Open by asking: 'Whales live in the ocean and look like giant fish — so why do they have tiny hip and leg bones buried in their bodies?' Show that modern whales retain a vestigial pelvis and, in some fossils (Pakicetus, Ambulocetus, Basilosaurus), full hind limbs that shrank over ~50 million years. Then hit them with the molecular twist: whale DNA is more similar to hippos than to any fish. Ask: 'If whales look like fish but their DNA says hippo, which evidence wins — and why?' Let 3–4 students respond. Land the point: relationships are inferred from MULTIPLE lines of evidence (fossils, anatomy, DNA), and today we will learn how scientists put that evidence together into a tree.

1. 10m

   How to read a phylogenetic tree

   Content

   A phylogenetic tree is a hypothesis about common ancestry. The tips are living or fossil taxa. Each internal node is a most recent common ancestor (MRCA) — a real ancestral population that split into two lineages. Two taxa are more closely related when their MRCA is more recent (closer to the tips), not when they sit closer on the page. You can rotate any branch around a node like a mobile without changing the relationships — so reading left-to-right as 'primitive to advanced' is wrong. There is no 'higher' or 'lower' organism; every tip is a modern endpoint of its own lineage. Walk a concrete example: on a tree with shark, salamander, lizard, mouse, and chimp, the MRCA of mouse and chimp is more recent than the MRCA of mouse and lizard, so mouse is more closely related to chimp than to lizard — even though a mouse looks more like a lizard than a chimp does. A cladogram shows only branching order; a phylogenetic tree may also scale branch length to time or genetic change.

   Delivery

   Use the slide's tree to point at nodes, not tips, when you say 'related.' Ask the class: 'Which two taxa share the most recent common ancestor?' and make them point to the NODE. Pre-empt the ladder misconception directly: say 'the tree is not a ranking — rotate any branch and the story does not change.' Also head off the sister-species confusion: closely branched species did NOT evolve from each other; they both evolved from the shared ancestor at their node. Do a 30-second turn-and-talk: 'Is a mouse more related to a lizard or a chimp? Defend using the node.'

2. 10m

   Five lines of evidence for common ancestry

   Content

   Scientists do not rely on one type of evidence — they triangulate. (1) Fossil record: transitional forms like Tiktaalik (fish → tetrapod, ~375 mya) and Archaeopteryx (dinosaur → bird) show step-by-step change over time. (2) Comparative anatomy: homologous structures share underlying bone pattern (one bone — two bones — many small bones — digits) across the human arm, whale flipper, bat wing, and cat leg, because all four inherited the tetrapod limb from a shared ancestor. Contrast with analogous structures — bird wings and insect wings both fly but have nothing in common structurally, so they are convergence, not shared ancestry. (3) Comparative embryology: vertebrate embryos all show pharyngeal arches and a post-anal tail early in development, even species that lose them later. (4) Biogeography: Australia's marsupials (kangaroo, koala, wombat) share a common marsupial ancestor and radiated after the continent isolated ~180 mya. (5) Molecular evidence: cytochrome c, hemoglobin, and rRNA sequences differ by roughly the amount predicted by fossil dates — humans and chimps differ by ~1–2% in DNA, humans and mice by ~15%, humans and yeast by far more. When multiple independent lines agree, the common-ancestry hypothesis is strongly supported.

   Delivery

   Walk through each line of evidence with ONE named example — the slide anchors each with a canonical image (tetrapod limb homology, Tiktaalik, Australian marsupials, aligned cytochrome c). Emphasize the logic: 'similar' is not enough — you must check whether the similarity is HOMOLOGOUS (inherited) or ANALOGOUS (convergent). Pre-empt the 'similarity = relatedness' misconception by contrasting shark/dolphin (look alike, distant) with hippo/whale (look different, close). Ask: 'Which two lines of evidence would you want before you trust a tree?' Students should say fossils + molecular, or anatomy + molecular.

3. 10m

   Cladograms and shared derived characters

   Content

   A cladogram is built by grouping taxa according to shared derived characters (synapomorphies) — traits that first appeared in a common ancestor and are inherited by all descendants of that ancestor. You start with an outgroup, a taxon known to have branched off before the others (e.g., lancelet for vertebrates), which lets you decide which trait state is ancestral and which is derived. Work an example with lamprey, shark, salamander, lizard, mouse: 'vertebral column' is shared by all five and defines the vertebrate clade; 'jaws' is derived and unites shark, salamander, lizard, mouse; 'four limbs (tetrapod)' unites salamander, lizard, mouse; 'amniotic egg' unites lizard and mouse. Each derived trait marks a NODE — the ancestor where that trait first appeared. A clade is a node plus everything above it: 'reptiles + birds' is a clade; 'fish' (excluding tetrapods) is NOT a clade because it leaves out descendants. Sister taxa share the most recent node but did not evolve from each other — both evolved from the common ancestor at that node.

   Delivery

   The slide shows the vertebrate cladogram with the four traits mapped to their nodes. Walk finger from outgroup up: 'vertebrae here — jaws here — limbs here — amniotic egg here.' Emphasize the outgroup's job: it tells us which state is ancestral. Pre-empt the 'sister species evolved from each other' error by pointing at the node between lizard and mouse: 'Neither one came from the other; both came from THAT population.' Have students identify one clade and one non-clade on the slide.

4. 8m

   Are viruses alive? The evidence

   Content

   The standard checklist for life includes: made of cells, maintains homeostasis, uses energy (metabolism), grows, responds to stimuli, reproduces on its own, and evolves. Viruses satisfy some of these but fail the most fundamental ones. A virus is a protein capsid (sometimes with a lipid envelope) wrapping a genome of DNA or RNA — no cell, no membrane-bound cytoplasm, no ribosomes. Outside a host, a virus is inert: it does not eat, respire, or maintain internal conditions. It cannot reproduce on its own — it must hijack a host cell's ribosomes and enzymes to make copies. On the 'alive' side, viruses have genetic material, they evolve rapidly by mutation and selection (flu strains, SARS-CoV-2 variants), and they interact with living systems. Where do they sit on a tree? Because viruses can grab genes from hosts and from each other (horizontal transfer), and because different virus families likely arose independently, viruses are usually placed in a SEPARATE category — often shown off to the side of the tree of cellular life, not as a clade within it. Introduce the counter-case briefly: giant viruses like Mimivirus and Pandoravirus have hundreds of genes and blur the line, keeping the debate alive.

   Delivery

   Frame this as a scientific claim-evidence problem, not a trivia question. Put the criteria on the slide and score viruses out loud together — check, X, X, X, check, X, check. Ask the class where the strongest 'not alive' evidence sits (no cells + cannot reproduce alone) and where the strongest 'alive' evidence sits (evolves; has a genome). Pre-empt the misconception that 'viruses are clearly alive because they make us sick' — causing disease is not one of the criteria for life. Tell them Activity 2 will be a structured debate on exactly this question, so they should be collecting evidence now.

1. 40m

   Activity 1 — Build a Cladogram from a Character Matrix and Check It Against DNA

   Students work in pairs. Distribute the handout below. Part 1 asks them to build a cladogram from a shared-derived-character matrix. Part 2 gives them an aligned DNA fragment for the same taxa and asks them to check whether the molecular evidence supports their morphological tree. Walk around and check: are they using the outgroup (lancelet) to decide ancestral vs. derived? Are they placing each derived trait on the correct NODE? Are they counting sequence differences carefully (each mismatched base = 1 difference)? Expect about 20 min on Part 1, 15 min on Part 2, 5 min debrief. In the debrief, ask one pair to defend their tree using two lines of evidence (traits + DNA) — this is exactly the standard's assessment style. Student handout — Part 1: Build the cladogram You have data on six animals: lancelet (outgroup), lamprey, tuna, frog, lizard, and mouse. A '1' means the trait is present, a '0' means it is absent. Character matrix - Lancelet: vertebrae 0, jaws 0, bony skeleton 0, four limbs 0, amniotic egg 0, hair 0 - Lamprey: vertebrae 1, jaws 0, bony skeleton 0, four limbs 0, amniotic egg 0, hair 0 - Tuna: vertebrae 1, jaws 1, bony skeleton 1, four limbs 0, amniotic egg 0, hair 0 - Frog: vertebrae 1, jaws 1, bony skeleton 1, four limbs 1, amniotic egg 0, hair 0 - Lizard: vertebrae 1, jaws 1, bony skeleton 1, four limbs 1, amniotic egg 1, hair 0 - Mouse: vertebrae 1, jaws 1, bony skeleton 1, four limbs 1, amniotic egg 1, hair 1 1. Because the lancelet is the outgroup, any trait it lacks is ancestral absent. Any '1' in the other taxa is a derived trait. 2. List the taxa in order from fewest to most derived traits: ______, ______, ______, ______, ______. 3. Draw a cladogram with lancelet on the far left. At each node, label the derived trait that first appears there. - Node 1 (after lancelet): ______ - Node 2 (after lamprey): ______ - Node 3 (after tuna): ______ - Node 4 (after frog): ______ - Node 5 (after lizard): ______ 4. Which two taxa are sister taxa? ______ and ______. 5. Is 'fish' (lamprey + tuna, excluding tetrapods) a clade? Explain in one sentence: ______. Student handout — Part 2: Check against DNA Below is an aligned 20-base fragment of a homologous gene. Count the number of positions where each taxon differs from the mouse. - Mouse: A T G C C A T G A A C T G G A T C C A G - Lizard: A T G C C A T G A A C T G G A T C C A T - Frog: A T G C C A T G A A C T G C A T G C A T - Tuna: A T G C C A A G A A C T C C A A G C A T - Lamprey: A T G C G A A G T A C C C C A A G G A T - Lancelet: A A G C G A A G T T C C C C T A G G A A 6. Differences from mouse (count mismatches): - Lizard: ______ - Frog: ______ - Tuna: ______ - Lamprey: ______ - Lancelet: ______ 7. Does the ORDER of DNA similarity to mouse (fewest → most differences) match the order predicted by your Part 1 cladogram? Yes / No. Explain: ______. 8. Claim–Evidence–Reasoning: In 3–4 sentences, make the claim that these six taxa share a common ancestor. Cite BOTH the morphological evidence (Part 1) and the molecular evidence (Part 2). Reason: why does agreement between two independent lines of evidence strengthen the claim?

   Materials

   • Printed handout (below)
   • Pencil
   • Ruler

   Example outputs

   • Correct cladogram: (lancelet, (lamprey, (tuna, (frog, (lizard, mouse))))). Node labels: vertebrae → jaws + bony skeleton → four limbs → amniotic egg → hair. Sister taxa: lizard and mouse. 'Fish' is NOT a clade because it excludes the tetrapod descendants of the shared ancestor.
   • Expected mismatch counts vs. mouse: lizard 1, frog 4, tuna 6, lamprey 10, lancelet 13. Order matches the morphological cladogram, so the two independent lines of evidence agree — this is exactly why the common-ancestry hypothesis is strongly supported.
2. 25m

   Activity 2 — Structured Debate: Are Viruses Alive?

   Split the class in half: 'Alive' side and 'Not alive' side (assign — do not let them pick). Give 8 minutes for each side to fill out the evidence organizer using the criteria for life plus the biology of viruses. Run a 12-minute structured debate: 2 min opening from each side, 4 min cross-examination (each side asks the other two questions), 2 min closing from each side. Then 3 minutes — students drop their assigned side and write their PERSONAL claim with evidence. Walk around during prep and push students past 'they make us sick' — that is not a life criterion. If a student cites SARS-CoV-2 variants, that is strong evidence for the 'alive' side (evolution) — make sure the 'not alive' side has an answer (evolution happens in populations, not necessarily requiring the individual to be alive). Student handout — Virus evidence organizer Criteria for life: made of cells · maintains homeostasis · uses energy (metabolism) · grows · responds to stimuli · reproduces independently · evolves Virus facts you may use: - A virus particle (virion) is a protein capsid around a DNA or RNA genome; some have a lipid envelope stolen from the host membrane. - Viruses have NO ribosomes, NO metabolic enzymes of their own, and NO cell membrane enclosing cytoplasm. - Outside a host, a virus is chemically inert — it can be crystallized like a mineral. - Viruses replicate ONLY by hijacking a host cell's ribosomes, nucleotides, and ATP. - Viral genomes mutate rapidly; flu and SARS-CoV-2 evolve new variants within months. - Giant viruses (Mimivirus ~1.2 million bp, ~1000 genes) approach the gene count of small bacteria. - Viruses acquire genes horizontally from hosts and from each other, so they do not form a single clean clade on the tree of cellular life. Part A — Fill in your assigned side. My assigned side: Alive / Not alive List 3 strongest pieces of evidence for your side: 1. ______ 2. ______ 3. ______ Anticipate the other side's strongest argument, and prepare a rebuttal: - Their argument: ______ - Our rebuttal: ______ Part B — After the debate, write your PERSONAL position (2–3 sentences). Claim: Viruses are / are not / should be placed in a separate category from living things. Evidence (cite at least 2 specific facts): ______ Reasoning (why does this evidence support your claim?): ______

   Materials

   • Printed handout (below)
   • Pencil

   Example outputs

   • 'Not alive' evidence: (1) no cells or ribosomes, so no metabolism of their own; (2) cannot reproduce without hijacking a host — they are obligate intracellular parasites; (3) can be crystallized outside a host like a non-living chemical. Rebuttal to 'they evolve': evolution can act on any replicator, even non-living ones, so evolution alone is not sufficient evidence of life.
   • 'Alive' or 'separate category' position: Viruses have genetic material, they evolve rapidly (SARS-CoV-2 variants), and giant viruses like Mimivirus have ~1000 genes — but they lack cells and cannot reproduce independently. Best answer: place viruses in a SEPARATE category — they meet some but not all criteria, and they do not fit cleanly onto the tree of cellular life because of horizontal gene transfer.

1. On a cladogram showing shark → salamander → lizard → mouse → chimp (in that branching order, with shark as the outgroup), which pair shares the most recent common ancestor, and what does that mean about their relatedness compared with lizard and mouse?

   short answerMouse and chimp share the most recent common ancestor (the node closest to the tips). This means mouse and chimp are more closely related to each other than either is to lizard, because their MRCA is more recent than the MRCA that unites them with lizard.

2. A student claims that dolphins are more closely related to sharks than to hippos because 'they both live in the ocean and have fins.' Using at least TWO lines of evidence from this lesson, evaluate the claim.

   short answerThe claim is wrong. (1) Comparative anatomy: dolphin flippers show the homologous tetrapod limb bone pattern (humerus, radius/ulna, carpals, digits) while shark fins do not — the similarity to sharks is analogous (convergent for swimming), not homologous. (2) Molecular evidence: dolphin DNA and proteins are far more similar to hippo than to shark; whales and hippos share a recent common ancestor. (3) Fossil evidence: transitional whale fossils (Pakicetus, Ambulocetus) show four-legged land-mammal ancestors. Similarity alone is not enough — you must check whether it is homologous.

3. On a cladogram of vertebrates, 'amniotic egg' is marked at the node uniting lizards and mammals. Which of the following is the BEST interpretation? A) Lizards evolved from mammals. B) Mammals evolved from lizards. C) Lizards and mammals both evolved from a common ancestor that had the amniotic egg. D) The amniotic egg evolved twice, once in lizards and once in mammals.

   multiple choiceC. The node represents a shared common ancestor in which the amniotic egg first appeared; both lizards and mammals inherited it. A and B are the 'ladder' misconception; D would require the trait to be analogous, but the cladogram shows it as a single shared derived character.

4. A classmate says, 'Viruses are clearly alive because they reproduce and evolve into new strains like the flu.' Using at least TWO criteria for life, argue for placing viruses in a SEPARATE category from living organisms.

   short answerViruses fail several core criteria for life. (1) They are not made of cells — a virion is only a protein capsid around a DNA or RNA genome, with no cytoplasm, no membrane-bound organelles, and no ribosomes. (2) They have no metabolism of their own — no enzymes to make ATP or build molecules. (3) They cannot reproduce independently — they must hijack a host cell's ribosomes and nucleotides. Yes, they evolve and have a genome, but that is not sufficient. Because they meet some but not all criteria, and because they acquire genes horizontally and do not form a single clade on the tree of cellular life, viruses are best placed in a separate category.

• ☐Printed Activity 1 handout (character matrix + DNA alignment + CER prompt) — one per student
• ☐Printed Activity 2 handout (virus evidence organizer) — one per student
• ☐Pencils
• ☐Rulers (for drawing cladogram branches cleanly)
• ☐Projector for the slide deck (cladogram, homologous limb bones, aligned sequences, virus structure)