Source: https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-3-speciation

[Revision notes](https://senecalearning.com/en-gb/revision-notes.md) › [International A Level](https://senecalearning.com/en-gb/revision-notes/international-a-level.md) › [Biology](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology.md) › [Cambridge International](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international.md) › Speciation

# Speciation

Biology: Cambridge International A Level, topic 17.3.3

## Overview of Speciation

Speciation is the where two new species arise from a single species. This happens when two populations are prevented from interbreeding.

### Reproductive isolation

![Reproductive isolation](https://image-v2.cdn.app.senecalearning.com/2018-08/d42c8ab3-e082-4583-8e0a-e559d0260bb1/Evolution.jpg)

- If two populations are prevented from interbreeding, differences begin to accumulate in the two gene pools.
- Gene flow is the movement of alleles between a population. When two populations are reproductively isolated, the gene flow of the populations is restricted.
- The two populations are said to be reproductively isolated.

### Accumulation of differences

![Accumulation of differences](https://image-v2.cdn.app.senecalearning.com/2018-08/d42c8ab3-e082-4583-8e0a-e559d0260bb1/Evolution.jpg)

- If the gene pools are reproductively isolated for an extended period of time, they will eventually accumulate enough genetic differences that the two populations can no longer interbreed.
- If individuals from the two populations can no longer interbreed to produce fertile offspring, the two populations are considered separate species.

### Speciation

![Speciation](https://image-v2.cdn.app.senecalearning.com/2018-08/d42c8ab3-e082-4583-8e0a-e559d0260bb1/Evolution.jpg)

- Speciation is when two (or more) populations have been reproductively isolated to produce two (or more) separate species.
- This is how the thousands of species that exist today have been produced.

## Allopatric Speciation

Allopatric speciation is a type of speciation where two (or more) populations are geographically isolated from each other.

### Geographical isolation

![Geographical isolation](https://image-v2.cdn.app.senecalearning.com/2018-04/f387c23d-381f-43c9-918e-e1adc3cf932a/shutterstock_371694880.jpg)

- Populations of a species can split into two or more populations.
- This may be caused by environmental changes (e.g. emergence of a river) or random events (e.g. a storm causing a population of birds to be carried to another island).
- When two populations are physically separated, they are geographically isolated.

### Selection pressures

![Selection pressures](https://image-v2.cdn.app.senecalearning.com/2018-07/77ddd23d-5436-4c93-996c-c724aceffb0b/food-healthy.jpg)

- Populations in different environments will be exposed to different environmental conditions (e.g. climate, food availability).
- The different environments will place different selection pressures on the populations.
- The different selection pressures will drive natural selection in different directions.

### Allele frequencies

![Allele frequencies](https://image-v2.cdn.app.senecalearning.com/courseImages/physics/AQA%20New%20Modules/7.1.3/frequency%20oscillations.jpg)

- The different selection pressures on the populations cause the allele frequencies in the two populations to change.
- If the populations continue to be geographically isolated for a long time, eventually the allele frequencies will change so much that if the populations are brought back together they can no longer interbreed.

### Speciation

![Speciation](https://image-v2.cdn.app.senecalearning.com/courseImages/biology/5.3.5-%20Hormones%20for%20infertility%20treatment/meerkat-255564_640-min.jpg)

- Species are defined as a group of actually or potentially interbreeding individuals. 
- If two populations that have been geographically isolated can no longer interbreed when they are brought back together, they are considered to be two species.
- This is allopatric speciation.

## Sympatric Speciation

Sympatric speciation is a type of speciation where two (or more) populations are not geographically isolated from each other and involves reproductive isolation.

### Reproductive isolation

![Reproductive isolation](https://image-v2.cdn.app.senecalearning.com/2018-09/6e939ad1-7de9-4a6b-be36-2ac23fcb8bc2/smoke-alone-smoker-.jpg)

- Scientists organise reproductive isolation into two groups: prezygotic and postzygotic. 
 - Recall that a zygote is a fertilised egg: the first cell of the development of an organism that reproduces sexually. 
- So, a prezygotic barrier blocks reproduction from taking place such as barriers that prevent fertilisation when organisms attempt reproduction.
- A postzygotic barrier occurs after zygote formation, such as organisms that don’t survive the embryonic stage and those born sterile.

### Reproductive isolation mutations

![Reproductive isolation mutations](https://image-v2.cdn.app.senecalearning.com/courseImages/biology/5.4.1%20-%20Plant%20hormones/flower-2919284_640-min.jpg)

- Sympatric speciation could be caused by mutations that influence many different factors:
 - Polyploidy - where a cell or organism has an extra set, or sets, of chromosomes. Polyploidy organisms are reproductively isolated from diploid organisms.
 - Mutations may influence the flowering times or mating times of individuals.
 - Mutations may alter the reproductive organs.
 - Mutations may alter mating behaviours.

### Prezygotic barriers

![Prezygotic barriers](https://image-v2.cdn.app.senecalearning.com/2018-09/fcc0f999-68dc-4554-8a7b-dd143029592c/frog-king-prince-crown-.jpg)

- Differences in breeding schedules, called temporal isolation, can act as a form of **reproductive isolation**.
 -  For example, two species of frogs inhabit the same area, but one reproduces from January to March, whereas the other reproduces from March to May. 
- In some cases, populations of a species move or are moved to a new habitat and take up residence in a place that no longer overlaps with the other populations of the same species. 
 - This situation is called **habitat isolation**.

### Prezygotic barriers  - 2

![Prezygotic barriers  - 2](https://image-v2.cdn.app.senecalearning.com/2018-08/57c77c4b-896b-40c0-8f39-69cf4dfd4df2/fashion-lights-show.jpg)

- **Behavioural isolation** occurs when a specific behaviour prevents reproduction from taking place. 
 - For example, male fireflies use specific light patterns to attract females. If a male of one species tried to attract the female of another, she would not recognize the light pattern. 
- Other prezygotic barriers work when differences in their gamete cells (eggs and sperm) prevent fertilization from taking place; this is called a **gametic barrier**.

### Postzygotic barriers

![Postzygotic barriers](https://image-v2.cdn.app.senecalearning.com/2018-09/6eeac374-72fd-463b-938d-9894096aeb34/mailbox-white-box-mail-post-.jpg)

- When fertilisation takes place and a zygote forms, postzygotic barriers can prevent reproduction.
- Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages.
 - This is called hybrid inviability because the hybrid organisms simply are not viable.
- In another postzygotic situation, reproduction leads to the birth and growth of a hybrid that is sterile and unable to reproduce offspring of their own. 
 - This is called hybrid sterility.

### Speciation

![Speciation](https://image-v2.cdn.app.senecalearning.com/2018-08/81445204-fdeb-40ca-8438-7d99b47a51ff/baby-monkey.jpg)

- The biological definition of a species is a group of individuals that can actually or potentially interbreed. 
- The emergence of a mutation that prevents two (or more) populations from interbreeding is called sympatric speciation.

## Test yourself on Speciation

- Which of these is necessary for speciation to take place?
- Which of the following statements are true for allopatric speciation?
- Which of these is NOT essential for allopatric speciation?
- What is the definition of a species?
- Which of the following statements about sympatric speciation are true?

[Practise this topic for free](https://app.senecalearning.com/classroom/course/8d1a05c1-df92-4b13-9a69-b60acc7af79a/section/23f4a35c-7771-42c9-9930-7e13e7bde009/session)

- Previous topic: [17.3.2 Evidence for Evolution](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-2-evidence-for-evolution.md)
- Next topic: [17.3.4 Extinction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-4-extinction.md)

## Biology: Cambridge International A Level contents

### 1 Cell Structure
- 1.1 Cell Structure
  - [1.1.1 Studying Cells - Microscopes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-1-studying-cells-microscopes.md)
  - [1.1.2 Introduction to Eukaryotic & Prokaryotic Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-2-introduction-to-eukaryotic-and-prokaryotic-cells.md)
  - [1.1.3 Ultrastructure of Eukaryotic Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-3-ultrastructure-of-eukaryotic-cells.md)
  - [1.1.4 Ultrastructure of Eukaryotic Cells 2](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-4-ultrastructure-of-eukaryotic-cells-2.md)
  - [1.1.5 Ultrastructure of Eukaryotic Cells 3](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-5-ultrastructure-of-eukaryotic-cells-3.md)
  - [1.1.6 Prokaryotic Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-6-prokaryotic-cells.md)
  - [1.1.7 Viruses](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-7-viruses.md)
  - [1.1.8 End of Topic Test - Cell Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-8-end-of-topic-test-cell-structure.md)
  - [1.1.9 Exam-Style Question - Microscopes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-9-exam-style-question-microscopes.md)
  - [1.1.10 A-A* (AO2/3) - Cell Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-10-a-a-ao23-cell-structure.md)

### 2 Biological Molecules
- 2.1 Testing for Biological Modules
  - [2.1.1 Tests for Sugars & Starch](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-1-1-tests-for-sugars-and-starch.md)
  - [2.1.2 Tests for Lipids & Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-1-2-tests-for-lipids-and-proteins.md)
- 2.2 Carbohydrates & Lipids
  - [2.2.1 Monomers & Polymers](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-1-monomers-and-polymers.md)
  - [2.2.2 Carbohydrates](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-2-carbohydrates.md)
  - [2.2.3 Examples of Carbohydrates](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-3-examples-of-carbohydrates.md)
  - [2.2.4 Triglycerides & Phospholipids](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-4-triglycerides-and-phospholipids.md)
  - [2.2.5 Types of Fatty Acids](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-5-types-of-fatty-acids.md)
  - [2.2.6 Exam-Style Question - Carbohydrates](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-6-exam-style-question-carbohydrates.md)
  - [2.2.7 End of Topic Test - Carbohydrates & Lipids](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-7-end-of-topic-test-carbohydrates-and-lipids.md)
- 2.3 Proteins
  - [2.3.1 Amino Acids & Overview of Protein Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-1-amino-acids-and-overview-of-protein-structure.md)
  - [2.3.2 Primary & Secondary Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-2-primary-and-secondary-structure.md)
  - [2.3.3 Tertiary & Quaternary Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-3-tertiary-and-quaternary-structure.md)
  - [2.3.4 Globular & Fibrous Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-4-globular-and-fibrous-proteins.md)
  - [2.3.5 Haemoglobin](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-5-haemoglobin.md)
  - [2.3.6 Collagen](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-6-collagen.md)
  - [2.3.7 End of Topic Test  - Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-7-end-of-topic-test-proteins.md)
- 2.4 Water
  - [2.4.1 Water Structure & Function](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-4-1-water-structure-and-function.md)
  - [2.4.2 End of Topic Test - Water](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-4-2-end-of-topic-test-water.md)

### 3 Enzymes
- 3.1 Enzymes
  - [3.1.1 Enzyme Action](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-1-enzyme-action.md)
  - [3.1.2 Factors Affecting Enzyme Activity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-2-factors-affecting-enzyme-activity.md)
  - [3.1.3 Factors Affecting Enzyme Activity 2](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-3-factors-affecting-enzyme-activity-2.md)
  - [3.1.4 A-A* (AO2/3) - Enzymes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-4-a-a-ao23-enzymes.md)

### 4 Cell Membranes & Transport
- 4.1 Biological Membranes
  - [4.1.1 Fluid Mosaic Model](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-1-fluid-mosaic-model.md)
  - [4.1.2 Cell Signalling](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-2-cell-signalling.md)
  - [4.1.3 End of Topic Test - Biological Membranes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-3-end-of-topic-test-biological-membranes.md)
  - [4.1.4 Exam-Style Question - Membranes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-4-exam-style-question-membranes.md)
- 4.2 Movement Into & Out of Cells
  - [4.2.1 Passive Movement Across the Membrane](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-1-passive-movement-across-the-membrane.md)
  - [4.2.2 Active Movement Across the Membrane](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-2-active-movement-across-the-membrane.md)
  - [4.2.3 Osmosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-3-osmosis.md)
  - [4.2.4 Osmosis & Plant Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-4-osmosis-and-plant-cells.md)
  - [4.2.5 Surface Area to Volume Ratio](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-5-surface-area-to-volume-ratio.md)
  - [4.2.6 Adaptions to Surface Area to Volume Ratio](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-6-adaptions-to-surface-area-to-volume-ratio.md)
  - [4.2.7 Calculations - Volume](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-7-calculations-volume.md)
  - [4.2.8 Calculations - Surface Area](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-8-calculations-surface-area.md)

### 5 The Mitotic Cell Cycle
- 5.1 Replication & Division of Nuclei
  - [5.1.1 Chromosomes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-1-chromosomes.md)
  - [5.1.2 The Cell Cycle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-2-the-cell-cycle.md)
  - [5.1.3 Mitosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-3-mitosis.md)
  - [5.1.4 Role of Mitosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-4-role-of-mitosis.md)
  - [5.1.5 Stem Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-5-stem-cells.md)
  - [5.1.6 Mitosis & Cancer](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-6-mitosis-and-cancer.md)
  - [5.1.7 End of Topic Test - Replication & Division](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-7-end-of-topic-test-replication-and-division.md)
  - [5.1.8 A-A* (AO2/3) - Cell Cycle & Transport](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-8-a-a-ao23-cell-cycle-and-transport.md)

### 6 Nucleic Acids & Protein Synthesis
- 6.1 Nucleic Acids
  - [6.1.1 Introduction to DNA & RNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-1-introduction-to-dna-and-rna.md)
  - [6.1.2 Nucleotides](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-2-nucleotides.md)
  - [6.1.3 Polynucleotides](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-3-polynucleotides.md)
  - [6.1.4 ADP & ATP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-4-adp-and-atp.md)
  - [6.1.5 DNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-5-dna.md)
  - [6.1.6 DNA Replication](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-6-dna-replication.md)
  - [6.1.7 RNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-7-rna.md)
  - [6.1.8 End of Topic Test - Nucleic Acids & ATP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-8-end-of-topic-test-nucleic-acids-and-atp.md)
- 6.2 Protein Synthesis
  - [6.2.1 The Genetic Code](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-1-the-genetic-code.md)
  - [6.2.2 Polypeptide Synthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-2-polypeptide-synthesis.md)
  - [6.2.3 RNA Processing in Eukaryotes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-3-rna-processing-in-eukaryotes.md)
  - [6.2.4 Mutations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-4-mutations.md)
  - [6.2.5 Exam-Style Question - Protein Synthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-5-exam-style-question-protein-synthesis.md)
  - [6.2.6 A-A* (AO2/3) - Coronavirus Translation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-6-a-a-ao23-coronavirus-translation.md)

### 7 Transport in Plants
- 7.1 Transport in Plants
  - [7.1.1 Plant Vessels](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-1-plant-vessels.md)
  - [7.1.2 Xylem](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-2-xylem.md)
  - [7.1.3 Investigating Transpiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-3-investigating-transpiration.md)
  - [7.1.4 Adaptations to Water Availability](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-4-adaptations-to-water-availability.md)
  - [7.1.5 Phloem](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-5-phloem.md)
  - [7.1.6 End of Topic Test - Transport in Animals & Plants](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-6-end-of-topic-test-transport-in-animals-and-plants.md)
  - [7.1.7 A-A* (AO2/3) - Transport in Animals & Plants](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-7-a-a-ao23-transport-in-animals-and-plants.md)

### 8 Transport in Mammals
- 8.1 Circulatory System
  - [8.1.1 Double Circulation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-1-double-circulation.md)
  - [8.1.2 Blood Vessels](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-2-blood-vessels.md)
  - [8.1.3 Important Blood Vessels](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-3-important-blood-vessels.md)
  - [8.1.4 Blood Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-4-blood-cells.md)
  - [8.1.5 Tissue Fluid](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-5-tissue-fluid.md)
- 8.2 Transport of Oxygen & Carbon Dioxide
  - [8.2.1 Haemoglobin](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-2-1-haemoglobin.md)
  - [8.2.2 Oxygen Dissociation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-2-2-oxygen-dissociation.md)
- 8.3 The Heart
  - [8.3.1 Structure of the Heart](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-3-1-structure-of-the-heart.md)
  - [8.3.2 The Cardiac Cycle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-3-2-the-cardiac-cycle.md)
  - [8.3.3 Coordination of Heart Action](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-3-3-coordination-of-heart-action.md)

### 9 Gas Exchange
- 9.1 Gas Exchange System
  - [9.1.1 Mammalian Gas Exchange](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/9-1-1-mammalian-gas-exchange.md)
  - [9.1.2 Smoking](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/9-1-2-smoking.md)

### 10 Infectious Diseases
- 10.1 Infectious Diseases
  - [10.1.1 Transmission of Disease](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-1-transmission-of-disease.md)
  - [10.1.2 Tuberculosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-2-tuberculosis.md)
  - [10.1.3 HIV](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-3-hiv.md)
  - [10.1.4 Malaria](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-4-malaria.md)
  - [10.1.5 Cholera](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-5-cholera.md)
  - [10.1.6 Measles](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-6-measles.md)
- 10.2 Antibiotics
  - [10.2.1 Antibiotics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-2-1-antibiotics.md)

### 11 Immunity
- 11.1 The Immune System
  - [11.1.1 Phagocytes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-1-phagocytes.md)
  - [11.1.2 Antigens](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-2-antigens.md)
  - [11.1.3 Lymphocytes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-3-lymphocytes.md)
  - [11.1.4 Primary & Secondary Response](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-4-primary-and-secondary-response.md)
- 11.2 Antibodies & Vaccination
  - [11.2.1 Antibodies](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-1-antibodies.md)
  - [11.2.2 Monoclonal Antibodies](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-2-monoclonal-antibodies.md)
  - [11.2.3 Immunity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-3-immunity.md)
  - [11.2.4 Vaccines](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-4-vaccines.md)
  - [11.2.5 Exam-Style Question - Immune System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-5-exam-style-question-immune-system.md)
  - [11.2.6 A-A* (AO2/3) - Disease & Immune System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-6-a-a-ao23-disease-and-immune-system.md)

### 12 Energy & Respiration (A2 Only)
- 12.1 Energy
  - [12.1.1 Energy & ATP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-1-1-energy-and-atp.md)
  - [12.1.2 Respiratory Substrates & Quotient](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-1-2-respiratory-substrates-and-quotient.md)
  - [12.1.3 Respirometers](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-1-3-respirometers.md)
- 12.2 Respiration
  - [12.2.1 Introduction to Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-1-introduction-to-respiration.md)
  - [12.2.2 Anaerobic Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-2-anaerobic-respiration.md)
  - [12.2.3 Aerobic Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-3-aerobic-respiration.md)
  - [12.2.4 Chemiosmosis & Coenzymes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-4-chemiosmosis-and-coenzymes.md)
  - [12.2.5 Mitochondria](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-5-mitochondria.md)
  - [12.2.6 Respiration Experiments](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-6-respiration-experiments.md)
  - [12.2.7 End of Topic Test - Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-7-end-of-topic-test-respiration.md)
  - [12.2.8 A-A* (AO3/4) - Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-8-a-a-ao34-respiration.md)

### 13 Photosynthesis (A2 Only)
- 13.1 Photosynthesis
  - [13.1.1 Introduction to Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-1-introduction-to-photosynthesis.md)
  - [13.1.2 Light-Dependent Reactions](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-2-light-dependent-reactions.md)
  - [13.1.3 Light-Independent Reactions](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-3-light-independent-reactions.md)
  - [13.1.4 Limiting Factors](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-4-limiting-factors.md)
  - [13.1.5 Investigating Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-5-investigating-photosynthesis.md)
  - [13.1.6 End of Topic Test - Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-6-end-of-topic-test-photosynthesis.md)
  - [13.1.7 A-A* (AO3/4) - Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-7-a-a-ao34-photosynthesis.md)

### 14 Homeostasis (A2 Only)
- 14.1 Homeostasis
  - [14.1.1 Homeostasis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-1-1-homeostasis.md)
  - [14.1.2 Negative Feedback](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-1-2-negative-feedback.md)
- 14.2 The Kidney
  - [14.2.1 Kidneys & Osmoregulation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-2-1-kidneys-and-osmoregulation.md)
- 14.3 Cell Signalling
  - [14.3.1 Messengers & cAMP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-3-1-messengers-and-camp.md)
- 14.4 Blood Glucose Concentration
  - [14.4.1 Blood Glucose Concentration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-4-1-blood-glucose-concentration.md)
  - [14.4.2 Controlling Blood Glucose](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-4-2-controlling-blood-glucose.md)
  - [14.4.3 Urinalysis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-4-3-urinalysis.md)
- 14.5 Homeostasis in Plants
  - [14.5.1 Stomata & Guard Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-5-1-stomata-and-guard-cells.md)

### 15 Control & Coordination (A2 Only)
- 15.1 Control & Coordination in Mammals
  - [15.1.1 Neurones](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-1-neurones.md)
  - [15.1.2 Receptors](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-2-receptors.md)
  - [15.1.3 Taste](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-3-taste.md)
  - [15.1.4 Reflexes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-4-reflexes.md)
  - [15.1.5 Action Potentials](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-5-action-potentials.md)
  - [15.1.6 Saltatory Conduction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-6-saltatory-conduction.md)
  - [15.1.7 Synapses](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-7-synapses.md)
  - [15.1.8 Cholinergic Synnapses](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-8-cholinergic-synnapses.md)
  - [15.1.9 Neuromuscular Junction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-9-neuromuscular-junction.md)
  - [15.1.10 Skeletal Muscle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-10-skeletal-muscle.md)
  - [15.1.11 Sliding Filament Theory Contraction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-11-sliding-filament-theory-contraction.md)
  - [15.1.12 Sliding Filament Theory Contraction 2](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-12-sliding-filament-theory-contraction-2.md)
  - [15.1.13 Menstruation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-13-menstruation.md)
  - [15.1.14 Contraceptive Pill](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-14-contraceptive-pill.md)
- 15.2 Control & Co-Ordination in Plants
  - [15.2.1 Venus Flytrap](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-2-1-venus-flytrap.md)
  - [15.2.2 Auixns](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-2-2-auixns.md)
  - [15.2.3 Gibberellins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-2-3-gibberellins.md)

### 16 Inherited Change (A2 Only)
- 16.1 Passage of Information to Offspring
  - [16.1.1 Meiosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-1-1-meiosis.md)
  - [16.1.2 Gametogenesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-1-2-gametogenesis.md)
  - [16.1.3 Meiosis & Genetic Diversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-1-3-meiosis-and-genetic-diversity.md)
- 16.2 Genes & Phenotype
  - [16.2.1 Key Terms in Genetics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-1-key-terms-in-genetics.md)
  - [16.2.2 Inheritance](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-2-inheritance.md)
  - [16.2.3 Linkage](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-3-linkage.md)
  - [16.2.4 Multiple Alleles](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-4-multiple-alleles.md)
  - [16.2.5 Chi-Squared Test](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-5-chi-squared-test.md)
  - [16.2.6 Mutations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-6-mutations.md)
  - [16.2.7 Specific Mutations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-7-specific-mutations.md)
  - [16.2.8 End of Topic Test - Genetics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-8-end-of-topic-test-genetics.md)
  - [16.2.9 A-A* (AO2/3) - Genetics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-9-a-a-ao23-genetics.md)
- 16.3 Gene Control
  - [16.3.1 lac Operon](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-3-1-lac-operon.md)
  - [16.3.2 Transcription Factors](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-3-2-transcription-factors.md)
  - [16.3.3 Gibberellins & DELLA Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-3-3-gibberellins-and-della-proteins.md)

### 17 Selection & Evolution (A2 Only)
- 17.1 Variation
  - [17.1.1 Phenotypic Variation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-1-1-phenotypic-variation.md)
  - [17.1.2 Continuous & Discontinuous Variation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-1-2-continuous-and-discontinuous-variation.md)
  - [17.1.3 t-Tests](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-1-3-t-tests.md)
- 17.2 Natural & Artificial Selection
  - [17.2.1 Natural Selection & Evolution](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-1-natural-selection-and-evolution.md)
  - [17.2.2 Types of Selection](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-2-types-of-selection.md)
  - [17.2.3 Types of Selection Summary](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-3-types-of-selection-summary.md)
  - [17.2.4 Genetic Drift & Founder Effect](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-4-genetic-drift-and-founder-effect.md)
  - [17.2.5 Populations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-5-populations.md)
  - [17.2.6 Hardy-Weinberg Principle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-6-hardy-weinberg-principle.md)
  - [17.2.7 Artificial Selection](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-7-artificial-selection.md)
- 17.3 Evolution
  - [17.3.1 Evolution by Natural Selection](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-1-evolution-by-natural-selection.md)
  - [17.3.2 Evidence for Evolution](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-2-evidence-for-evolution.md)
  - [17.3.3 Speciation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-3-speciation.md)
  - [17.3.4 Extinction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-4-extinction.md)

### 18 Classification & Conservation (A2 Only)
- 18.1 Biodiversity
  - [18.1.1 Key Terms](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-1-key-terms.md)
  - [18.1.2 Biodiversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-2-biodiversity.md)
  - [18.1.3 Sampling Methods](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-3-sampling-methods.md)
  - [18.1.4 Correlation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-4-correlation.md)
  - [18.1.5 Spearman’s Rank](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-5-spearmans-rank.md)
  - [18.1.6 Measuring Biodiversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-6-measuring-biodiversity.md)
- 18.2 Classification
  - [18.2.1 3-Domain Classification System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-2-1-3-domain-classification-system.md)
  - [18.2.2 Linnaean Classification System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-2-2-linnaean-classification-system.md)
- 18.3 Conservation
  - [18.3.1 Importance of Biodiversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-1-importance-of-biodiversity.md)
  - [18.3.2 Conservation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-2-conservation.md)
  - [18.3.3 Zoos, Seedbanks, & Other Conservation Methods](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-3-zoos-seedbanks-and-other-conservation-methods.md)
  - [18.3.4 Assisted Reproduction & Contraception](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-4-assisted-reproduction-and-contraception.md)
  - [18.3.5 Non-Governmental Organisations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-5-non-governmental-organisations.md)

### 19 Genetic Technology (A2 Only)
- 19.1 Manipulating Genomes
  - [19.1.1 Recombinant DNA & Genetic Engineering](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-1-recombinant-dna-and-genetic-engineering.md)
  - [19.1.2 Restriction Endonucleases](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-2-restriction-endonucleases.md)
  - [19.1.3 Gel Electrophoresis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-3-gel-electrophoresis.md)
  - [19.1.4 DNA Probes & Microarrays](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-4-dna-probes-and-microarrays.md)
  - [19.1.5 End of Topic Test - Manipulating Genomes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-5-end-of-topic-test-manipulating-genomes.md)
  - [19.1.6 A-A* (AO2/3) - Manipulating Genomes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-6-a-a-ao23-manipulating-genomes.md)
- 19.2 Genetic Technology Applied to Medicine
  - [19.2.1 Bioinformatics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-1-bioinformatics.md)
  - [19.2.2 Applications of Recombinant DNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-2-applications-of-recombinant-dna.md)
  - [19.2.3 Genetic Screening](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-3-genetic-screening.md)
  - [19.2.4 Gene Therapy](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-4-gene-therapy.md)
  - [19.2.5 Ethics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-5-ethics.md)
  - [19.2.6 DNA Profiling & Forensics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-6-dna-profiling-and-forensics.md)
- 19.3 Genetically Modified Organisms in Agriculture
  - [19.3.1 GM Crops & Livestock](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-3-1-gm-crops-and-livestock.md)
  - [19.3.2 Ethics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-3-2-ethics.md)