Natural Selection · Grade 10 Science
Chapter: Evolution, Ancestry & BiodiversityNatural Selection
Learning goals
- I can explain the four factors — heritable variation, overproduction, competition, and differential survival and reproduction — that together drive natural selection.
- I can use the concept of fitness to predict how the traits of a population will shift over many generations in a given environment.
Readings for this unit
Before this unit
Distinguish individual survival from population change.
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain. Trace the explained visual parts and write a first model, then attempt the illustrated and changed independent cases. In the third session explain a correction and revise the opening account using evidence.
Start with something to explain
Why can a population change without every individual changing its own inherited trait?
Illustrative evidence case. Heritable variants contribute different numbers of reproductive descendants under a supplied condition.
Notice and wonder
Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.
Why can a population change without every individual changing its own inherited trait?
Your first model
Connect initial variation to survival/reproduction and later proportions.
Build the language from the idea
- natural selection
- The process by which individuals whose heritable traits are better suited to their environment tend to survive and reproduce at higher rates, so those traits become more common in the population over generations.
- variation
- The differences in heritable traits among the individuals of a population, arising ultimately from mutation and, in sexually reproducing species, the reshuffling of genes.
- overproduction
- The tendency of populations to produce far more offspring than the environment can support, which guarantees competition for limited resources.
- fitness
- A measure of an individual's reproductive success — the number of surviving, fertile offspring it leaves relative to others in the population, not its strength or size.
- adaptation
- A heritable trait, shaped by natural selection over many generations, that improves an organism's fitness in a particular environment.
- selective pressure
- An environmental factor — such as predation, climate, disease, or scarce food — that causes some variants in a population to survive and reproduce more than others.
In 1859, after decades of collecting evidence and famously five weeks studying finches and tortoises in the Galápagos Islands, Charles Darwin published a mechanism that could explain how life changes over time — and the naturalist Alfred Russel Wallace arrived at nearly the same idea independently at almost the same moment. That mechanism is natural selection, and its real power is that it needs no designer and no goal. It follows automatically from a handful of facts that are true of almost every population on Earth.
Start with those facts. First, the individuals in a population are not identical — they show variation in traits like size, speed, color, and biochemistry, and much of that variation is *heritable*, meaning it can be passed to offspring through genes. Second, populations tend toward overproduction: a single oak drops thousands of acorns and a frog lays hundreds of eggs, far more than could ever survive, so individuals inevitably compete for limited food, space, mates, and safety. Put those two facts together and a third follows: survival and reproduction cannot be random. The individuals whose particular traits happen to suit the current environment leave, on average, more offspring than the rest.
Because those helpful traits are heritable, the offspring inherit them too, and the makeup of the whole population shifts a little in the next generation — then a little more in the generation after that. This is the crucial move: evolution is not something an individual does; it is a change in a population across generations. No single beetle turns darker to hide, and no giraffe stretches its neck longer and passes the stretch to its calves. Individuals are simply born with the traits they inherited, and the environment does the 'selecting' by determining who survives to reproduce.
The engine of that selecting is the selective pressure — the specific environmental challenge doing the filtering. When a drought on a Galápagos island left only large, tough seeds, finches that happened to have deeper, stronger beaks could crack them and survive, while shallow-beaked birds starved; within a single generation the average beak in the population grew measurably deeper. When bacteria meet an antibiotic, the rare cells carrying a resistance gene survive and multiply. In each case the pressure is different, but the logic is identical.
This is also where the phrase 'survival of the fittest' gets badly misread. In biology, fitness does not mean strong, fast, or aggressive — it means *reproductively successful*. An organism that lives to a ripe old age but never reproduces has a fitness of zero, while a smaller, weaker individual that leaves many surviving offspring has high fitness. Fitness is always measured relative to a *specific* environment, too: a polar bear's thick white coat is superb fitness in the Arctic and a lethal handicap in a desert. There is no such thing as being 'fit' in the abstract.
The lasting result of this slow filtering is adaptation — a trait so well matched to its environment that it looks almost designed: the camouflage of a stick insect, the water-storing tissue of a cactus, the streamlined body of a shark. It is worth remembering what natural selection can and cannot do. It can only work with the variation already present in a population; it cannot summon a helpful trait on demand, and if the right variation never appears, a population can go extinct despite the 'need.' Selection edits; it does not invent from nothing.
Natural selection requires heritable variation and differences in reproductive success in a particular environment. Individuals experience selection, while allele or trait frequencies change in populations across generations. Organisms do not choose useful mutations because they need them.
Fitness is reproductive contribution in context, not simply physical strength or lifespan. A trait beneficial in one environment can be neutral or harmful in another. Connect the trait to a mechanism and measured survival or reproduction.
Selection is one evolutionary process alongside drift, mutation and gene flow. A population change alone does not identify which process caused it. Compare evidence about inheritance, environment and reproduction before attributing every change to adaptation.
Build your understanding
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain.
Part 1 · Three conditions, one outcome
Natural selection changes inherited variants’ representation when they differ in reproductive success under conditions. Variation must be present, relevant features must be heritable, and survival or reproduction must differ. The selected individuals do not intentionally manufacture the needed allele after noticing a challenge. A population proportion can change even if no individual transforms its own inherited category. Measure the initial variants, offspring contributions and conditions rather than infer all three requirements from a later color difference alone.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Does the later marker distribution mean each blue individual turned gold because it needed to?
Visual reasoning — after your attempt
No. Differential contribution of inherited variants can change proportions across generations without that individual transformation.
Natural selection follows from three conditions. Individuals in a population vary. Some of that variation is heritable. And some of it affects how many offspring an individual leaves in its environment. Where all three hold, the proportions of the heritable variants change from one generation to the next, because the variants that help their bearers reproduce become more common.
Nothing in that account requires intention, effort, or a goal. It is an arithmetic consequence of unequal reproduction among differing, heritable types. Removing any one condition stops the process: without variation there is nothing to select, without heritability nothing is transmitted, and without an effect on reproduction the proportions do not shift.
Pause and explain
A population varies in a trait caused entirely by diet rather than by inheritance. Can selection on that trait change the population across generations?
Check your reasoning after answering
No. The variation is not heritable, so even if it affects survival and reproduction, offspring do not inherit their parents' version of the trait and the next generation's composition is unchanged.
Part 2 · Fitness means offspring, not strength
Fitness concerns reproductive contribution in a specified environment and population context. Equal starting numbers of parents can produce different numbers of surviving reproductive descendants. Size, strength or survival until one moment alone is not the definition. A variant that contributes more offspring in one condition can contribute fewer in another; frequency-dependent interactions and tradeoffs can matter. State what offspring stage is counted, the starting group and the interval before interpreting a measured advantage or comparing datasets.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Could a physically smaller variant have greater measured fitness here?
Visual reasoning — after your attempt
Yes. Fitness is the stated reproductive contribution, not a ranking by physical strength or size.
Fitness in this context means reproductive success in a particular environment: the number of surviving offspring an individual contributes. It is not strength, size, speed, or longevity, although those may contribute to it in some environments. A small, short-lived organism that leaves many surviving offspring has higher fitness than a large, long-lived one that leaves few.
Fitness is also relative to conditions. A trait that raises reproductive success in one environment can lower it in another — a thick coat in a cold climate against a warm one — so there is no such thing as a generally superior type. This is why the phrase "survival of the fittest" misleads: it suggests a fixed ranking, when what matters is reproduction under present conditions.
Pause and explain
Why is the strongest individual in a population not necessarily the fittest in the evolutionary sense?
Check your reasoning after answering
Fitness is measured by surviving offspring contributed to the next generation. Strength raises fitness only if it increases reproduction in that environment; a weaker individual that leaves more surviving offspring has higher fitness.
Part 3 · Selection sorts; mutation supplies
A mutation changes sequence; recombination rearranges existing material; inheritance transmits it. Selection concerns differential contribution among variants under conditions, not the purposeful creation of a helpful change. Some mutation processes have biases, yet this does not mean a population reliably generates exactly the trait it needs. Drift and migration also change frequencies. A changed frequency supports an evolutionary observation, while identifying its cause requires suitable comparisons and evidence of reproductive differences or other mechanisms.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Does observing a useful new allele prove that need caused that precise mutation?
Visual reasoning — after your attempt
No. Its origin and later contribution are distinct questions; utility after selection does not establish purposeful mutation.
Selection cannot produce the variation it acts on. Mutation during DNA replication, together with the recombination and independent assortment of meiosis, generates heritable variation without regard to what would be useful. Selection then sorts among whatever is present. Keeping these two roles separate resolves most confusion about how evolution works.
Antibiotic resistance is the clearest demonstration, because the timing can be checked. Resistant cells arise by mutation before any antibiotic is applied, which has been shown by finding resistant cells in populations never exposed to the drug. Exposure then removes susceptible cells, so the resistant ones come to dominate. The drug changed which cells reproduced; it did not create resistance.
Pause and explain
What observation would distinguish "the antibiotic caused resistance" from "the antibiotic selected pre-existing resistance"?
Check your reasoning after answering
Testing a population that has never been exposed to the antibiotic. Finding resistant individuals already present shows the variation existed independently of the drug, so exposure selected rather than created it.
Part 4 · Measuring change in a population
An allele frequency is the fraction of relevant gene copies belonging to a version; a phenotype frequency counts an observed category and can differ. The supplied frequency rises in one condition, while a comparison fluctuates near its initial value. This supports a condition-related pattern but requires controls, sampling and reproduction evidence to identify selection. Drift, migration and changed detection can supply alternatives. A model’s generation axis is not an individual’s age, and a rising variant is not necessarily beneficial in every environment.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
What additional measurement would strengthen a selection explanation?
Visual reasoning — after your attempt
Compare reproductive contributions of inherited variants under the defined conditions, with replicated populations and alternative mechanisms considered.
Because evolution is a change in a population's composition, the evidence takes the form of frequencies measured across generations. If a trait is present in 8 percent of one generation and 24 percent two generations later, the population has changed, and the size of the change can be stated precisely. An individual's own characteristics are fixed at fertilization, so no individual evolves.
Interpreting such frequencies requires care about sampling and about alternative causes. Small samples fluctuate; immigration can change frequencies without any selection; and chance alone shifts frequencies in small populations. Attributing a measured change to selection therefore means ruling out those alternatives, not just observing that the frequency moved.
Pause and explain
A trait's frequency rises from 10 to 30 percent in a small population over three generations. Name two explanations besides selection.
Check your reasoning after answering
Chance fluctuation, which has large effects in small populations, and immigration of individuals carrying the trait from elsewhere. Ruling these out requires estimates of population size and of movement between populations.
Apply the whole unit · Account for a resistance record
Illustrative monitoring data from a bacterial population treated with one antibiotic. Before treatment: 2 percent of 10,000 sampled cells resistant. After 1 day of treatment: 38 percent of 2,000 cells. After 3 days: 90 percent of 1,200 cells. A separate sample taken from the same source population two years earlier, with no antibiotic ever applied, contained 11 resistant cells in 5,000.
Your task
Calculate the resistant proportion in the historical sample, describe the change in resistant frequency across the treatment period, and explain which measurement establishes that resistance was not created by the antibiotic. Then explain why the falling total cell counts matter to the interpretation, and state what the record cannot tell you.
What your response should show
- Choose the appropriate biological model or relationship and state the conditions it assumes.
- Show the counts, rates, ratios, or intermediate reasoning the question requires.
- Use the supplied evidence to justify the conclusion and separate observation from inference.
- Identify a meaningful limitation, uncertainty, or next measurement.
Response guidance — after your attempt
Historical sample: 11 / 5,000 = 0.22 percent resistant, in a population never exposed to the antibiotic.
Treatment period: resistant frequency rises from 2 percent to 38 percent to 90 percent, a 45-fold increase in share over three days.
Identify the decisive measurement: the unexposed historical sample, which shows resistant cells present before any exposure, so the variation pre-existed the drug.
Interpret the falling totals: sampled cell numbers drop from 10,000 to 1,200, so the resistant share rose partly because susceptible cells were killed rather than because resistant cells multiplied enormously.
Check that reasoning quantitatively: 2 percent of 10,000 is about 200 resistant cells, and 90 percent of 1,200 is about 1,080 — a roughly fivefold increase in resistant cells against a fall in the susceptible population from about 9,800 to about 120.
State the limits: the record does not identify the mutation responsible, does not show whether resistance carries a cost when the antibiotic is absent, and samples may not be representative of the whole population.
The historical sample contained 0.22 percent resistant cells with no antibiotic ever applied, which is the decisive measurement: resistance existed in the population before exposure, so the drug selected rather than created it. During treatment the resistant share rose from 2 to 38 to 90 percent, a 45-fold increase. The falling total counts matter because they show how that increase came about: resistant cells rose from roughly 200 to roughly 1,080, about fivefold, while susceptible cells fell from about 9,800 to about 120. Most of the change in proportion therefore came from susceptible cells dying rather than from a dramatic expansion of resistant ones — a distinction invisible if only percentages are reported. The record cannot identify which mutation confers resistance, cannot show whether resistance costs anything in the drug's absence, and rests on samples that may not represent the whole population; measuring frequencies after treatment ends would address the second question.
Scientific references and adapted source figures
Parent / teacher teaching notes
Two phrasings are worth correcting on sight: that organisms adapt during their lives in ways they pass on, and that a threat causes the resistance that defeats it. Both dissolve once mutation and selection are kept in separate roles — mutation supplies variation blindly, selection sorts it. The percentage-against-count distinction in the application is the quantitative point, and it is the kind of reasoning that also applies to any reported change in proportions elsewhere in the course. Ask for an explanation of each label, arrow and axis before calculation. Keep first and revised models; attempt the changed case before consulting guidance. Distinguish invented instructional records from named historical sources. Source chromosome symbols are allele-category models, not DNA sequences. These independently authored high-school supplements are not the full OpenSciEd teacher editions or an endorsement.
The key idea
Natural selection follows automatically from four facts — heritable variation, overproduction and competition, and differences in survival and reproduction — so that traits improving an individual's fitness (reproductive success) in its environment become more common in the population over generations.
Worked examples
Example 1
- Beak depth varies among individuals and is heritable (passed from parents to offspring).
- The drought is a selective pressure: only birds able to crack hard seeds get enough food to survive.
- Deeper-beaked finches survive and reproduce more than shallow-beaked ones (higher fitness in this environment).
- Because beak depth is heritable, their offspring tend to have deeper beaks, so the population's average beak depth increases over generations.
Example 2 · Worked application
- Variants differ before exposure.
- Treatment reduces survival or reproduction of susceptible bacteria.
- Resistant lineages contribute more descendants under those conditions.
Resistance can also spread through gene transfer; the scenario alone does not specify every mechanism.
Example 3
- In biology, fitness means reproductive success — the number of surviving, fertile offspring — not size, strength, or lifespan.
- Individual A leaves 2 surviving offspring; Individual B leaves 9.
- Therefore B contributes far more of its genes to the next generation.
Example 4
- Birds with that form leave more offspring under these conditions.
- Inheritance can increase the form's frequency across generations.
Example 5 · Find and fix the mistake
- Test the claim against the stated conditions: Resistance can also spread through gene transfer; the scenario alone does not specify every mechanism.
- Mutation is not directed by need; selection acts on variants, with other processes also possible.
Resistance can also spread through gene transfer; the scenario alone does not specify every mechanism.
Example 6 · Reason from the visual evidence
- A mutation changes sequence; recombination rearranges existing material; inheritance transmits it. Selection concerns differential contribution among variants under conditions, not the purposeful creation of a helpful change. Some mutation processes have biases, yet this does not mean a population reliably generates exactly the trait it needs. Drift and migration also change frequencies. A changed frequency supports an evolutionary observation, while identifying its cause requires suitable comparisons and evidence of reproductive differences or other mechanisms.
- No. Its origin and later contribution are distinct questions; utility after selection does not establish purposeful mutation.
Original process-role diagram, not a mutation-rate model or exhaustive population-genetic equation.
Avoid these mistakes
- Individuals do not intentionally evolve a needed trait during their lifetime.
Check your understanding
Drought favors birds with a heritable beak form that helps open available seeds. Explain the next-generation change.
Discuss the answer
Selection can shift the population distribution; birds did not change beaks by deciding to adapt.
Why does a benefit in one environment not guarantee a benefit everywhere?
Discuss the answer
Fitness depends on reproductive contribution under conditions, with tradeoffs and interactions. A feature advantageous here can be disadvantageous elsewhere.
Return to the phenomenon
Return to the initial case. Why can a population change without every individual changing its own inherited trait? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
Selection sorts contributions across generations; mutation origins, drift and sampling remain distinct questions.
What should we investigate next?
Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.
Investigate, explain and revise
Complete these tasks after the teaching parts. Preserve your first model and a supported revision; use guidance after a real attempt.
Activity 1 · Return to the opening case
Use the opening question, defined terms, Parts 1–4 and your notebook. The supplied-data route needs no equipment or external account.
Your task
Return to the initial case. Why can a population change without every individual changing its own inherited trait? Revise your drawing or explanation using two named ideas and one stated limitation.
Success criteria
- Use relevant stated observations.
- Explain the mechanism with a labeled model.
- Distinguish quantities and assumptions.
- Describe a revision and a useful next test.
Hint
Follow objects, materials and information separately. Check what each measured quantity means.
Response guidance — discuss after your attempt
Selection sorts contributions across generations; mutation origins, drift and sampling remain distinct questions.
Activity 2 · Transfer to a changed case
Cover the worked guidance and use the unit explanations/figures to solve a changed problem.
Your task
Equal groups of 20 parents of two heritable variants produce 40 and 70 surviving reproductive offspring under one condition. Compare per-parent output and explain why this does not prove a universal advantage.
Success criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Hint
Name the stated conditions, select the relevant mechanism and show intermediate reasoning.
Response guidance — discuss after your attempt
Outputs are 2 and 3.5 offspring per parent; the second is 1.75 times the first here. Other environments, starting frequencies and interactions can change the result, so the advantage is conditional.
Words to know
- natural selection
- The process by which individuals whose heritable traits are better suited to their environment tend to survive and reproduce at higher rates, so those traits become more common in the population over generations.
- variation
- The differences in heritable traits among the individuals of a population, arising ultimately from mutation and, in sexually reproducing species, the reshuffling of genes.
- overproduction
- The tendency of populations to produce far more offspring than the environment can support, which guarantees competition for limited resources.
- fitness
- A measure of an individual's reproductive success — the number of surviving, fertile offspring it leaves relative to others in the population, not its strength or size.
- adaptation
- A heritable trait, shaped by natural selection over many generations, that improves an organism's fitness in a particular environment.
- selective pressure
- An environmental factor — such as predation, climate, disease, or scarce food — that causes some variants in a population to survive and reproduce more than others.
Two men, one theory: Darwin and Wallace
Charles Darwin spent more than twenty years quietly building his case after returning from his voyage on HMS Beagle, famously nervous about how the world would react. He was jolted into action in 1858 when Alfred Russel Wallace, a younger naturalist working in the Malay Archipelago, mailed him an essay outlining almost exactly the same idea of natural selection. Rather than compete, their work was presented jointly at the Linnean Society of London in 1858, and Darwin published On the Origin of Species the next year. It remains one of science's most striking cases of two people independently uncovering the same profound idea.
Why the giraffe-stretching myth won't die
Before Darwin, Jean-Baptiste Lamarck proposed that traits acquired during life could be inherited — a giraffe stretches its neck, so its offspring are born with longer necks. This is wrong: stretching your muscles doesn't rewrite the DNA in your egg or sperm cells. Natural selection works differently — giraffes were born with varying neck lengths, and the longer-necked ones simply reproduced more. Beware a common modern mix-up: epigenetics (chemical tags that switch genes on or off) is a real and fascinating field, but it does not resurrect Lamarck, because it does not create the new heritable DNA variation that long-term evolution requires.
Evolution caught in the act: the Grants' finches
Biologists Peter and Rosemary Grant spent decades measuring every finch on the tiny Galápagos island of Daphne Major. When a severe drought in 1977 left only large, tough seeds, birds with deeper, stronger beaks survived far better — and in just a single generation, the population's average beak depth measurably increased. Their work is one of the clearest demonstrations that natural selection isn't only a story about the distant past; it can be watched, measured, and predicted in real time.
Survival isn't everything: sexual selection
Some traits shaped by selection actually make survival harder — think of a peacock's enormous, conspicuous tail, which makes fleeing predators more difficult. These persist through sexual selection, a special case where a trait spreads because it boosts mating success rather than survival. One idea, the handicap principle, suggests that only a genuinely healthy, high-quality male can afford to grow and carry such a costly ornament, so the handicap itself becomes an honest advertisement of good genes that potential mates can trust.
Natural selection in the clinic
The bacteria that cause infections reproduce in as little as twenty minutes, so natural selection can reshape them within months. Every time antibiotics are overused, the rare resistant cells survive and multiply, producing 'superbugs' like MRSA that ordinary drugs can no longer kill. This is why the entire field of evolutionary medicine exists — and why doctors insist on using antibiotics carefully, since carelessly applied selection pressure is exactly what breeds resistance.
In the real world
Antibiotic resistance is natural selection in fast-forward: whenever antibiotics are overused, the rare bacterial cells carrying resistance genes survive and multiply while the rest die, which is exactly why doctors urge people to finish prescriptions and limit unnecessary antibiotic use to slow the evolution of drug-resistant 'superbugs.'
Exit check
What must be true for a survival difference to contribute to natural selection across generations?
Check your explanation after answering
The relevant variation must be heritable and affect reproductive contribution; survival alone without inheritance or reproduction is insufficient.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.