What is natural selection and how does it relate to evolution in the IB MYP Science curriculum?

Natural selection is a fundamental mechanism of evolution, where organisms better adapted to their environment tend to survive and produce more offspring. In the IB MYP Science curriculum, natural selection is studied as a key process that drives evolutionary change, explaining how species evolve over time through variations that enhance survival and reproduction.

How does natural selection lead to the adaptation of species?

Natural selection leads to adaptation by favoring traits that improve an organism's chances of survival and reproduction in a particular environment. Over generations, these advantageous traits become more common within the population, resulting in species that are well-suited to their environment. This process of adaptation is a central concept in understanding how species evolve.

What are some examples of natural selection in action?

Examples of natural selection include the development of antibiotic resistance in bacteria, where only resistant bacteria survive and reproduce, and the evolution of the peppered moth in England, where moths with coloration that matched their environment were less likely to be eaten by predators. These examples illustrate how natural selection can lead to significant changes in populations over time.

What role do genetic variations play in natural selection?

Genetic variations are crucial for natural selection because they provide the raw material for evolution. Variations arise through mutations, gene shuffling during reproduction, and other genetic processes. Natural selection acts on these variations, favoring those that confer a survival or reproductive advantage, leading to evolutionary change over time.

How does natural selection differ from artificial selection?

Natural selection is a process driven by environmental pressures where organisms with advantageous traits are more likely to survive and reproduce. In contrast, artificial selection is a human-driven process where individuals with desirable traits are selectively bred to produce offspring with those traits. While both processes lead to changes in populations, natural selection occurs naturally, whereas artificial selection is intentional and guided by human choice.

Why is "Saying organisms 'need' or 'want' a trait so they evolve it." a common mistake in Natural Selection?

Why it happens: Lamarckian intuition (giraffes stretching their necks). How to avoid it: Variation is RANDOM. Natural selection just preserves what already happens to work. The giraffe didn't stretch; long-necked giraffes simply outlived short-necked ones.

Why is "Saying an individual evolves to be resistant." a common mistake in Natural Selection?

Why it happens: Confuses adaptation with evolution. How to avoid it: POPULATIONS evolve, INDIVIDUALS don't. A bacterium doesn't 'become' resistant — it either has the mutation or doesn't.

Why is "Saying the antibiotic causes resistance." a common mistake in Natural Selection?

Why it happens: Cause-and-effect intuition. How to avoid it: The mutation existed BEFORE the antibiotic was applied. The antibiotic SELECTS — it doesn't create.

EvolutionIB MYP Sciences (Years 1-5)

Natural Selection

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

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Short Study Notes — Natural Selection

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Natural Selection — IB MYP Sciences: how species change over generations

Darwin's big idea, explained step-by-step. This MYP Sciences note covers variation, struggle for survival, selection pressures, and modern examples (antibiotic resistance, peppered moths).

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — State Darwin's four points of natural selection.
MYP Sciences A — Define adaptation, selection pressure, fitness.
MYP Sciences C — Apply natural selection to explain a real example.
MYP Sciences D — Discuss antibiotic resistance as evolution in action.

Darwin's four-step argument

Variation + over-production + struggle + heritability → evolution.

Charles Darwin's theory of natural selection (1859) can be broken into four logical steps:

Variation — individuals in a species are not identical. Different beak sizes, fur colours, running speeds, etc.
Over-production — every species produces more offspring than the environment can support. Most die before reproducing.
Struggle for existence — limited food/water/space + predators + disease means only some survive.
'Survival of the fittest' — the survivors are not random. Individuals whose VARIATIONS happen to suit the environment are more likely to survive and reproduce.

Because traits are HERITABLE (passed via alleles), the next generation contains more of the "fit" variations. Over many generations the species drifts towards the trait that survives best. New species can arise when populations are split and adapt to different environments.

Natural selection in action: a selection pressure (predation by birds on a sooty background) shifts the population towards the better-camouflaged form.

A KEY POINT: natural selection does NOT decide what the organism needs and then create it. Variation already exists (from random mutation). Selection just removes the misfits.

Variation → over-production → struggle → heritable advantage.
Random mutation creates variation; environment selects.
Over time, populations change — that's evolution.

Classic and modern examples

Peppered moths, finches, antibiotic resistance — same mechanism.

Peppered moth (Biston betularia). Before industrial pollution (~1850), most moths were light grey to camouflage on lichen-covered trees. Soot killed lichens and blackened trunks. Suddenly dark moths were better hidden, light moths got eaten by birds. Within 50 years, dark moths dominated industrial cities. After the Clean Air Acts (1956), pollution fell and light moths came back. A real-time observation of natural selection.

Darwin's finches (Galápagos). A single ancestral species reached the islands and diversified into 13+ species, each with a beak suited to its food (seeds, insects, cactus, etc.). Different islands had different selection pressures — different beaks won.

Antibiotic resistance. A small fraction of bacteria carry random mutations giving resistance to a drug. When the drug is used, sensitive bacteria die; resistant ones survive and reproduce. Within a few generations the whole population is resistant. THIS IS EVOLUTION HAPPENING IN HOSPITALS RIGHT NOW.

Important nuances:

Antibiotics don't "create" resistance — the mutations are already there.
Misusing antibiotics (not finishing a course, taking them for viruses) speeds the selection.
MRSA (methicillin-resistant Staphylococcus aureus) is an example.

Peppered moth: pollution flips the colour balance.
Galápagos finches: one ancestor → 13+ species with different beaks.
Antibiotic resistance: natural selection in real time. A serious public-health issue.

Evidence for evolution

Fossils, anatomy, DNA — they all agree.

Multiple independent lines of evidence support evolution:

Fossil record. Older rocks contain simpler organisms; newer rocks contain modern forms. Transitional fossils (e.g. Archaeopteryx — feathers + teeth, between dinosaur and bird) show intermediate stages.
Comparative anatomy. Vertebrate forelimbs (human arm, whale flipper, bat wing) have the same bone arrangement — HOMOLOGOUS STRUCTURES — pointing to a common ancestor.
DNA sequencing. Closely related species share more DNA. Humans and chimps share ~98%. This is the strongest modern evidence.
Direct observation. Antibiotic resistance, peppered moths, lab bacteria experiments.

What evolution is NOT:

It is NOT 'just a theory' in the everyday sense. In science, a THEORY is an explanation supported by overwhelming evidence (like the theory of gravity).
It does NOT progress towards humans. There is no goal — just whatever survives.
It does NOT happen to individuals. INDIVIDUALS develop; POPULATIONS evolve.

Fossil record: simpler → more complex over time.
Homologous structures (same bones, different uses).
DNA: ~98% shared with chimps confirms close ancestry.
Evolution acts on populations, not individuals.

How it’s examined

Criterion A: state Darwin's points. Criterion C: apply to a new example. Criterion D: ethics of antibiotic over-use.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Natural Selection, ready to print or save as PDF.

Step-by-step worked examples — Natural Selection

Step-by-step solutions to past-paper-style questions on natural selection, written exactly the way a tutor would explain them at the board.

1State Darwin's four steps
Getting started• theory
Question
Outline the four steps of natural selection as described by Darwin.
Step-by-step solution
1. Step 1
  
  Variation: individuals differ (random mutation + sexual reproduction).
2. Step 2
  
  Over-production: more offspring are born than the environment can support.
3. Step 3
  
  Struggle for existence: limited resources mean only some survive.
4. Step 4
  
  Heritable advantage: survivors pass their useful alleles to offspring.
Answer
Variation → over-production → struggle for existence → heritable advantage (survival of the fittest).
2Antibiotic resistance
Building confidence• antibiotic resistance
Question
Use natural selection to explain how a population of bacteria becomes resistant to an antibiotic over a few generations.
Step-by-step solution
1. Step 1
  A few bacteria carry random mutations giving resistance — present BEFORE the antibiotic is used.
2. Step 2
  When antibiotic is applied, non-resistant bacteria die; resistant ones survive.
3. Step 3
  Resistant survivors reproduce — they pass the resistance allele on. Each bacterial generation is short, so within days the whole colony is resistant.
Answer
Variation (random mutation) gives some bacteria resistance. Antibiotic selects for them. They reproduce, transferring the resistance allele.
3Predict the moth outcome
Stretch• peppered moth
Question
After the Clean Air Acts, soot levels dropped and lichens grew back on tree trunks. Predict and explain what happened to the proportion of dark and light peppered moths.
Step-by-step solution
1. Step 1
  Lichen returned → trunks are pale again. Now LIGHT moths camouflage well; DARK moths stand out.
2. Step 2
  Birds eat the dark moths more than the light ones — selection pressure has flipped.
3. Step 3
  Light moths survive and reproduce more → light allele frequency rises over generations. This was observed in the UK from the 1960s onwards.
Answer
Light moths now camouflage; dark moths get eaten — light moths increase. Selection pressure has been reversed.

Key Definitions and Keywords — Natural Selection

Definitions to memorise and the exact keywords mark schemes credit for natural selection answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Natural selection
Examiner keyword
The process by which individuals with traits better suited to their environment survive and reproduce more than others, so those traits become more common in the next generation.
Variation
Examiner keyword
Differences between individuals in a species (caused by mutation and sexual reproduction).
Selection pressure
Examiner keyword
An environmental factor (predator, climate, food shortage, antibiotic) that gives one variation an advantage over another.
Fitness
Examiner keyword
The reproductive success of an organism — how many surviving offspring it produces. NOT how strong it is.
Evolution
Change in the inherited characteristics of a population over many generations.

Common Mistakes and Misconceptions — Natural Selection

The traps other students keep falling into on natural selection questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Saying organisms 'need' or 'want' a trait so they evolve it.
Why it happens
Lamarckian intuition (giraffes stretching their necks).
How to avoid it
Variation is RANDOM. Natural selection just preserves what already happens to work. The giraffe didn't stretch; long-necked giraffes simply outlived short-necked ones.
✕Saying an individual evolves to be resistant.
Why it happens
Confuses adaptation with evolution.
How to avoid it
POPULATIONS evolve, INDIVIDUALS don't. A bacterium doesn't 'become' resistant — it either has the mutation or doesn't.
✕Saying the antibiotic causes resistance.
Why it happens
Cause-and-effect intuition.
How to avoid it
The mutation existed BEFORE the antibiotic was applied. The antibiotic SELECTS — it doesn't create.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Natural Selection — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

EvolutionIB MYP Sciences (Years 1-5)

Natural Selection

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

PreviousNext

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Short Study Notes in the form of Slides

Read the notes first. If the method in a worked example clicks, you're ready for the questions.

Short Study Notes — Natural Selection

Start with these resources to cover the key concepts, then work through the practice questions.

Page 1 / 0

Detailed Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Natural Selection, ready to print or save offline.

Natural Selection — IB MYP Sciences: how species change over generations

Darwin's big idea, explained step-by-step. This MYP Sciences note covers variation, struggle for survival, selection pressures, and modern examples (antibiotic resistance, peppered moths).

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — State Darwin's four points of natural selection.
MYP Sciences A — Define adaptation, selection pressure, fitness.
MYP Sciences C — Apply natural selection to explain a real example.
MYP Sciences D — Discuss antibiotic resistance as evolution in action.

Darwin's four-step argument

Variation + over-production + struggle + heritability → evolution.

Charles Darwin's theory of natural selection (1859) can be broken into four logical steps:

Variation — individuals in a species are not identical. Different beak sizes, fur colours, running speeds, etc.
Over-production — every species produces more offspring than the environment can support. Most die before reproducing.
Struggle for existence — limited food/water/space + predators + disease means only some survive.
'Survival of the fittest' — the survivors are not random. Individuals whose VARIATIONS happen to suit the environment are more likely to survive and reproduce.

Natural selection in action: a selection pressure (predation by birds on a sooty background) shifts the population towards the better-camouflaged form.

A KEY POINT: natural selection does NOT decide what the organism needs and then create it. Variation already exists (from random mutation). Selection just removes the misfits.

Variation → over-production → struggle → heritable advantage.
Random mutation creates variation; environment selects.
Over time, populations change — that's evolution.

Classic and modern examples

Peppered moths, finches, antibiotic resistance — same mechanism.

Important nuances:

Antibiotics don't "create" resistance — the mutations are already there.
Misusing antibiotics (not finishing a course, taking them for viruses) speeds the selection.
MRSA (methicillin-resistant Staphylococcus aureus) is an example.

Peppered moth: pollution flips the colour balance.
Galápagos finches: one ancestor → 13+ species with different beaks.
Antibiotic resistance: natural selection in real time. A serious public-health issue.

Evidence for evolution

Fossils, anatomy, DNA — they all agree.

Multiple independent lines of evidence support evolution:

Fossil record. Older rocks contain simpler organisms; newer rocks contain modern forms. Transitional fossils (e.g. Archaeopteryx — feathers + teeth, between dinosaur and bird) show intermediate stages.
Comparative anatomy. Vertebrate forelimbs (human arm, whale flipper, bat wing) have the same bone arrangement — HOMOLOGOUS STRUCTURES — pointing to a common ancestor.
DNA sequencing. Closely related species share more DNA. Humans and chimps share ~98%. This is the strongest modern evidence.
Direct observation. Antibiotic resistance, peppered moths, lab bacteria experiments.

What evolution is NOT:

It is NOT 'just a theory' in the everyday sense. In science, a THEORY is an explanation supported by overwhelming evidence (like the theory of gravity).
It does NOT progress towards humans. There is no goal — just whatever survives.
It does NOT happen to individuals. INDIVIDUALS develop; POPULATIONS evolve.

Fossil record: simpler → more complex over time.
Homologous structures (same bones, different uses).
DNA: ~98% shared with chimps confirms close ancestry.
Evolution acts on populations, not individuals.

How it’s examined

Criterion A: state Darwin's points. Criterion C: apply to a new example. Criterion D: ethics of antibiotic over-use.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Natural Selection, ready to print or save as PDF.

Step-by-step worked examples — Natural Selection

Step-by-step solutions to past-paper-style questions on natural selection, written exactly the way a tutor would explain them at the board.

1State Darwin's four steps
Getting started• theory
Question
Outline the four steps of natural selection as described by Darwin.
Step-by-step solution
1. Step 1
  
  Variation: individuals differ (random mutation + sexual reproduction).
2. Step 2
  
  Over-production: more offspring are born than the environment can support.
3. Step 3
  
  Struggle for existence: limited resources mean only some survive.
4. Step 4
  
  Heritable advantage: survivors pass their useful alleles to offspring.
Answer
Variation → over-production → struggle for existence → heritable advantage (survival of the fittest).
2Antibiotic resistance
Building confidence• antibiotic resistance
Question
Use natural selection to explain how a population of bacteria becomes resistant to an antibiotic over a few generations.
Step-by-step solution
1. Step 1
  A few bacteria carry random mutations giving resistance — present BEFORE the antibiotic is used.
2. Step 2
  When antibiotic is applied, non-resistant bacteria die; resistant ones survive.
3. Step 3
  Resistant survivors reproduce — they pass the resistance allele on. Each bacterial generation is short, so within days the whole colony is resistant.
Answer
Variation (random mutation) gives some bacteria resistance. Antibiotic selects for them. They reproduce, transferring the resistance allele.
3Predict the moth outcome
Stretch• peppered moth
Question
After the Clean Air Acts, soot levels dropped and lichens grew back on tree trunks. Predict and explain what happened to the proportion of dark and light peppered moths.
Step-by-step solution
1. Step 1
  Lichen returned → trunks are pale again. Now LIGHT moths camouflage well; DARK moths stand out.
2. Step 2
  Birds eat the dark moths more than the light ones — selection pressure has flipped.
3. Step 3
  Light moths survive and reproduce more → light allele frequency rises over generations. This was observed in the UK from the 1960s onwards.
Answer
Light moths now camouflage; dark moths get eaten — light moths increase. Selection pressure has been reversed.

Key Definitions and Keywords — Natural Selection

Definitions to memorise and the exact keywords mark schemes credit for natural selection answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Natural selection
Examiner keyword
The process by which individuals with traits better suited to their environment survive and reproduce more than others, so those traits become more common in the next generation.
Variation
Examiner keyword
Differences between individuals in a species (caused by mutation and sexual reproduction).
Selection pressure
Examiner keyword
An environmental factor (predator, climate, food shortage, antibiotic) that gives one variation an advantage over another.
Fitness
Examiner keyword
The reproductive success of an organism — how many surviving offspring it produces. NOT how strong it is.
Evolution
Change in the inherited characteristics of a population over many generations.

Common Mistakes and Misconceptions — Natural Selection

The traps other students keep falling into on natural selection questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Saying organisms 'need' or 'want' a trait so they evolve it.
Why it happens
Lamarckian intuition (giraffes stretching their necks).
How to avoid it
Variation is RANDOM. Natural selection just preserves what already happens to work. The giraffe didn't stretch; long-necked giraffes simply outlived short-necked ones.
✕Saying an individual evolves to be resistant.
Why it happens
Confuses adaptation with evolution.
How to avoid it
POPULATIONS evolve, INDIVIDUALS don't. A bacterium doesn't 'become' resistant — it either has the mutation or doesn't.
✕Saying the antibiotic causes resistance.
Why it happens
Cause-and-effect intuition.
How to avoid it
The mutation existed BEFORE the antibiotic was applied. The antibiotic SELECTS — it doesn't create.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Natural Selection — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Natural Selection

Short Study Notes in the form of Slides

Short Study Notes — Natural Selection

Detailed Notes

What you’ll learn

How it’s examined

1State Darwin's four steps

2Antibiotic resistance

3Predict the moth outcome

Natural selection

Variation

Selection pressure

Fitness

Evolution

✕Saying organisms 'need' or 'want' a trait so they evolve it.

✕Saying an individual evolves to be resistant.

✕Saying the antibiotic causes resistance.

Practice questions

Past paper style quiz

Assess your understanding

Natural Selection — frequently asked questions

Natural Selection

Short Study Notes in the form of Slides

Short Study Notes — Natural Selection

Detailed Notes

What you’ll learn

How it’s examined

1State Darwin's four steps

2Antibiotic resistance

3Predict the moth outcome

Natural selection

Variation

Selection pressure

Fitness

Evolution

✕Saying organisms 'need' or 'want' a trait so they evolve it.

✕Saying an individual evolves to be resistant.

✕Saying the antibiotic causes resistance.

Practice questions

Past paper style quiz

Assess your understanding

Natural Selection — frequently asked questions