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Higher Level Revision Notes
Leaving Cert Higher Level Biology
Chapter 10: Evolution & Natural Selection
Natural Selection
Subtopics Covered
- Learning Outcomes
- Big Picture: Why Evolution Matters
- Key Definitions
- Sources of Variation
- Natural Selection: The Mechanism
- Worked Example: Antibiotic Resistance
What This Pack Includes
- Structured chapter notes formatted for ExamsLogic website reading
- Exam-focused diagrams, definitions, and worked examples
- Interactive practice sections carried over from the source notes
- Independent study guidance based on the official curriculum
- Print-friendly layout for future PDF export when needed
This publication is an independent educational resource developed by ExamsLogic and compiled by experienced educators. It is based on publicly available official curricula, including Cambridge, Pearson Edexcel, IB, and the Irish Leaving Certificate. This product is not endorsed by, affiliated with, or sponsored by any examination board or governing authority. All registered trademarks remain the property of their respective owners.
1. Learning Outcomes
- Define evolution as a change in inherited characteristics of populations over time.
- Explain variation, mutation, adaptation and natural selection.
- Describe the evidence for evolution, including fossils, comparative anatomy and DNA evidence.
- Explain how new species can form by isolation and selection.
- Apply natural selection to unfamiliar examples such as antibiotic resistance and insecticide resistance.
- Distinguish Lamarck's theory from Darwin and Wallace's theory of natural selection.
HL focus: In exam answers, link variation to survival, survival to reproduction, reproduction to inherited alleles, and alleles to population change over many generations.
2. Big Picture: Why Evolution Matters
Explains Diversity
Evolution explains how millions of species are related and why organisms are adapted to their environments.
Predicts Change
Populations change when environmental pressures favour some inherited traits over others.
Medicine & Farming
Antibiotic resistance, pesticide resistance and selective breeding all depend on evolutionary principles.
Core idea: Individuals do not evolve during their lifetime. Populations evolve over generations.
3. Key Definitions
| Term | Meaning | Exam detail |
|---|---|---|
| Evolution | Change in inherited characteristics of a population over time. | Always mention population and time. |
| Variation | Differences between individuals of the same species. | Can be genetic or environmental. |
| Mutation | A change in DNA sequence. | Creates new alleles and can increase genetic variation. |
| Adaptation | An inherited feature that improves survival or reproduction in a particular environment. | Must be linked to environment. |
| Natural selection | Process where individuals with advantageous inherited traits survive and reproduce more successfully. | Changes allele frequencies in a population. |
| Species | A group of organisms that can interbreed to produce fertile offspring. | Useful in speciation questions. |
Examiner trap: Do not write “organisms evolve because they need to.” Evolution is not a deliberate choice. Selection acts on existing variation.
4. Sources of Variation
Genetic Variation
- Mutation creates new alleles.
- Meiosis shuffles alleles through independent assortment and crossing over.
- Random fertilisation produces new allele combinations.
Environmental Variation
- Diet, temperature, light, disease and training can affect phenotype.
- Environmental variation is not always inherited.
- Some traits involve both genes and environment.
Exam Tip: When asked for sources of genetic variation, use mutation, meiosis and fertilisation. Do not use “practice” or “diet” unless the question asks about environmental variation.
5. Natural Selection: The Mechanism
Natural selection happens when some individuals are better suited to the environment than others because of inherited variation.
Variation exists→Selection pressure→Best adapted survive→They reproduce→Alleles passed on→Population changes
| Stage | What happens | Correct exam wording |
|---|---|---|
| Variation | Individuals have different traits. | Some individuals have inherited advantageous alleles. |
| Selection pressure | Environment creates a challenge. | Predation, disease, competition, drought, antibiotics or pesticides. |
| Differential survival | Better adapted individuals survive more often. | They are more likely to survive to reproductive age. |
| Differential reproduction | Survivors produce more offspring. | Advantageous alleles are passed on. |
| Population change | Trait becomes more common. | Allele frequency changes over generations. |
Common mistake: “The giraffe stretched its neck and passed on a longer neck.” This is Lamarckian thinking, not modern natural selection.
6. Worked Example: Antibiotic Resistance
- A bacterial population contains variation.
- A mutation gives some bacteria resistance to an antibiotic.
- The antibiotic kills non-resistant bacteria.
- Resistant bacteria survive and reproduce rapidly.
- The resistance allele becomes more common in the population.
HL wording: Antibiotics do not cause bacteria to become resistant. They select for resistant bacteria already present or produced by mutation.
7. Evidence for Evolution
| Evidence | What it shows | Example |
|---|---|---|
| Fossils | Organisms have changed over geological time. | Transitional fossils show intermediate features. |
| Comparative anatomy | Similar body structures can suggest common ancestry. | Pentadactyl limb in humans, whales, bats and horses. |
| Embryology | Similar early development can suggest relatedness. | Vertebrate embryos show similarities. |
| DNA / protein evidence | More similar DNA suggests closer evolutionary relationship. | Humans and chimpanzees share many DNA similarities. |
| Biogeography | Species distribution reflects ancestry and isolation. | Island species often evolve unique features. |
Examiner Secret: DNA evidence is powerful because it directly compares inherited information. The more similar the DNA, the more recent the common ancestor is likely to be.
8. Speciation
Speciation is the formation of a new species. It often happens when populations become isolated and evolve separately.
One population→Isolation→Different selection pressures→Different mutations selected→Reproductive isolation→New species
| Type of isolation | Meaning | Example |
|---|---|---|
| Geographical isolation | Physical barrier separates populations. | Mountain, river, island or glacier. |
| Reproductive isolation | Groups can no longer interbreed to produce fertile offspring. | Different mating calls, breeding seasons or incompatible gametes. |
| Ecological isolation | Populations occupy different habitats or niches. | Different food sources or microhabitats. |
Examiner Trap: Isolation alone does not make a new species immediately. It allows separated populations to accumulate genetic differences over many generations.
9. Darwin, Wallace and Lamarck
| Scientist | Main idea | Modern evaluation |
|---|---|---|
| Lamarck | Acquired characteristics are passed to offspring. | Mostly rejected. Acquired traits like muscle from training are not usually inherited. |
| Darwin | Natural selection explains adaptation and evolution. | Supported by evidence and modern genetics. |
| Wallace | Independently developed natural selection theory. | Important co-discoverer with Darwin. |
Exam Tip: If the question asks for Darwinian selection, include inherited variation and differential reproduction. These are the high-value marks.
10. Interactive Simulator: Natural Selection Builder
Build the Sequence
Current stage: Variation exists in the population.
Feedback
Start: Variation exists
Choose the next step in natural selection.
Progress: 0/5
11. Interactive Simulator: Evidence Detective
Identify the Evidence
Clue: Similar limb bones in mammals with different functions.
Feedback
Ready
Use the clue to identify the evidence type.
12. High-Value Exam Guidance
Exam Tip 1: Use the phrase “advantageous inherited variation” in natural selection answers.
Exam Tip 2: Always include “more likely to survive and reproduce” for selection questions.
Exam Tip 3: Finish with “allele becomes more common in the population over generations.”
Examiner Trap 1: Do not say the environment “decides” or “wants” traits. Selection is not conscious.
Examiner Trap 2: Antibiotics kill non-resistant bacteria; they do not teach bacteria how to resist.
Examiner Trap 3: Individuals are selected; populations evolve.
Common Mistake 1: “The species adapted because it needed to survive.” Correct: individuals with favourable alleles survived and reproduced more.
Common Mistake 2: Forgetting reproduction. Survival alone is not enough for evolutionary change.
Examiner Secret 1: Most full-mark answers are cause-and-effect chains, not definitions.
Examiner Secret 2: Speciation questions usually require isolation plus different selection pressures plus reproductive isolation.
13. MCQs with Instant Answers
1. Evolution is best defined as:
Answer: B. Evolution is population change over generations.
2. Which creates new alleles?
Answer: A. Mutation changes DNA and can create new alleles.
3. In natural selection, advantageous alleles become more common because individuals with them:
Answer: B. Differential survival and reproduction drive allele frequency change.
4. Similar bone structure in whale flippers and human arms is evidence from:
Answer: A. Homologous structures are comparative anatomy evidence.
14. Structured Questions
- Define evolution. [2]
- Explain natural selection using antibiotic resistance as an example. [8]
- Describe three sources of genetic variation. [6]
- Explain how geographical isolation can lead to speciation. [6]
- Give two pieces of evidence for evolution and explain how each supports evolution. [6]
15. Mark Scheme
Q1. Change in inherited characteristics [1] of a population over time/generations [1].
Q2. Variation exists in bacteria [1]; mutation may produce resistant bacteria [1]; antibiotic is selection pressure [1]; non-resistant bacteria die [1]; resistant bacteria survive [1]; resistant bacteria reproduce [1]; resistance allele passed on [1]; resistant bacteria/allele become more common over generations [1].
Q3. Mutation creates new alleles [2]; meiosis produces new combinations by crossing over/independent assortment [2]; random fertilisation combines alleles from two parents [2].
Q4. Population split by barrier [1]; gene flow stops [1]; different selection pressures act [1]; mutations/allele frequencies change differently [1]; groups become reproductively isolated [1]; new species formed [1].
Q5. Fossils show change over geological time [2]; comparative anatomy shows common ancestry [2]; DNA/protein similarities show relatedness [2]. Accept any two explained pieces of evidence.
Q2. Variation exists in bacteria [1]; mutation may produce resistant bacteria [1]; antibiotic is selection pressure [1]; non-resistant bacteria die [1]; resistant bacteria survive [1]; resistant bacteria reproduce [1]; resistance allele passed on [1]; resistant bacteria/allele become more common over generations [1].
Q3. Mutation creates new alleles [2]; meiosis produces new combinations by crossing over/independent assortment [2]; random fertilisation combines alleles from two parents [2].
Q4. Population split by barrier [1]; gene flow stops [1]; different selection pressures act [1]; mutations/allele frequencies change differently [1]; groups become reproductively isolated [1]; new species formed [1].
Q5. Fossils show change over geological time [2]; comparative anatomy shows common ancestry [2]; DNA/protein similarities show relatedness [2]. Accept any two explained pieces of evidence.
16. Mastery Checklist
- I can define evolution accurately.
- I can explain variation and mutation.
- I can write a full natural selection answer.
- I can apply selection to antibiotic or pesticide resistance.
- I can describe evidence for evolution.
- I can explain how isolation can lead to speciation.
- I can avoid Lamarckian wording in exam answers.