor.
• Bigger: compare the same stretches of DNA from whales, hippos and other hoofed mammals, and count the differences; the fewest should be between whales and hippos.

Evidence: DNA

The same stretch of DNA, 24 bases long, from five species. It is made up to show the method, in the pattern real comparisons find.

A. Count the differences

Highlight every base that is different from the base in the same place in the human sequence. Then count them.

B. Order and tree

2. Name the five tips of the tree. The two most closely related species go at the first join, on the left.

C. Questions

Exit tickets

One tree of life

All living things are related through common ancestors, further and further back. An evolutionary tree shows it: the tips are living species, each join is a common ancestor, and the lower the join, the longer ago it lived. New species form when populations of one species are kept apart, by a sea, a mountain range or a desert. Each population meets its own selection pressures and builds up its own mutations until the two can no longer interbreed. Some of the oldest evidence of life on Earth, layered stromatolites about 3.5 billion years old, is in the Pilbara of Western Australia; the variety of life today is the result of speciation over that whole time. Students read a tree, explain how one species becomes two, and sort twelve pieces of evidence by kind.

We are learning to

• read an evolutionary tree to compare how closely species are related,
• explain how separated populations can become new species,
• sort evidence for evolution into fossils, anatomy, and DNA and proteins.

Success criteria

• I can find the two animals on a tree that share the most recent common ancestor.
• I can explain, in steps, how one species can become two.

You need

• the evidence cards (third sheet), one set per pair, with scissors,
• the worksheet (next sheet), one per student.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the words, the board and Part A in Session A, and Parts B and C in Session B.

Watch for these ideas

• “Evolution is a ladder with humans at the top.” It is a branching tree; every living species is the tip of a branch just as long.
• “The platypus is less evolved.” Its line has been evolving for exactly as long as ours since the two split.
• “A new species appears in one generation.” Differences usually build up over many generations of separation.

Make it easier, make it bigger

• Easier: trace two animals’ branches down with a finger until they meet; the higher the meeting point, the closer the relatives.
• Bigger: Bali and Lombok are only about 35 km apart, yet their animals are very different, as Alfred Russel Wallace noticed. Find out why.

Answers and look-fors

• Part A: 1. The kangaroo and the koala. 2. The kangaroo: they share a more recent ancestor than the platypus and the chicken do. 3. The human: the kangaroo’s line meets ours above the join with the platypus. 4. On the branch to all the mammals: the platypus, kangaroo, koala, mouse and human.
• Part B: the two lizard populations are kept apart, so they cannot interbreed; each meets different pressures (food, predators, climate) and gathers its own mutations; over many generations the differences build up; in the end they cannot breed together even if they meet, and they are two species.
• Part C: fossils B, E, G, I, L; anatomy A, D, F, J; dna and proteins C, H, K.
• Bigger: a deep channel of sea between the two islands stayed open even when sea levels fell, so the animals on each side were never joined by land.

One tree of life

A. Read the tree

Each join is a common ancestor; the lower the join, the longer ago it lived. The gold squares mark where two features first appeared.

B. One species becomes two

C. Sort the evidence cards

Fossils, anatomy, or DNA and proteins?

Cut out the three headings and the twelve cards. Sort each piece of evidence under the kind it is.

Teacher note: the answer letters are in the plan. The three piles are different sizes.

Show what we know

The term comes together. In the task, students meet the cane toads brought to Queensland in 1935, which have spread across northern Australia ever faster: about 10 km a year in the 1940s to 1960s, and more than 50 km a year by the 2000s. Toads at the front of the spread have longer legs than toads where the spread began. Students describe example data in that pattern, explain it with natural selection, correct a news claim, and plan how to test that leg length is inherited. Then they sit the eight-question final check, and the class plays the unit quiz as a game on the board.

We are learning to

• apply the four steps of natural selection to a new case,
• use data to support an explanation, and judge a claim,
• plan a test that separates inherited variation from the effect of the environment.

Success criteria

• I can explain why toads at the front have longer legs, without saying any toad grew them on purpose.
• I can answer the final check on my own.

You need

• the task (next sheet) and the final check (third sheet), one each per student,
• the interactive unit open on the board for the quiz; mini whiteboards if you have them.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the steps and the task in Session A, and the final check and the quiz in Session B.

Watch for these ideas

• Students who say the toads stretched their legs, or needed longer ones: ask which toads were already at the front, and why.
• Students who treat the example numbers as a measurement they can quote: the task says they show the pattern, not the real values.
• Students who name only one line of evidence in the check: ask for a second that was gathered in a different way.

Make it easier, make it bigger

• Easier: give the four step names as sentence starters for task question 2.
• Bigger: the toads at the front also spread out in straighter lines. Explain how the same selection could favour that.

Answers and look-fors

• Task: 1. Leg length rises with the year the toads arrived: from 37% of body length at the site reached in 1945 to 44% at the site reached in 2005. 2. Toads vary in leg length, and the trait is inherited; at the front, the fastest toads arrive first, with fewer rivals for food and space; they breed with each other, and their young inherit long legs; generation after generation, the front is made of ever longer-legged toads. 3. No toad grew longer legs because it needed to; the toads that already had longer legs reached the front and bred there. 4. Raise tadpoles from the front and from an old population in the same conditions, then measure the young toads: if the front’s young still have longer legs, the difference is inherited. 5. It should stay fast or get faster, because the front keeps selecting the fastest toads. 6. Real measurements from many toads at many sites, published and checked by other scientists, ideally repeated by another team.
• Final check: 1. inherited variation. 2. Green dots were hard to see on green cloth, so fewer were taken and more survived to have green offspring. 3. Variation, selection pressure, survive and reproduce, change over generations. 4. homologous. 5. Any two, such as fossils (the order of groups in rock layers), anatomy (the same bones in a bat wing and a human arm), or DNA (fewer differences between close relatives). 6. how common a trait is in the population. 7. The lower one, as long as the layers have not been folded or disturbed. 8. The bacteria varied; the antibiotic killed the ones it could; the resistant ones survived and multiplied; now most of them are resistant.
• Bigger: toads that wander in curves cover less new ground; those that move in straighter lines reach the front first and breed there.
• Mark the task against the rubric in the assessment kit; the check covers variation, selection, evidence and data.

Toads on the march

In 1935 cane toads were brought to Queensland to control beetles in the sugar cane. They have spread across northern Australia ever since, and ever faster: about 10 km a year in the 1940s to 1960s, and more than 50 km a year by the 2000s. Scientists measured toads at sites the toads had reached in different years. The numbers below are example data in the pattern they found.

Show what we know