Open with a real, current phenomenon: MRSA (methicillin-resistant Staphylococcus aureus). Tell students that in 1960, nearly all S. aureus infections were killed by methicillin; by 2005, over 50% of hospital S. aureus samples in the U.S. were methicillin-resistant, and MRSA now kills more Americans each year than HIV. Pose the driving question on the board: 'Did the bacteria learn to fight the antibiotic, or did something else happen?' Take a quick hand-vote: (A) bacteria toughened up when exposed, (B) resistant bacteria were already there and survived, (C) not sure. Do NOT reveal the answer — tell students that by the end of the block they will use data to defend an answer. Tie in a second hook image: an African elephant with no tusks. In Gorongosa National Park (Mozambique), tuskless females were 18.5% before the civil war (1977) and jumped to 50.9% after 15 years of ivory poaching. Ask: 'Did the elephants choose to grow without tusks? Or is something else going on?' Both puzzles are the same phenomenon — selection changing a population.

1. 8m

   Variation and mutation: the raw material

   Content

   Selection cannot act on a population unless individuals differ from one another, and those differences must be heritable. That heritable variation comes from mutation — random changes in DNA sequence, most often from DNA-replication errors when a cell copies its genome. In humans, about 1 in every 100 million bases is miscopied per division; in bacteria dividing every 20 minutes, a single overnight culture of ~10⁹ cells will contain many independent mutations at every gene. Most mutations are neutral or harmful, but some create a new allele that happens to help — for example, a point mutation in the S. aureus pbp gene that changes the shape of a protein methicillin binds to. The key idea: mutations are random with respect to what the organism 'needs.' The antibiotic does not cause the useful mutation; the mutation is already there when the antibiotic arrives.

   Delivery

   Anchor this on the whiteboard question from the hook. Emphasize the word random — this is where the biggest misconception lives. Ask: 'If I raise a bacterial colony in a plain nutrient broth with no antibiotic, will resistant cells exist in it?' (Yes — mutation is constant.) Foreshadow the Luria-Delbrück fluctuation-test result if students are strong: resistant colonies appear even before exposure. Pre-empt the misconception that organisms develop traits because they 'need' them.

2. 12m

   Natural selection: the four-part logic

   Content

   Natural selection follows from four observations that together force a conclusion. (1) Variation: individuals in a population differ in traits. (2) Heritability: some of that variation is passed to offspring through alleles. (3) Overproduction and competition: populations produce more offspring than the environment can support, so not all survive and reproduce. (4) Differential reproductive success: individuals whose traits fit the current environment leave more offspring — they have higher fitness. Over generations, the alleles behind those traits rise in frequency. Work a concrete numeric example: imagine 1,000 S. aureus cells; 999 are methicillin-sensitive (mecA⁻) and 1 carries the resistance allele (mecA⁺). Add methicillin. Sensitive cells die; the one resistant cell divides. After ~10 generations (about 3 hours), that lineage is now ~1,000 cells and dominates the culture. The allele frequency of mecA⁺ went from 0.001 to ~1.0 — not because bacteria learned, but because only the resistant lineage reproduced. Fitness is context-dependent: in a hospital with methicillin, mecA⁺ is high-fitness; in a plain environment, mecA⁺ often carries a small metabolic cost and is lower-fitness. Nothing here has a goal. Selection is not aiming at 'perfection'; it is just a bookkeeping consequence of who reproduces.

   Delivery

   Walk through the four points slowly and have students recite them back — VHOD (Variation, Heritability, Overproduction, Differential success) is a useful mnemonic. Do the 1,000-cell math on the board with them; the frequency change is the payoff. Emphasize twice: individuals do NOT evolve; populations do. The single resistant cell did not 'become more resistant'; its descendants simply outnumber the rest. Pre-empt teleology: strike phrases like 'the bacteria wanted to survive' and rephrase them as 'the resistant cells left more offspring.'

3. 8m

   Artificial selection: humans as the selective pressure

   Content

   Artificial selection works by the same four-step logic as natural selection, but the selective pressure is a human choice about who gets to reproduce. Every domesticated plant and animal is a case study. Corn (Zea mays) descends from teosinte, a Mexican grass whose 'ears' were about 2–3 cm long with 5–12 hard-cased kernels; over ~9,000 years of farmers replanting seeds from the largest, softest-kernel ears, modern maize ears are ~30 cm with hundreds of exposed kernels. Dogs (Canis familiaris) descend from a gray-wolf ancestor; ~15,000 years of selection for tameness, size, and coat produced Chihuahuas and Great Danes — same species. Broccoli, cauliflower, kale, cabbage, kohlrabi, and Brussels sprouts are ALL Brassica oleracea, selected from one wild mustard for different edible parts (flower buds, leaves, stems, terminal buds). The critical link: artificial selection is not 'random change.' It is directed reproduction, and it demonstrates that when you change who breeds, you change the population's allele frequencies — exactly what natural selection does when the environment changes who breeds.

   Delivery

   The point is the parallel structure, not a list of breeds. After naming teosinte → maize and wolf → dog, ask students: 'Who is the selective pressure in these cases?' (The farmer/breeder.) Then ask: 'Who is the selective pressure with MRSA?' (The antibiotic environment — no one is 'choosing,' but the environment still filters who reproduces.) Head off the misconception that artificial selection is random or unrelated to natural selection — it is the same mechanism with a different filter.

4. 10m

   Reading selection from data: moths, elephants, bacteria

   Content

   The clearest evidence for selection is a shift in trait frequency across generations tied to a change in the selective pressure. Three canonical data sets: (1) Peppered moth, Biston betularia, near Manchester, England. Before the Industrial Revolution (~1848), the dark 'carbonaria' form was <2% of the population; light moths were camouflaged on lichen-covered trees. By 1895, after coal soot killed the lichens and blackened tree bark, carbonaria reached ~98% in industrial areas — birds preferentially ate the now-conspicuous light moths. After the UK Clean Air Act (1956), lichens returned and carbonaria frequency dropped back below 20% by 2000. Same alleles, opposite direction, different environment. (2) African elephants, Loxodonta africana, Gorongosa. Poachers targeted tusked elephants for ivory during the 1977–1992 civil war. Frequency of tuskless females rose from 18.5% (pre-war) to 50.9% (post-war). Recent genome work identified a sex-linked allele (AMELX region) that makes tusklessness lethal in males but viable in females — which explains why the shift is in females. (3) Antibiotic resistance: incremental exposure produces a stepwise rise in the resistance allele frequency each generation; the classic 'MEGA-plate' experiment (Baym et al., 2016) shows E. coli evolving 1,000× resistance in ~10 days as they cross a plate with stepped antibiotic concentrations.

   Delivery

   For each case, name the selective pressure explicitly and connect it to which trait rises. This is exactly what students will do in the activities. Ask: 'What would happen if the pressure reversed?' — moths already show the answer. Flag that tuskless elephants are NOT elephants deciding not to grow tusks; the allele existed before poaching, and poaching just changed who reproduced. Reinforce the misconception target: no individual moth changed color; no individual elephant lost its tusks; no individual bacterium 'learned.' Populations shifted.

1. 30m

   Predator–Prey Bean Selection LabLab

   Groups of 4 run a 4-generation predator–prey simulation on a burlap habitat. Three students are 'bird predators' (forceps only, no fingers); one student is the ecosystem manager (scatters beans, times rounds, records). The tan burlap acts as the environment — pinto beans are camouflaged, white and red are conspicuous, black is intermediate. Student handout — Predator–Prey Bean Lab Part 1 — Setup - Spread the burlap flat. Manager scatters 20 of EACH color bean (80 total) randomly across the burlap without looking. - Predators stand back and close their eyes for 5 seconds while beans are scattered. Part 2 — Generation 1 hunt - On 'GO,' each predator has 15 seconds to pick up as many beans as possible using ONLY forceps, one bean at a time, into their cup. - No grabbing handfuls and no using fingers. - After 15 seconds, STOP. Count the SURVIVORS (beans still on burlap) by color and record below. Part 3 — Reproduction rule - Each surviving bean 'reproduces' — the manager adds 1 new bean of the same color for every survivor of that color (double the survivors). If you cannot double (out of that color), add as many as you have. - Rescatter all beans on the burlap. This is Generation 2. Part 4 — Repeat - Run Generations 2, 3, and 4 the same way (15-second hunt, then doubling). Data table — Survivors by color at end of each generation - Generation 1: white ______ , red ______ , black ______ , pinto ______ - Generation 2: white ______ , red ______ , black ______ , pinto ______ - Generation 3: white ______ , red ______ , black ______ , pinto ______ - Generation 4: white ______ , red ______ , black ______ , pinto ______ Part 5 — Graph - On graph paper, plot generation (x-axis, 1–4) vs. number of survivors (y-axis). Use one colored line per bean color. Part 6 — Analysis questions (answer in complete sentences) 1. Which color's frequency INCREASED across generations? Which DECREASED? Why? 2. Identify each of the four ingredients of natural selection (variation, heritability, overproduction, differential reproduction) in THIS simulation. What represents each? 3. A student says, 'The white beans learned to hide better by Generation 4.' Rewrite this sentence using correct evolution language. 4. Predict: if in Generation 3 we swapped the burlap for a WHITE sheet, which color would rise? Explain using the term selective pressure. 5. Where does new variation come from in a real population? (One sentence — connect to mutation.)

   Materials

   • 1 large tray or shallow box per group (representing habitat)
   • ~30 white beans (navy/lima), ~30 red kidney beans, ~30 black beans, ~30 pinto beans per group
   • 1 sheet of tan/brown fabric or burlap per group as habitat backdrop
   • Plastic forceps or tweezers (one per 'predator' student)
   • Timer / stopwatch
   • Data sheet (provided in description)
   • Colored pencils for graphing

   Example outputs

   • Sample data: pinto survivors 12 → 20 → 30 → 42; white survivors 8 → 5 → 2 → 0. Ratio pinto:white shifts from roughly 1:1 to overwhelmingly pinto by Gen 4.
   • Sample answer to Q3: 'White beans did not learn anything. Predators removed white beans faster than pinto beans each generation, so the pinto allele frequency rose while the white frequency fell. The population changed, not the individuals.'
2. 30m

   Case-Study Data Analysis: Moths, Elephants, and Antibiotics

   Students work in pairs through three real data sets and write claim-evidence-reasoning responses. This activity comes AFTER the bean lab so students bring the mechanism with them. Student handout — Selection in Real Populations Case 1 — Peppered moths near Manchester, England Data: percent of Biston betularia caught that were the dark (carbonaria) form. - 1848: 2% - 1860: 15% - 1880: 62% - 1895: 98% - 1956 (Clean Air Act passed): 94% - 1980: 55% - 2000: 18% 1. Plot year (x-axis, 1840–2010) vs. percent dark form (y-axis, 0–100). 2. Mark two turning points on your graph: (a) peak industrial soot ~1895, (b) Clean Air Act 1956. 3. In 1848, what color was better camouflaged and why? In 1895? 4. In 2–3 sentences, explain the RISE from 1848 to 1895 using the terms selective pressure, variation, and fitness. 5. Why did the dark form DECLINE after 1956? Same mechanism — different pressure? Case 2 — Gorongosa National Park elephants (Mozambique) Data: percent of adult FEMALE African elephants (Loxodonta africana) that are tuskless. - Pre-war (1972 census): 18.5% - Post-war (1994 census, after 15 years of ivory poaching): 50.9% - Females born after the war (born 1995–2004): 33% Background: recent genome work found the tuskless allele is X-linked and dominant, but LETHAL in males (males with the allele do not survive to birth). 6. Calculate: how much did tuskless-female frequency change, in percentage points, between 1972 and 1994? ______ 7. What was the selective pressure? Be specific. 8. A student says, 'The elephants stopped growing tusks so poachers would leave them alone.' Identify TWO errors in this claim and rewrite it correctly. 9. Why is the shift seen ONLY in females, not males? (Use the word allele.) Case 3 — Antibiotic resistance in Staphylococcus aureus Data: percent of U.S. hospital S. aureus samples resistant to methicillin (MRSA). - 1960: <1% - 1975: 2.4% - 1985: 17% - 1995: 33% - 2005: 55% - 2015 (after antibiotic-stewardship programs): 43% 10. Plot year vs. % resistant. Describe the trend. 11. Where did the FIRST resistant bacteria come from? (Circle one: a) they mutated because methicillin was present, b) mutations were already there before methicillin was used.) Justify your choice in one sentence. 12. Explain the SLIGHT DECLINE after 2005. What changed about the selective pressure? 13. Claim–Evidence–Reasoning: In a full paragraph, argue whether the MRSA case is (A) individual bacteria learning to resist or (B) a population evolving by natural selection. Cite at least one number from the data and connect it to at least two of: variation, heritability, differential reproduction, allele frequency.

   Materials

   • Student handout (content below)
   • Graph paper
   • Calculators
   • Colored pencils

   Example outputs

   • Q8 sample: 'Error 1 — individual elephants did not stop growing tusks; individuals cannot change their own genes. Error 2 — evolution has no goal; elephants did not act to avoid poachers. Correct: poachers selectively killed tusked females, so tuskless females were the ones that survived and reproduced, raising the tuskless-allele frequency in the population.'
   • Q13 sample CER: 'Claim: MRSA is a population evolving by natural selection. Evidence: resistant frequency rose from <1% in 1960 to 55% in 2005 under methicillin use. Reasoning: mutations in the pbp gene existed in a few cells before methicillin; those cells had higher fitness in the antibiotic environment, reproduced, and passed the resistance allele to offspring, raising its frequency. No individual cell 'learned' — the population's allele frequency shifted.'

1. A colony of Escherichia coli is grown in a flask of nutrient broth with NO antibiotic. A researcher then adds ampicillin to the flask, and 6 hours later finds a dense population of ampicillin-resistant cells. Which statement BEST explains this result? A) The ampicillin caused the bacteria to mutate and develop resistance because they needed it. B) The individual bacteria toughened up and learned to survive ampicillin. C) A few resistant cells were already present due to random mutation; ampicillin killed the sensitive cells, and the resistant ones reproduced. D) The bacteria evolved as individuals in response to the ampicillin.

   multiple choiceC. Mutations arise randomly during DNA replication BEFORE the selective pressure appears. The antibiotic does not create the resistance mutation — it filters who reproduces. Answers A and B express the 'need' / 'lifetime learning' misconception; D confuses individual change with population change.

2. In 1848, only 2% of peppered moths near Manchester were the dark form; by 1895, 98% were dark. Explain this shift in 3–4 sentences using ALL of the following terms: variation, selective pressure, fitness, allele frequency.

   short answerVariation for dark and light color already existed in the moth population before industrialization. Coal soot killed lichens and darkened tree bark, becoming a new selective pressure — birds now spotted and ate light moths more easily than dark ones. This gave dark moths higher fitness (more surviving offspring). Over ~50 generations, the allele frequency for the dark form rose from ~2% to ~98% in the population.

3. A rancher breeds only the fastest horses in each generation. After 20 generations, the average sprint speed of the herd has increased by 15%. Which type of selection is this, and how is it similar to and different from the peppered moth case?

   short answerThis is artificial selection — the rancher (human) is the selective pressure choosing who reproduces. It is SIMILAR to the peppered moth case because the mechanism is identical: heritable variation exists, some individuals leave more offspring than others, and the population's allele frequencies shift over generations. It DIFFERS in the filter — with peppered moths, the environment (predation on conspicuous moths) does the filtering; with horses, a human intentionally selects breeders.

4. At the start of an experiment, a population of 10,000 bacteria contains 50 cells carrying a resistance allele (R) and 9,950 carrying the sensitive allele (r). What is the initial allele frequency of R? After antibiotic exposure kills all sensitive cells and R cells reproduce to reach 10,000, what is the new frequency of R?

   calculationInitial frequency of R = 50 / 10,000 = 0.005 (or 0.5%). After exposure, all surviving cells carry R, so the new frequency of R ≈ 10,000 / 10,000 = 1.0 (or 100%). The allele frequency changed by a factor of 200 in one selection event — because only R cells reproduced.

• ☐Burlap or tan fabric sheets (1 per group of 4)
• ☐Plastic trays or shallow boxes (1 per group)
• ☐Dried beans: white/navy, red kidney, black, and pinto (~20 of each color per group)
• ☐Plastic forceps/tweezers (3 per group)
• ☐Stopwatches or phone timers
• ☐Cups for 'predator' bean collection (3 per group)
• ☐Graph paper (2 sheets per student)
• ☐Colored pencils
• ☐Calculators
• ☐Printed student handout: Predator–Prey Bean Lab (1 per student)
• ☐Printed student handout: Selection in Real Populations case studies (1 per student)
• ☐Printed formative assessment (1 per student)