Variation — Genetic and Environmental

Variation is the differences that exist between individuals. This lesson covers the causes of variation, the distinction between genetic and environmental variation, and how these concepts relate to evolution. This is a key topic in the AQA GCSE Combined Science Trilogy specification (8464).

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What Is Variation?

Variation means the differences between individuals of the same species. No two organisms are exactly alike (except identical clones). Variation can be classified as:

1. Genetic variation — caused by differences in DNA
2. Environmental variation — caused by conditions in which an organism lives
3. A combination of both — most characteristics are influenced by genes AND the environment

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Genetic Variation

Genetic variation arises from differences in DNA. It is inherited from parents to offspring.

Sources of Genetic Variation

Source	How It Causes Variation
Meiosis	Produces gametes with different combinations of alleles due to independent assortment and crossing over
Random fertilisation	Any sperm can fertilise any egg, producing unique combinations of alleles
Mutations	Random changes in the DNA base sequence can create new alleles

Examples of Genetically Determined Characteristics

Characteristic	Details
Blood group	Determined entirely by genes (A, B, AB or O)
Eye colour	Primarily genetic (though shade can vary)
Ability to roll tongue	Genetic
Inherited disorders	e.g. cystic fibrosis, polydactyly
Natural hair colour	Genetic (though people can dye their hair)
Earlobe shape	Free or attached — determined by alleles

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Environmental Variation

Environmental variation is caused by the conditions in which an organism develops and lives. These are not inherited.

Environmental Factor	Effect on Variation
Diet and nutrition	Affects height, weight and health
Climate and temperature	Affects plant growth, animal fur thickness
Exercise and physical activity	Affects muscle development and fitness
Accidents and injuries	Can cause permanent changes (e.g. scars)
Sunlight exposure	Affects skin colour (tanning)
Soil conditions	Affects plant growth, size and colour
Disease	Can stunt growth or change appearance

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Combined Variation: Genes AND Environment

Most characteristics are influenced by both genetic and environmental factors. This is an important point that AQA frequently tests.

Characteristic	Genetic Contribution	Environmental Contribution
Height	Genes determine potential maximum height	Diet, health and exercise affect actual height reached
Weight	Genes influence metabolism and body type	Diet and exercise determine actual weight
Skin colour	Genes determine base skin colour	Sun exposure causes tanning
Intelligence	Genes play a role in cognitive potential	Education, nutrition and environment influence outcomes
Plant height	Genes determine growth potential	Light, water, minerals and temperature affect actual growth

Exam Tip: AQA (8464) loves to ask whether a characteristic is due to genes, environment, or both. If the answer is "both", make sure you explain HOW each factor contributes. For example: "A person's height is influenced by their genes, which set the maximum potential height, but environmental factors such as diet and health during childhood determine whether that potential is reached."

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Continuous and Discontinuous Variation

Variation can also be classified by how it is distributed:

Continuous Variation

Continuous variation shows a range of values with no distinct categories. It is usually influenced by many genes and the environment (polygenic).

Examples	Features
Height	Any value within a range
Weight	Gradual spread of values
Foot size	No clear groups
Hand span	Measured on a continuous scale

Continuous variation is usually displayed as a histogram or line graph, often producing a bell-shaped (normal distribution) curve.

Discontinuous Variation

Discontinuous variation shows distinct categories with no intermediates. It is usually controlled by a single gene.

Examples	Features
Blood group (A, B, AB, O)	Distinct groups only
Tongue rolling (can or cannot)	Two categories
Earlobe shape (free or attached)	Two categories
Eye colour (to some extent)	Distinct categories

Discontinuous variation is usually displayed as a bar chart.

graph LR
    A["Variation"] --> B["Continuous<br/>• Range of values<br/>• Many genes + environment<br/>• e.g. height, weight"]
    A --> C["Discontinuous<br/>• Distinct categories<br/>• Usually one gene<br/>• e.g. blood group"]
    style A fill:#bbdefb,stroke:#1565c0
    style B fill:#c8e6c9,stroke:#2e7d32
    style C fill:#fff9c4,stroke:#f9a825

Exam Tip: If you are given data in an exam and asked to identify whether it is continuous or discontinuous, look at whether the data falls into distinct groups (discontinuous) or a range of values (continuous).

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Worked Example: Investigating Variation

A student measured the heights of 30 students in their class and recorded the following:

Height range (cm)	Number of students
150–154	2
155–159	5
160–164	9
165–169	8
170–174	4
175–179	2

This is an example of continuous variation because:

• Height can take any value within the range
• The data shows a spread of values forming a roughly normal distribution
• Height is influenced by both genes and the environment

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Common Mistakes

Mistake	Correction
Saying environmental variation is inherited	Environmental variation is not passed on to offspring
Saying all variation is genetic	Most characteristics are influenced by both genes and environment
Confusing continuous and discontinuous	Continuous = range of values; discontinuous = distinct categories
Saying mutations always cause variation	Most mutations have no effect; only some create new alleles that lead to variation

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Summary