What is continuous variation?

Continuous variation shows a range of phenotypes with no distinct categories and is controlled by many genes (polygenic inheritance) plus environmental factors.

What is discontinuous variation?

Discontinuous variation shows distinct phenotypic categories with no intermediates and is typically controlled by one or a few genes with little environmental influence.

Define stabilising selection.

Stabilising selection favours intermediate phenotypes and reduces variation, occurring when the environment is stable.

What effect does stabilising selection have on phenotypic variation?

It reduces variation by selecting against extreme phenotypes.

Define directional selection.

Directional selection favours one extreme phenotype, shifting the mean of the population in one direction, occurring when environmental conditions change.

Under what conditions does directional selection typically occur?

When environmental conditions change, favouring individuals with phenotypes at one extreme.

OCR A-Level Biology H420

Cell Structure

Topic overview Microscopy, slide preparation and staining Biological drawings, magnification and resolution Eukaryotic cell ultrastructure The secretory pathway and the cytoskeleton Prokaryotic and eukaryotic cells compared

Biological Molecules

Topic overview The properties of water Monomers, polymers and the elements of life Monosaccharides and disaccharides Polysaccharides and their functions Lipids Amino acids and protein structure Globular and fibrous proteins Inorganic ions The biochemical tests Colorimetry and chromatography

Nucleotides and Nucleic Acids

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Enzymes

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Exchange Surfaces

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Transport in Plants

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Communicable Diseases, Disease Prevention and the Immune System

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Biodiversity

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Classification and Evolution

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Communication and Homeostasis

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Excretion as an Example of Homeostatic Control

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Neuronal Communication

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Photosynthesis

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Respiration

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Cellular Control

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Patterns of Inheritance

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Manipulating Genomes

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Practical Skills and Mathematical Requirements

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Patterns of Inheritance

Selection, genetic drift and the founder effect

2 things to know
Components 02 and 03

Overview

Stabilising and directional selection and what each does to variation, and the drift, bottlenecks and founder effects that act on small populations.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Compare stabilising and directional selection.

Key term

What is genetic drift, and in which populations does it have the greatest effect?

Questions from this subtopic

Each one with the model answer underneath.

1
What is continuous variation?
Model answer
Continuous variation shows a range of phenotypes with no distinct categories and is controlled by many genes (polygenic inheritance) plus environmental factors.
2
What is discontinuous variation?
Model answer
Discontinuous variation shows distinct phenotypic categories with no intermediates and is typically controlled by one or a few genes with little environmental influence.
3
Define stabilising selection.
Model answer
Stabilising selection favours intermediate phenotypes and reduces variation, occurring when the environment is stable.
4
What effect does stabilising selection have on phenotypic variation?
Model answer
It reduces variation by selecting against extreme phenotypes.
5
Define directional selection.
Model answer
Directional selection favours one extreme phenotype, shifting the mean of the population in one direction, occurring when environmental conditions change.
6
Under what conditions does directional selection typically occur?
Model answer
When environmental conditions change, favouring individuals with phenotypes at one extreme.

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