Introduction

If you measured the height of every student in a school, you would find a wide range of values. Some students would be short, some tall, and many somewhere in between. However, if you recorded everyone's blood group, you would find only a few distinct categories such as A, B, AB, and O.

These two patterns represent the two main types of biological variation: continuous variation and discontinuous variation. Understanding the difference between them helps scientists study genetics, inheritance, populations, and evolution.

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What Is Continuous Variation?

Continuous variation occurs when a characteristic can take any value within a range.

There are no clear categories because individuals show many small differences.

Examples include:

• Height.
• Body mass.
• Hand span.
• Foot length.
• Leaf length.
• Milk production in cows.

Most individuals fall near the middle of the range, with fewer individuals at the extremes.

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Figure 1. Continuous variation shows a smooth range of values with many intermediate forms.

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Features of Continuous Variation

Characteristics showing continuous variation:

• Have many possible values.
• Usually produce a smooth distribution.
• Are often measured with numbers.
• Are influenced by many genes.
• Are also affected by the environment.

Examples:

• A person's height depends on:
  • Genes.
  • Nutrition.
  • Health.
  • Lifestyle.

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Graphing Continuous Variation

Continuous variation is often displayed using a histogram or frequency distribution.

The graph usually forms a bell-shaped curve (also called a normal distribution).

Most individuals have average values, while relatively few have extremely low or extremely high values.

Example of continuous variation

Figure 2. Continuous variation often produces a bell-shaped distribution, with most individuals having average values.

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What Is Discontinuous Variation?

Discontinuous variation occurs when characteristics fall into distinct categories.

There are no intermediate values between the groups.

Examples include:

• Blood group (A, B, AB, O).
• Biological sex (in many species).
• Ability to roll the tongue (in simplified genetics models).
• Attached or detached earlobes (simplified example).

Individuals belong to one category or another.

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Figure 3. Discontinuous variation consists of distinct categories with no intermediate values.

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Features of Discontinuous Variation

Characteristics showing discontinuous variation:

• Have only a few categories.
• Cannot take intermediate values.
• Are usually controlled by one or a few genes.
• Are affected very little by the environment.

For example:

A person's blood group is determined by inherited genes and is not changed by diet, exercise, or climate.

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Comparing Continuous and Discontinuous Variation

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Why Do Some Traits Show a Range of Values?

Many characteristics show continuous variation because they are influenced by:

• Multiple genes (polygenic inheritance).
• Environmental factors.

For example, height is affected by:

• Many different genes.
• Nutrition.
• Exercise.
• Health during childhood.

The combined effects of these factors create many possible values instead of just a few categories.

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Figure 4. Continuous variation usually results from the combined effects of many genes and environmental factors.

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Examples in Nature

Continuous Variation

• Tree height.
• Wing length in birds.
• Body mass of mammals.
• Seed size.
• Human height.

Discontinuous Variation

• Human blood groups.
• Flower colour in some plant species.
• Coat colour determined by a single gene in some animals.
• Certain inherited genetic disorders.

Both types of variation occur naturally in populations.

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Why Variation Matters

Both continuous and discontinuous variation contribute to biodiversity.

Variation allows:

• Populations to adapt to changing environments.
• Scientists to study inheritance.
• Breeders to improve crops and livestock.
• Natural selection to act over generations.

Understanding the type of variation helps biologists investigate how characteristics are inherited.

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Figure 5. Living organisms display both continuous and discontinuous variation.

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Worked Example

Question

Classify each characteristic as continuous or discontinuous.

• Height
• Blood group
• Body mass
• Eye colour categories
• Hand span

Solution

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Real-World Connection

Doctors use continuous variation when monitoring a patient's height, body mass, blood pressure, and cholesterol levels. These measurements can take many values and are influenced by both genetics and lifestyle. In contrast, characteristics such as blood group are discontinuous and remain the same throughout a person's life.

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Did You Know?

Scientists estimate that human height is influenced by hundreds of different genes, as well as nutrition, health, and environmental conditions during growth. This is why height shows such a wide range of values rather than falling into just a few categories.

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Key Terms

Continuous variation – Variation in which characteristics show a continuous range of values with many intermediate forms.

Discontinuous variation – Variation in which characteristics fall into distinct categories with no intermediate values.

Environmental factor – A condition in an organism's surroundings that influences its characteristics.

Normal distribution – A bell-shaped pattern often seen when graphing continuous variation.

Polygenic inheritance – Inheritance in which a characteristic is controlled by many different genes.

Trait (Characteristic) – A feature or quality of an organism.

Variation – Differences in characteristics between individuals of the same species.

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Key Takeaways

• Continuous variation shows a range of values, while discontinuous variation consists of distinct categories.
• Continuous variation is usually influenced by many genes and environmental factors.
• Discontinuous variation is usually controlled by one or a few genes and is only slightly affected by the environment.
• Continuous variation is commonly displayed using histograms or bell-shaped distributions.
• Characteristics such as height and body mass show continuous variation, while blood group is an example of discontinuous variation.
• Understanding these two types of variation helps explain inheritance, diversity, and evolution.