Science is not a collection of facts — it is a way of thinking. In this lesson you will learn how scientists construct powerful arguments using claims, evidence and reasoning; how to tell reliable sources from misleading ones; and how to communicate your own conclusions with the clarity and precision that scientific questions demand.
Imagine you are trying to convince a friend that smartphones affect sleep. You might say "They just do" or "Everyone knows it." But a scientist would need something stronger.
Now answer: What three things would make an argument about genetics or evolution convincing to you? How would you decide whether a website or video about gene editing is trustworthy?
Every scientific argument has the same backbone. Whether a biologist is arguing that antibiotic resistance is increasing, or a geneticist is defending the safety of CRISPR, the structure is identical: claim, evidence, reasoning — known as CER.
Claim is your answer to the question. It must be specific and debatable. "Evolution happens" is too vague. "Antibiotic resistance in Staphylococcus aureus has increased in Australian hospitals since 2000" is a strong claim because it is precise and testable.
Evidence is the data that supports your claim. Evidence must be observable, measurable and reproducible. It can come from experiments, field studies, meta-analyses or large datasets. A single anecdote is not evidence. A controlled trial with 500 participants is.
Reasoning is the bridge between evidence and claim. It explains why the evidence matters, using scientific principles. Reasoning answers the question: "So what? Why does this evidence prove my claim?" Without reasoning, evidence is just a pile of facts.
Australian researchers use CER every day in public health debates. When Professor Fiona Stanley argued that folate fortification of bread would reduce neural tube defects, she presented a claim (fortification reduces spina bifida rates), evidence (randomised controlled trials showing 50–70% reduction) and reasoning (folate is essential for neural tube closure during early embryonic development). The argument was so compelling that Australia mandated folate fortification in 2009. This is how science changes policy — not through shouting, but through structured, evidence-based argumentation.
In an age of viral posts and AI-generated content, being able to evaluate sources is one of the most important scientific skills you can develop. A reliable source in science has several defining features:
Bias can appear in subtle ways. A pharmaceutical company funding a study on its own drug has a financial conflict of interest. A blog selling organic supplements may overstate risks of GM foods to boost sales. Even well-meaning sources can be biased by confirmation bias — only citing evidence that supports a pre-existing belief.
A well-written scientific argument reads like a logical staircase: each step follows naturally from the one before. The language is precise, the tone is measured, and uncertainty is acknowledged rather than hidden.
Start with your claim in the first sentence. Then present your evidence in order of strength, using data rather than vague descriptions. "Many people got better" is weak. "73% of participants showed reduced symptoms compared to 31% in the control group" is strong. Finally, explain your reasoning using scientific principles. Connect your evidence to concepts like natural selection, genetic inheritance, or molecular structure.
Scientific writing also acknowledges limitations. No study is perfect. Sample sizes may be small, confounding variables may exist, and correlation does not prove causation. A scientist who admits uncertainty is more trustworthy than one who claims absolute certainty.
Wrong: "If a source is published online, it must be reliable."
Right: Anyone can publish online. Reliability depends on peer review, author expertise, citations and transparency — not on whether something looks professional or appears high in search results.
Wrong: "In science, a 'theory' is just a guess that has not been proven."
Right: In science, a theory is a well-substantiated explanation supported by extensive evidence from multiple independent lines of inquiry. The theory of evolution by natural selection is as well supported as the theory of gravity. A "guess" in science is called a hypothesis.
1 Identify at least three problems with this argument (e.g., weak evidence, logical fallacies, unreliable source).
2 Rewrite the argument as a proper CER response about whether GM foods should be regulated in Australia. Include a clear claim, at least one piece of strong evidence, and reasoning.
3 The flawed argument uses an anecdote ("my uncle felt sick") as evidence. Explain why anecdotes are scientifically weak, and describe what kind of evidence would be stronger.
1 Question: Should Australia allow gene editing in human embryos to prevent inherited diseases? Construct a CER argument with a clear claim, two pieces of evidence, and reasoning.
2 Question: A social media post claims that "vaccines alter your DNA." Evaluate this claim using source reliability criteria. Is the claim supported by scientific evidence? Explain.
3 Question: Scientists have observed that cane toads in Australia are evolving longer legs and spreading faster than when they were first introduced. Construct a CER argument explaining why this is an example of evolution by natural selection.
1. In a claim-evidence-reasoning argument, what is the "reasoning"?
2. Which of the following is a reliable source for a scientific argument?
3. A student argues: "Evolution is just a theory, so it is not proven." What is wrong with this argument?
4. Which claim is best supported by evidence?
5. A website claims "GM foods cause cancer" but cites no studies and sells organic supplements. What bias is present?
6. Explain the three parts of a claim-evidence-reasoning (CER) argument. Use an example about evolution or genetics. 4 MARKS
7. Read this claim: "Humans are not evolving anymore because we have medicine and technology." Identify the claim, evaluate whether the evidence supports it, and explain your reasoning. 4 MARKS
8. Describe two criteria you would use to evaluate whether a source about genetic technologies is reliable. Explain why each criterion matters. 4 MARKS
Go back to your Think First responses at the top of the lesson.
1. Problem 1: The evidence is a single anecdote ("my uncle felt sick"), which is not reproducible, measurable or representative of a population [1 mark]. Problem 2: The argument commits a logical fallacy — "natural things are always better" is an appeal to nature, not a scientific principle [1 mark]. Problem 3: The source is a blog with no citations, no peer review and no identifiable author, making it unreliable [1 mark].
2. Claim: GM foods in Australia should be regulated through rigorous safety testing and clear labelling [1 mark]. Evidence: Multiple peer-reviewed studies, including meta-analyses published in Journal of Animal Science, have found no consistent evidence of harm from GM foods approved for human consumption [1 mark]. Reasoning: Scientific regulation requires reproducible evidence of safety or harm. Without large-scale, controlled studies showing risk, banning GM foods is not justified by the current evidence base, though ongoing monitoring is scientifically appropriate [1 mark].
3. Anecdotes are scientifically weak because they involve a single observation with no control group, no blinding and no way to rule out coincidence or other causes [1 mark]. The uncle might have felt sick for unrelated reasons (food poisoning, virus, anxiety). Stronger evidence would be a randomised controlled trial or a meta-analysis of multiple independent studies measuring health outcomes in large populations [1 mark].
1. Claim: Australia should permit gene editing in human embryos only to prevent serious inherited diseases, under strict regulatory oversight [1 mark]. Evidence 1: In 2019, a Chinese scientist edited human embryos without oversight, causing international condemnation and demonstrating that unregulated use causes harm [1 mark]. Evidence 2: The UK's Human Fertilisation and Embryology Authority has approved controlled research on embryo editing for mitochondrial diseases, showing that regulated research can proceed safely [1 mark]. Reasoning: Gene editing offers the potential to eliminate devastating genetic conditions, but the technology also raises ethical concerns about unintended effects and eugenics. Strict regulation balances scientific benefit against ethical risk, which is why Australia should follow a regulated, disease-limited approach [1 mark].
2. Source reliability: The claim lacks peer-reviewed citations and contradicts established molecular biology [1 mark]. Scientific evidence: Vaccines contain mRNA or weakened pathogens that stimulate immune response, but no vaccine licensed in Australia integrates into human DNA. The mRNA in COVID-19 vaccines remains in the cytoplasm and is degraded within days; it never enters the nucleus where chromosomes are stored [1 mark]. Conclusion: The claim is not supported by scientific evidence and appears to originate from misinformation sources rather than peer-reviewed research [1 mark].
3. Claim: The rapid spread of longer-legged cane toads in Australia is an example of evolution by natural selection [1 mark]. Evidence: Field studies by researchers at the University of Sydney have shown that toads at the invasion front have significantly longer legs than toads at the source population, and they travel faster [1 mark]. Reasoning: Longer-legged toads can cover more ground in a given time, reaching new habitats and resources ahead of shorter-legged toads. Because leg length is heritable, the offspring of faster, longer-legged toads also tend to have longer legs. Over generations, the proportion of longer-legged toads increases in the population — the definition of natural selection acting on heritable variation [1 mark].
1. C — Reasoning explains why the evidence supports the claim. Evidence is the data, not the reasoning. The question is separate from the reasoning, and the source list is part of referencing, not reasoning.
2. B — Peer-reviewed journal articles from university research teams undergo expert evaluation and are the gold standard for scientific information. Social media posts, product blogs and sensational headlines lack these safeguards.
3. D — In science, a theory is a well-supported, comprehensive explanation backed by extensive evidence. The theory of evolution is supported by genetics, palaeontology, comparative anatomy and biogeography. "Just a theory" misrepresents the scientific meaning of the word.
4. A — This claim is specific, cites worldwide hospital data, and acknowledges a correlation with a known mechanism. The other options rely on anecdotes, make absolute unsupported predictions, or commit logical fallacies.
5. B — The website has a financial conflict of interest (selling organic supplements) and provides no peer-reviewed evidence. These are serious red flags for bias and unreliability.
Q6 (4 marks): A CER argument has three parts. Claim is a specific, testable statement that answers the question [1 mark]. Evidence is observable, measurable data from experiments, field studies or reliable datasets [1 mark]. Reasoning is the logical explanation of why the evidence supports the claim, using scientific principles [1 mark]. Example: Claim — "The peppered moth population in industrial England evolved darker colouration due to natural selection" [0.5 mark]. Evidence — "Bird predation experiments showed dark moths survived better on soot-covered trees" [0.5 mark]. Reasoning — "Because colour is heritable and predators selectively removed light moths from dark backgrounds, the allele frequency for dark colour increased over generations" [1 mark].
Q7 (4 marks): The claim is that "humans are not evolving anymore because we have medicine and technology" [1 mark]. This claim is poorly supported by evidence. While medicine and technology have reduced selection pressure for some traits (e.g., resistance to certain infections), humans continue to evolve in measurable ways [1 mark]. Evidence includes the rapid increase in lactase persistence in some populations, changes in height and skin pigmentation across generations, and genetic adaptation to high-altitude environments [1 mark]. Reasoning: Evolution simply requires heritable variation, differential survival/reproduction and time. Medicine does not eliminate these three conditions; it changes which traits are selected for. Therefore the claim overstates the effect of technology and ignores ongoing evolutionary processes [1 mark].
Q8 (4 marks): Criterion 1 — Peer review: I would check whether the source has been peer-reviewed. Peer review means independent experts have evaluated the methods, data and conclusions before publication [1 mark]. This matters because it catches errors, biases and unsupported claims that non-experts might miss [1 mark]. Criterion 2 — Author expertise and affiliation: I would check whether the authors have relevant qualifications and are affiliated with recognised institutions (e.g., universities, CSIRO, medical research institutes) [1 mark]. This matters because expertise ensures the authors understand the complex science involved, and institutional affiliation provides accountability and access to resources for rigorous research [1 mark].
Climb platforms using your knowledge of CER frameworks, source evaluation and scientific reasoning. Pool: Lesson 19.
Tick when you have finished all activities and checked your answers.