17.2.2
Types of Selection
Stabilising Selection
Stabilising Selection
Stabilising selection is the process where natural selection favours an average phenotype.


Selection pressure
Selection pressure
- Natural selection acts on an individual by imposing a selection pressure.
- The selection pressure is an external factor that influences the reproductive success of an individual.
- E.g. Disease acts as a selection pressure that increases the reproductive success for individuals who are immune to the disease.


Stabilising selection
Stabilising selection
- In stabilising selection, natural selection favours an average phenotype.
- The selection pressures in stabilising selection select against the extreme phenotypes.
- Stabilising selection takes place in environments that do NOT change.


E.g. Mouse camouflage
E.g. Mouse camouflage
- A population of mice live on the floor of the woods.
- Natural selection will favour individuals who best match the brown colour of the floor. This helps avoid detection by predators.
- Mice with brown fur are most likely to survive and reproduce, passing on their genes for their brown coat.
- Mice that carry alleles that make their fur lighter or darker will stand out against the ground and be more likely to be eaten by predators. Stabilising selection favours the average phenotype: brown.
Directional Selection
Directional Selection
Directional selection is the process where natural selection favours an extreme phenotype.


Directional selection
Directional selection
- In directional selection, natural selection favours one extreme phenotype.
- The selection pressures in directional selection select against all other phenotypes.
- Directional selection takes place after an environment has experienced a change.


E.g. Peppered moth
E.g. Peppered moth
- Before the Industrial Revolution, peppered moths were predominantly white. This allowed them to camouflage with the light-coloured trees and lichens in their habitat.
- The environment changed when the Industrial Revolution darkened the trees with soot.
- The white moths became easier for predators to spot and the white colour was now selected against.
- Natural selection caused the frequency of the black-coloured moth to increase because the black colouration became more favourable.
Disruptive Selection
Disruptive Selection
Disruptive selection is the process where natural selection favours the two extremes of a phenotype.


Disruptive selection
Disruptive selection
- In disruptive selection, natural selection favours the two extremes of a phenotype.
- The selection pressures in disruptive selection select against the average phenotype.
- Disruptive selection takes place in an environment that favours more than one phenotype.
- Disruptive selection can eventually lead to the production of two new species.


Multiple male forms
Multiple male forms
- Some populations of animals have multiple male forms.
- Large, dominant alpha males obtain mates by brute force.
- Small males can sneak in for secret copulations with the females in an alpha male’s territory.
- The two phenotypes both have a selective advantage.
- Natural selection selects for both phenotypes.
- Medium-sized males are selected against because they are not big enough to obtain mates using size but they aren't small enough for "sneaking".
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
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