English:Natural Selection
Natural Selection
Introduction
Why do some insects blend into tree bark while others are easy for birds to spot? Why can a population of bacteria become harder to kill with an antibiotic? Why do related birds on different islands have different beaks?
These questions can be explained by natural selection. Natural selection is a process in which individuals with certain heritable traits tend to survive and reproduce more successfully than other individuals in the same environment. Across many generations, those helpful traits can become more common in the population. Natural selection is one of the main mechanisms of evolution.
In this aiMOOC for Grades 7–8, you will explore variation, inheritance, selection pressure, biological fitness, adaptation, and evolution. You will use examples from beetles, peppered moths, Galápagos finches, and bacteria. You will also investigate how scientists build explanations from evidence.
Learning Goals
By the end of this aiMOOC, you should be able to explain natural selection in your own words and connect it to evolution.
You should be able to distinguish between a trait that is simply different and a trait that gives an organism an advantage in a particular environment.
You should be able to explain why variation and heredity are necessary for natural selection.
You should be able to describe how a selection pressure can affect which individuals leave more offspring.
You should be able to interpret examples such as camouflage, finch beaks, and antibiotic resistance without claiming that organisms change because they "need" to.
You should be able to use observations or simple data to make a claim, support it with evidence, and explain your reasoning.
The Big Idea of Natural Selection
A population contains individuals that are not exactly alike. Some differences are caused by genes and can be inherited. If a heritable difference affects survival or reproduction in a particular environment, individuals with that trait may leave more offspring. Their offspring can inherit the trait. Over generations, the trait may become more common.
Natural selection therefore acts on individual differences, but the long-term evolutionary change is seen in a population. An individual organism does not evolve during its lifetime.
Natural selection is not a plan and does not work toward a perfect final form. Environments change, and a trait that is useful in one place or time may be neutral or harmful somewhere else.
Key Vocabulary
Population: A group of organisms of the same species living in the same area and able to reproduce with one another.
Variation: Differences among individuals in a population, such as body size, color, beak shape, or resistance to a disease.
Heritable trait: A characteristic that can be passed from parents to offspring through genetic information.
Selection pressure: An environmental factor that affects survival or reproduction, such as predators, drought, temperature, disease, or competition for food.
Fitness: In biology, the relative success of an organism in surviving and producing offspring in a particular environment. It does not simply mean being strong or athletic.
Adaptation: A heritable feature that improves survival or reproductive success in a particular environment and has become established in a population through evolution.
Mutation: A change in genetic material. Mutations can create new genetic variants. Their effects can be helpful, harmful, or neutral depending on the environment.
Evolution: Change in the heritable characteristics of populations across generations.
The Ingredients of Natural Selection
Natural selection requires several connected conditions. You can think of them as parts of one cause-and-effect process rather than as separate facts to memorize.
Variation
Individuals in a population differ. Some differences are visible, such as fur color or beak depth. Other differences are less visible, such as resistance to a toxin or the ability to digest a certain food.
Variation can arise from mutations and from the reshuffling of genetic information during sexual reproduction. Not every difference is heritable. A scar, for example, is usually not passed genetically to offspring.
Without heritable variation, natural selection cannot consistently favor one inherited version of a trait over another.
Inheritance
For natural selection to produce evolutionary change, at least some of the useful differences must be heritable. If parents carry genetic variants related to a trait, their offspring may inherit those variants.
This does not mean that offspring are exact copies of their parents. Offspring receive genetic information from their parents, but new combinations and mutations can add further variation.
More Offspring Than Can All Survive
Organisms often produce more offspring than can all survive and reproduce. Food, water, space, shelter, and mates may be limited. Predators, parasites, disease, and harsh weather can also reduce survival.
Because resources and conditions are limited, individuals do not all have the same chance of leaving offspring.
Differential Survival and Reproduction
If a heritable trait helps an individual survive long enough to reproduce, find a mate, produce more offspring, or help offspring survive, that trait may be passed on more often.
This difference in reproductive success is central to natural selection. The key question is not simply "Who survives?" but "Who contributes more offspring to the next generation?"
Change Across Generations
When individuals with a helpful heritable trait repeatedly leave more offspring, the trait can become more common over many generations. That population-level change is evolution by natural selection.
The environment does not directly give organisms the traits they need. Instead, existing heritable variation is filtered by environmental conditions. New mutations may also add variation, but mutations do not appear because an organism wants or needs a particular change.
Natural Selection as a Four-Part Process
A useful way to explain a natural-selection example is to connect four ideas in order.
Variation: Individuals begin with different heritable traits.
Selection pressure: A feature of the environment makes some traits more useful than others.
Differential reproduction: Individuals with helpful traits tend to leave more surviving offspring.
Population change: Across generations, the helpful trait becomes more common.
When you explain an example, try to include all four parts. Saying only that "the best adapted survive" is incomplete because it leaves out inheritance, reproduction, and population change.
Case Study: Peppered Moths
The peppered moth is a classic example used to study natural selection. Peppered moths can have light or dark coloration. Their color affects how easily predators such as birds can see them when the moths rest on tree surfaces.
During the Industrial Revolution in parts of Britain, air pollution darkened many tree surfaces and reduced pale lichens. In some polluted areas, dark moths became better camouflaged than light moths. Birds were more likely to detect poorly camouflaged moths, so dark moths often had a survival advantage in those environments. Because coloration is heritable, the dark form became more common in many polluted populations.
When air pollution later decreased and tree surfaces became lighter again, the advantage could shift back toward lighter moths. This shows an important rule: an adaptation is useful only in relation to a particular environment.
Think like a scientist: If you found that dark moths were common in one forest and light moths were common in another, what evidence would you need before concluding that camouflage and predation caused the difference?
Case Study: Galápagos Finches
The Galápagos finches are a group of related birds living on the Galápagos Islands. Different species and populations have different beak shapes and sizes. Beak form can affect which foods a bird can handle efficiently.
Long-term studies of finch populations by scientists including Peter and Rosemary Grant showed that environmental changes can alter which beak traits are favored. During a severe drought, for example, the types and sizes of available seeds may change. Birds whose beaks are better suited to the remaining food can have a better chance of surviving and reproducing. If beak traits are heritable, the average beak characteristics of the population can shift in later generations.
This does not mean that an individual finch grows a new beak because food becomes difficult to find. The population already contains variation. Natural selection changes which inherited traits become more common over generations.
Case Study: Antibiotic Resistance
Natural selection can happen quickly in organisms with short generation times, such as bacteria.
A bacterial population may already contain genetic variation. A few bacteria may carry resistance to an antibiotic because of mutations or because resistance genes were acquired from other bacteria. When an antibiotic is used, susceptible bacteria are more likely to die, while resistant bacteria are more likely to survive. The survivors reproduce, so resistance can become more common in the bacterial population.
The antibiotic does not "teach" an individual bacterium to become resistant on purpose. The antibiotic is a selection pressure. It changes which variants survive and reproduce.
This is one reason antibiotic resistance is an important real-world example of evolution and public health. Medical decisions about antibiotics should always follow guidance from qualified health professionals.
Darwin, Wallace, and the History of the Idea
Charles Darwin is strongly associated with natural selection. During and after his voyage on HMS Beagle, Darwin studied the geographic distribution, fossils, and variation of organisms. Over many years he developed an explanation for how species change.
Alfred Russel Wallace independently developed a very similar idea about natural selection after studying organisms in South America and the Malay Archipelago. In 1858, ideas from Darwin and Wallace were presented together to the Linnean Society of London. Darwin later explained natural selection in detail in On the Origin of Species, published in 1859.
Science does not depend on one person's authority. Natural selection became accepted because generations of scientists tested predictions, collected evidence, compared fossils and living organisms, studied inheritance, observed populations, and developed genetics.
What Natural Selection Does Not Mean
Natural selection is not "trying" to improve a species. The process has no goal or foresight. Environmental conditions simply affect reproductive success.
The fittest organism is not always the strongest. Biological fitness means leaving more surviving offspring in a particular environment.
Individuals do not evolve because they need to. A population evolves when heritable traits change in frequency across generations.
Useful traits are not always useful everywhere. Thick fur may help in a cold environment but may be costly in a hot one.
Not every trait is an adaptation. Some traits may be neutral, may be linked to other traits, or may have become common through processes other than natural selection.
Natural selection does not create perfect organisms. Evolution works with existing variation, historical constraints, trade-offs, and changing environments.
Natural Selection, Adaptation, and Evolution
These terms are related but not identical.
Natural selection is a process that causes some heritable variants to leave more offspring than others.
Adaptation can describe a heritable trait that improves reproductive success in a certain environment. The word can also describe the evolutionary process by which populations become better suited to their environments.
Evolution is the broader change in heritable characteristics of populations across generations. Natural selection is one mechanism of evolution. Other mechanisms also exist, including mutation, gene flow, and genetic drift.
For Grades 7–8, the most important connection is: heritable variation plus differences in reproductive success can cause a population to change over generations.
Build an Evidence-Based Explanation
Scientists often organize explanations using claim, evidence, and reasoning.
Claim: State what you think happened. Example: darker beetles became more common because they were better camouflaged in a dark environment.
Evidence: Use observations or data. Example: birds captured more light beetles than dark beetles on dark ground.
Reasoning: Connect the evidence to natural selection. Example: dark beetles survived more often, reproduced, and passed their heritable color trait to more offspring, so dark coloration increased across generations.
A strong explanation should name the variation, identify the selection pressure, describe the difference in reproductive success, and connect inheritance to population change.
Mini Investigation: Paper Prey
You can model selection without harming living organisms. Cut small paper shapes in two or more colors. Scatter equal numbers on a patterned surface such as a printed page or a patch of grass. Ask a partner to collect as many shapes as possible during a short, fixed time.
Count how many of each color were collected and how many remained. Repeat several trials, keeping the starting numbers and collection time the same. Then switch to a different background.
Ask yourself: Which color was easiest to detect? Did the result change with the background? How might surviving paper "prey" represent organisms that live long enough to reproduce? Which parts of real natural selection does this model represent well, and which parts does it leave out?
A model is useful because it helps you test an idea, but a model is not the same as a real ecosystem.
Interactive Tasks
Quiz: Test Your Knowledge
What must be present in a population for natural selection to act on inherited differences? (Heritable variation) (!Identical traits) (!Unlimited resources) (!A planned goal)
What does biological fitness mainly describe? (Reproductive success in an environment) (!Physical strength) (!Body size) (!Length of life alone)
Which statement best describes natural selection? (Helpful heritable traits can become more common across generations) (!Individuals choose the traits they need) (!Every organism changes in the same way) (!The environment creates perfect organisms)
What is a selection pressure? (An environmental factor that affects survival or reproduction) (!A trait inherited from a parent) (!A guaranteed improvement in a species) (!A measurement of body strength)
Why can camouflage affect natural selection? (It can change the chance of being seen by predators) (!It makes all organisms identical) (!It prevents inheritance) (!It always makes an organism stronger)
What happens when resistant bacteria survive antibiotic treatment and reproduce? (Resistance can become more common in the population) (!Every bacterium becomes resistant immediately) (!Resistance disappears from the population) (!The antibiotic becomes a gene)
What changes during evolution by natural selection? (The population across generations) (!One individual during its lifetime) (!The environment into a gene) (!Every species at the same speed)
What is the role of inheritance in natural selection? (It allows some traits to pass from parents to offspring) (!It makes all mutations helpful) (!It removes all variation) (!It guarantees survival)
Why can the same trait be helpful in one environment but not another? (Fitness depends on environmental conditions) (!Traits have fixed value everywhere) (!All environments select the same traits) (!Inheritance stops when environments change)
Which statement about mutations is correct? (They can introduce new genetic variation) (!They happen because organisms decide to change) (!They are always harmful) (!They always create adaptations immediately)
Memory Game
| Variation | Differences among individuals in a population |
| Heredity | Passing genetic information from parents to offspring |
| Selection pressure | Environmental factor that affects survival or reproduction |
| Fitness | Relative success in leaving surviving offspring |
| Adaptation | Heritable feature that improves success in a certain environment |
| Population | Members of one species living and reproducing in the same area |
| Mutation | Change in genetic material that can create a new variant |
| Evolution | Change in heritable population characteristics across generations |
Drag and Drop
| Match the correct terms. | Topic |
|---|---|
| Heritable variation | Differences that can be passed to offspring |
| Predation | A selection pressure involving animals that hunt other organisms |
| Differential reproduction | Some individuals leave more offspring than others |
| Camouflage | A trait that can reduce detection in a matching environment |
| Population change | A heritable trait becomes more or less common across generations |
...
Crossword Puzzle
| Variation | What word means differences among individuals in a population? |
| Fitness | What biology term means relative success in producing surviving offspring? |
| Adaptation | What word means a heritable feature that improves success in a particular environment? |
| Population | What group changes across generations during evolution? |
| Mutation | What genetic change can create a new variant? |
| Heredity | What word describes the passing of genetic information from parents to offspring? |
LearningApps
Cloze Text
Open-Ended Tasks
Easy
- Nature Observation Journal: Visit a safe outdoor place, observe variation in one kind of organism without collecting it, and create a one-page journal with sketches or photographs and three questions about how the environment might affect survival.
- Camouflage Model: Create two paper organisms with different colors, place them on two backgrounds, photograph the results, and explain which combination gives better camouflage and why.
- Natural Selection Comic: Produce a six-panel comic showing variation, a selection pressure, differential reproduction, and population change without giving the organisms human intentions.
- Vocabulary Audio Guide: Record a two-minute audio or video guide that clearly explains variation, heredity, fitness, adaptation, and evolution in your own words.
Standard
- Paper Moth Investigation: Carry out a repeated paper-prey experiment on light and dark backgrounds, record the numbers detected and remaining, graph your results, and explain how the model represents selection.
- Finch Evidence Story: Create an illustrated explanation of how a drought could change which finch beak traits are favored, making clear that individual birds do not change their beaks because they need to.
- Science Interview: Interview a biology teacher, ecologist, veterinarian, farmer, or other relevant professional about an example of variation or selection, then summarize what evidence would be needed to test the explanation.
- Selection Explainer Video: Produce a three-minute video that teaches natural selection using one original example and includes the four ideas of variation, selection pressure, differential reproduction, and population change.
Advanced
- Antibiotic Resistance Model: Design a physical or digital model showing how a resistant bacterial variant can become more common after antibiotic exposure, and add a paragraph explaining what the model simplifies.
- Selection Simulation: Create a spreadsheet, card game, or classroom simulation in which two heritable traits have different reproductive success, run several generations, and analyze how trait frequencies change.
- Evolution Evidence Visit: Visit a natural history museum, science center, university collection, zoo education exhibit, or high-quality virtual museum and produce a report connecting one exhibit to natural selection using evidence.
- Scientific Argument: Research a real natural-selection case from reliable sources, write a claim-evidence-reasoning argument, compare at least two sources, and identify one uncertainty or limitation in the evidence.
Learning Assessment
- Selection Scenario Analysis: Given a new habitat change and a population with two heritable traits, predict which trait may become more common and justify your prediction using variation, selection pressure, reproduction, and inheritance.
- Misconception Correction: Rewrite the statement "animals evolve the traits they need" so that it accurately explains how natural selection changes populations across generations, then explain why the original statement is misleading.
- Evidence Evaluation: Compare two short data sets from different environments and decide whether they support a natural-selection explanation, naming at least one alternative explanation that would need to be tested.
- Model Critique: Evaluate a classroom camouflage model by identifying two useful features, two limitations, and one change that would make the model represent real populations more accurately.
- Transfer to Resistance: Explain how the same logic of natural selection can apply to antibiotic resistance even though bacteria and moths are very different organisms.
- Environment Change Challenge: Describe how a trait that is currently helpful could become harmful after an environmental change, and predict how natural selection might then affect the population.
Evidence of Learning
Knowledge: You can accurately explain variation, heredity, selection pressure, fitness, adaptation, and evolution, and you can connect them in a cause-and-effect explanation of natural selection.
Scientific reasoning: You can distinguish observations from explanations, use evidence to support a claim, and identify what additional data would strengthen or weaken a natural-selection hypothesis.
Data skills: You can organize simple results, calculate or compare proportions, make a clear graph, and describe how a trait changes across repeated trials or generations.
Products: Your evidence may include a journal, model, graph, comic, audio guide, video, interview summary, simulation, or claim-evidence-reasoning report.
Transfer: You can apply the same natural-selection framework to an unfamiliar example rather than memorizing only moths or finches.
Scientific language: You avoid goal-directed phrases such as "the species tried to adapt" and instead explain how heritable differences in reproductive success change populations.
Reflection: You can identify the limits of a model or data set and explain why scientific conclusions should match the available evidence.
OERs on the Topic
The English Wikipedia article below provides an additional overview of natural selection. Use it to extend your learning and compare its explanations with the simpler models in this course.
Linked Learning Areas
Natural selection connects genetics, ecology, evolution, environmental science, medicine, mathematics, and the history of science. The table below links the main ideas you can explore next.