1. What Is Variation?

Introduction

Look around any classroom, sports team, or family. Although everyone belongs to the same species, no two people are exactly alike. Some people are taller, some have curly hair, others have different eye colours, fingerprints, or blood groups. These differences are examples of variation.

Variation occurs in every species on Earth, from plants and animals to bacteria. It is one of the most important features of living organisms because it allows populations to adapt to changing environments over time. Without variation, species would struggle to survive environmental changes, diseases, and other challenges.

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What Is Variation?

Variation refers to the differences in characteristics between individuals of the same species.

Characteristics that can vary include:

• Height.
• Hair colour.
• Eye colour.
• Blood group.
• Leaf shape.
• Flower colour.
• Fur pattern.

Variation means that no two individuals are exactly the same (except identical twins, which are genetically almost identical).

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Figure 1. Individuals within the same species show many different forms of variation.

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Variation Within a Population

A population is a group of individuals of the same species living in the same area.

Within a population, individuals may differ in:

• Size.
• Colour.
• Behaviour.
• Speed.
• Disease resistance.
• Ability to tolerate different environmental conditions.

For example:

A population of rabbits may include individuals with:

• White fur.
• Brown fur.
• Larger ears.
• Smaller ears.
• Different body sizes.

These differences are examples of variation.

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Why Are Individuals Different?

Individuals are not identical because of two main factors:

• Genetics (inheritance).
• Environmental influences.

Many characteristics are affected by both.

For example:

Height depends on:

• The genes inherited from parents.
• Nutrition and overall health during growth.

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Inherited Characteristics

Inherited characteristics are passed from parents to offspring through genes.

Examples include:

• Eye colour.
• Natural hair colour.
• Blood group.
• Attached or detached earlobes.
• Ability to roll the tongue (in some genetics models).

These characteristics are determined mainly by DNA.

They cannot usually be changed by lifestyle or experience.

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Figure 2. Inherited characteristics are passed from parents to offspring through genes.

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Acquired Characteristics

Acquired characteristics develop during an individual's lifetime.

They are caused by:

• Lifestyle.
• Environment.
• Learning.
• Injury.
• Experience.

Examples include:

• A suntan.
• Muscle development from exercise.
• Scars.
• Pierced ears.
• Learning a new language.

These characteristics are not passed on genetically to offspring.

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Comparing Inherited and Acquired Characteristics

Inherited Characteristics	Acquired Characteristics
Passed from parents	Develop during life
Controlled mainly by genes.	Influenced by the environment or experience
Usually permanent	May be temporary or permanent
Passed to offspring	Not inherited by offspring

Understanding this difference is essential in genetics.

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Figure 3. Inherited characteristics come from genes, while acquired characteristics develop through life experiences and environmental influences.

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Continuous and Discontinuous Variation

Variation can be grouped into two broad categories.

Continuous Variation

Characteristics show a range of values.

Examples include:

• Height.
• Body mass.
• Hand span.
• Foot length.

There are many possible values with no clear groups.

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Discontinuous Variation

Characteristics fall into distinct categories.

Examples include:

• Blood group.
• Natural eye colour categories.
• Ability to roll the tongue (in simplified genetics examples).

Individuals belong to one category or another, with no intermediate values.

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Why Variation Is Important

Variation is essential because it helps populations survive when environments change.

For example:

If a disease affects a population:

• Some individuals may be naturally resistant.
• These individuals survive and reproduce.
• Their offspring may inherit the resistant characteristics.

Variation therefore increases the chances that a species will continue to exist.

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Figure 4. Variation increases the likelihood that some individuals will survive changing environmental conditions.

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Examples of Variation in Nature

Variation can be seen throughout the natural world.

Examples include:

• Different flower colours in the same plant species.
• Different fur colours in wolves.
• Different beak sizes in birds.
• Different shell patterns in snails.
• Different leaf shapes within a tree species.

Every population contains natural variation.

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Variation and Evolution

Variation provides the raw material for evolution.

Natural selection acts on variation by favouring individuals with characteristics that improve survival and reproduction.

Over many generations:

• Helpful inherited characteristics become more common.
• Populations gradually change.

Without variation, evolution could not occur.

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Why Scientists Study Variation

Scientists study variation to:

• Understand inheritance.
• Improve crop plants.
• Breed healthier livestock.
• Develop medicines.
• Protect endangered species.
• Investigate evolution.

Variation is one of the central ideas in biology.

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Figure 5. Variation occurs in all living organisms and is fundamental to genetics and evolution.

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Worked Example

Question

Classify each characteristic as inherited or acquired.

• Eye colour
• Scar from a bicycle accident
• Blood group
• Suntan
• Muscle growth from exercise

Solution

Characteristic.	Type
Eye colour	Inherited
Scar	Acquired
Blood group	Inherited
Suntan	Acquired
Muscle growth	Acquired

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Real-World Connection

Plant breeders make use of natural variation to develop crop varieties that produce higher yields, resist diseases, or tolerate drought. By selecting plants with desirable inherited characteristics and breeding them over many generations, scientists can develop crops that are better suited to different environments and help improve food security.

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Did You Know?

Your fingerprints are unique, even if you have an identical twin. Although identical twins share almost the same DNA, tiny differences in conditions inside the womb during development create unique fingerprint patterns for each individual.

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Key Terms

Acquired characteristic – A characteristic that develops during an individual's lifetime and is not inherited genetically.

Characteristic – A feature or trait of an organism.

Inherited characteristic – A characteristic passed from parents to offspring through genes.

Population – A group of individuals of the same species living in the same area.

Species – A group of organisms that can reproduce with one another and produce fertile offspring.

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

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Key Takeaways

• Variation refers to the differences between individuals of the same species.
• Individuals differ because of inherited genes, environmental influences, or a combination of both.
• Inherited characteristics are passed from parents to offspring, while acquired characteristics develop during an individual's lifetime and are not inherited.
• Variation can be continuous (a range of values) or discontinuous (distinct categories).
• Variation is important because it helps populations survive environmental changes and provides the basis for evolution by natural selection.
• Scientists study variation to improve agriculture, medicine, conservation, and our understanding of life on Earth.

 

 

 

2. Genetic Variation

Introduction

No two zebras have exactly the same stripe pattern, no two oak trees produce identical leaves, and no two people—except identical twins—have exactly the same genetic makeup. These differences arise because every individual inherits a unique combination of genes.

Genetic variation is the foundation of biodiversity. It makes every population slightly different and provides the raw material for natural selection and evolution. Without genetic variation, species would have little ability to adapt to changing environments, new diseases, or other challenges.

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What Is Genetic Variation?

Genetic variation refers to the differences in DNA and genes between individuals of the same species.

These differences produce variations in characteristics such as:

• Eye colour.
• Hair colour.
• Height.
• Blood group.
• Flower colour.
• Fur pattern.
• Disease resistance.

Because individuals inherit different combinations of genes, every population contains genetic diversity.

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Figure 1. Genetic variation arises because individuals inherit different combinations of genes.

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Genes and Variation

A gene is a section of DNA that contains instructions for making a particular protein or controlling a characteristic.

Most characteristics are influenced by one or more genes.

Different versions of the same gene are called alleles.

For example, a gene controlling flower colour may have alleles for:

• Purple flowers.
• White flowers.

Different combinations of alleles produce variation within a population.

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How Sexual Reproduction Increases Variation

Sexual reproduction combines genetic information from two parents.

Each parent contributes one set of chromosomes through their reproductive cells (gametes).

During fertilisation:

• A sperm cell joins with an egg cell.
• The offspring receives half its genes from each parent.

Because each offspring inherits a different combination of genes, brothers and sisters are genetically similar but not identical.

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Figure 2. Sexual reproduction mixes genetic information from two parents, producing unique offspring.

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Sources of Genetic Variation

Several processes create genetic variation.

1. Independent Assortment

During meiosis, chromosomes are distributed randomly into gametes.

Each gamete receives a different combination of chromosomes.

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2. Crossing Over

During meiosis, matching chromosomes exchange sections of DNA.

This process is called crossing over (or recombination).

It creates new combinations of genes that were not present in either parent.

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3. Random Fertilisation

Any sperm can fertilise any egg.

This random combination produces enormous numbers of possible genetic combinations.

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4. Mutations

Mutations create entirely new genetic variation by changing DNA itself.

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Mutations

A mutation is a change in the DNA sequence of a gene or chromosome.

Mutations may occur because of:

• Mistakes during DNA replication.
• Radiation.
• Certain chemicals.
• Some viruses.

Most mutations are:

• Neutral (no noticeable effect).
• Harmful.
• Occasionally beneficial.

Beneficial mutations may improve survival and reproduction.

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Figure 3. Mutations create new genetic variation by changing DNA.

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Mutations and Evolution

Mutations introduce new alleles into a population.

If a mutation gives an organism an advantage:

• It is more likely to survive.
• It is more likely to reproduce.
• The beneficial allele may become more common over many generations.

Without mutations, no new genetic information would enter a population.

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Why Genetic Variation Is Important

Genetic variation allows populations to:

• Adapt to environmental changes.
• Resist diseases.
• Survive climate changes.
• Respond to new predators.
• Evolve over time.

Populations with greater genetic variation are generally more likely to survive future challenges.

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Figure 4. Genetic variation provides the diversity needed for natural selection and adaptation.

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Genetic Variation and Evolution

Evolution occurs when inherited characteristics change in a population over many generations.

Natural selection acts on genetic variation.

Individuals with beneficial inherited characteristics are more likely to:

• Survive.
• Reproduce.
• Pass their genes to the next generation.

Over time:

• Helpful alleles become more common.
• Populations gradually evolve.

Genetic variation is therefore the raw material for evolution.

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Examples of Genetic Variation

Examples include:

Humans

• Blood groups.
• Eye colour.
• Natural hair colour.
• Height (partly genetic).

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Animals

• Fur colour in rabbits.
• Shell patterns in snails.
• Beak size in birds.
• Stripe patterns in zebras.

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Plants

• Flower colour.
• Leaf shape.
• Fruit size.
• Disease resistance.

Every species shows genetic variation.

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Figure 5. Genetic variation is found in every population of living organisms.

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Why Scientists Study Genetic Variation

Scientists investigate genetic variation to:

• Improve crops.
• Breed healthier livestock.
• Develop medicines.
• Conserve endangered species.
• Study evolution.
• Understand inherited diseases.

Modern genetics has become one of the most important fields in biology.

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Worked Example

Question

List four sources of genetic variation.

Solution

The four major sources are:

• Independent assortment during meiosis.
• Crossing over during meiosis.
• Random fertilisation.
• Mutations.

Together, these processes create the genetic diversity found in populations.

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Real-World Connection

Doctors and scientists monitor genetic variation in viruses such as influenza. As viruses reproduce, mutations can create new strains that spread more easily or avoid existing immunity. Studying these genetic changes helps researchers develop updated vaccines and improve public health responses to disease outbreaks.

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Did You Know?

Although the DNA of any two humans is about 99.9% identical, the remaining 0.1% contains millions of small genetic differences. These tiny variations contribute to differences in appearance, health, and many other inherited characteristics, making every person genetically unique (except for identical twins, who are genetically almost identical).

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Key Terms

Allele – A different version of a gene.

Crossing over (recombination) – The exchange of DNA between matching chromosomes during meiosis, creating new gene combinations.

Gene – A section of DNA that contains instructions for making a protein or controlling a characteristic.

Genetic variation – Differences in DNA and genes between individuals of the same species.

Mutation – A change in the DNA sequence that creates new genetic variation.

Natural selection – The process by which individuals with advantageous inherited characteristics are more likely to survive and reproduce.

Random fertilisation – The random fusion of sperm and egg cells during sexual reproduction.

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Key Takeaways

• Genetic variation refers to differences in genes and DNA between individuals of the same species.
• Genes and different versions called alleles contribute to inherited variation.
• Sexual reproduction increases variation by combining genetic information from two parents.
• The main sources of genetic variation are independent assortment, crossing over, random fertilisation, and mutations.
• Mutations create new genetic information and provide the raw material for evolution.
• Genetic variation is essential because it enables populations to adapt, survive environmental changes, and evolve over time.

 

 

 

 

3. Environmental Variation

Introduction

Not all differences between individuals are caused by genes. Two people may have similar genetic potential for height, but one may grow taller because they had a healthier diet during childhood. A plant grown in rich soil may be much larger than the same type of plant grown in poor soil, even though their genes are very similar.

These differences are examples of environmental variation. Environmental factors influence how organisms grow, develop, and survive throughout their lives. Many characteristics are shaped by both genetics and the environment, making environmental variation an important part of biology.

What Is Environmental Variation?

Environmental variation refers to differences in characteristics that are caused by environmental factors rather than inherited genes.

Environmental variation occurs because organisms experience different conditions during their lives.

Examples include:

• Body mass.
• Muscle strength.
• Suntan.
• Plant height.
• Language spoken.
• Scars.

These differences are not caused directly by inherited DNA.

 

 

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Figure 1. Environmental conditions can cause individuals with similar genes to develop differently.

Environmental Factors

Many environmental factors influence the characteristics of organisms.

Common factors include:

• Nutrition.
• Temperature.
• Light.
• Water availability.
• Exercise.
• Disease.
• Climate.
• Pollution.

Different environments can produce different outcomes, even within the same species.

Nutrition

Food provides the energy and nutrients needed for growth and development.

Examples:

• Children with balanced diets usually grow more effectively.
• Plants supplied with enough minerals often grow larger and healthier.

Poor nutrition can reduce growth even when genes would allow greater growth.

Climate and Temperature

Climate affects many living organisms.

Examples include:

• Animals in colder regions often develop thicker fur.
• Plants grow differently in tropical and desert environments.
• Temperature influences the growth rate of many organisms.

Environmental conditions can affect how genes are expressed.

 

 

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Figure 2. Environmental conditions such as climate can influence the appearance and survival of organisms.

Exercise and Lifestyle

Lifestyle choices influence many characteristics.

Examples include:

• Exercise increases muscle size and strength.
• Regular training improves fitness.
• Poor sleep can affect growth and health.
• Smoking may damage the lungs.
• Excessive sun exposure causes tanning.

These are examples of environmental variation because they develop during life.

Disease

Illness can influence growth and development.

Examples include:

• Disease reducing plant growth.
• Childhood illnesses affecting physical development.
• Parasites reducing body condition in animals.

Disease is an environmental factor because it is not inherited directly as a characteristic.

Genetic vs Environmental Variation

Many characteristics are influenced mainly by either genes or the environment.

Some characteristics depend on both.

Characteristics Influenced by Both

Many traits result from a combination of genes and environment.

Examples include:

Height

Affected by:

• Genes.
• Nutrition.
• Health.

Body Mass

Affected by:

• Genes.
• Diet.
• Exercise.
• Lifestyle.

Intelligence and Learning

Influenced by:

• Genetics.
• Education.
• Environment.
• Experience.

Most complex characteristics involve interactions between genes and environmental factors.

 

 

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Figure 3. Many characteristics result from both inherited genes and environmental influences.

Analysing Environmental Variation

When studying variation, scientists ask:

• Is the characteristic inherited?
• Is it caused by the environment?
• Is it influenced by both?

For example:

A person develops large muscles after years of weight training.

The increased muscle size is mainly due to exercise, making it an example of environmental variation.

Why Environmental Variation Is Important

Environmental variation allows organisms to respond to changing conditions.

It helps explain why:

• The same species looks different in different environments.
• Individuals grow at different rates.
• Lifestyle influences health.
• Organisms adapt to their surroundings during their lifetime.

Unlike genetic variation, environmental variation is not inherited by offspring.

Examples of Environmental Variation

Examples include:

Humans

• Suntans.
• Scars.
• Muscle growth.
• Fitness level.
• Language learned.

Plants

• Height.
• Number of leaves.
• Growth rate.
• Flower size.

Animals

• Body condition.
• Fur thickness (seasonal changes).
• Weight.
• Behaviour learned through experience.

Environmental variation occurs throughout the living world.

 

 

Figure 4. Environmental variation can affect the appearance, growth, and behaviour of many organisms.

Environmental Variation and Survival

Although environmental variation is not inherited, it can improve an individual's chances of survival.

Examples:

• Regular exercise improves fitness.
• Good nutrition promotes healthy growth.
• Adequate water helps plants survive drought.
• Learning helps animals avoid predators.

These changes benefit the individual but are generally not passed genetically to offspring.

Worked Example

Question

Classify each characteristic as:

• Genetic
• Environmental
• Both genetic and environmental

Characteristics:

• Blood group
• Height
• Suntan
• Muscle development
• Eye colour

Solution

Real-World Connection

Farmers carefully manage environmental conditions to help crops grow successfully. They control factors such as water, soil nutrients, sunlight, and temperature to improve plant growth and increase crop yields. Even plants with excellent genetic potential cannot reach their full size or produce healthy fruits if environmental conditions are poor.

Did You Know?

Identical twins have almost exactly the same DNA, yet they often become more different in appearance, health, and even personality as they grow older. Different diets, occupations, hobbies, illnesses, and life experiences create environmental variation, making each twin increasingly unique over time.

Key Terms

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

Environmental variation – Differences in characteristics caused by environmental factors rather than inherited genes.

Genetic variation – Differences in characteristics caused by inherited genes.

Lifestyle – The way an individual lives, including diet, exercise, sleep, and other habits that affect health.

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

Key Takeaways

• Environmental variation refers to differences in characteristics caused by environmental factors rather than inherited genes.
• Common environmental factors include nutrition, climate, exercise, disease, light, water, and pollution.
• Some characteristics are mainly genetic, some are mainly environmental, and many are influenced by both.
• Lifestyle and surroundings can strongly affect growth, health, and development.
• Environmental variation helps explain differences between individuals living in different conditions.
• Unlike genetic variation, environmental variation is generally not inherited by offspring.

4. Continuous and Discontinuous Variation

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

Illustrative frequency distribution of student heights showing a bell-shaped pattern.

 

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

Continuous Variation	Discontinuous Variation
Shows a range of values	Shows distinct categories
Many intermediate values	No intermediate values
Measured with numbers	Classified into groups
Influenced by many genes	Usually controlled by one or a few genes
Strongly affected by the environment.	Little or no environmental influence
Often forms a bell-shaped graph	Often shown as separate bars

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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

Characteristic	Type
Height	Continuous
Blood group	Discontinuous
Body mass	Continuous
Eye colour categories.	Discontinuous
Hand span	Continuous

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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.

5. Measuring Variation in Populations

Learning outcomes

• I can collect data about variation in a population.
• I can organize and present variation data.
• I can calculate averages from biological data.
• I can interpret graphs showing variation.
• I can draw conclusions from population data.

Introduction

Biologists often ask questions such as: How tall are the plants in this field?, What is the average wingspan of these birds?, or How much variation exists in this population of insects? To answer these questions, scientists collect measurements from many individuals and analyse the results.

Measuring variation helps scientists understand populations, compare species, monitor environmental changes, and investigate evolution. By organising data into tables and graphs and calculating simple statistics, patterns that are difficult to see at first become much clearer.

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What Is a Population?

A population is a group of individuals of the same species living in the same area.

Examples include:

• Oak trees in a forest.
• Fish in a lake.
• Rabbits in a field.
• Students in a classroom.

Scientists often measure characteristics within a population to study variation.

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Figure 1. A population consists of individuals of the same species living in the same area.

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Collecting Data

To study variation, scientists collect data from many individuals.

Common characteristics measured include:

• Height.
• Body mass.
• Leaf length.
• Beak length.
• Foot length.
• Wing span.

Good data collection should:

• Use a large sample size.
• Measure accurately.
• Use the same method for every individual.
• Record results carefully.

Larger samples usually give more reliable conclusions.

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Organising Data

Once measurements have been collected, they should be organised clearly.

Scientists commonly use:

• Tables.
• Frequency tables.
• Spreadsheets.

Example:

Student	Height (cm)
A	158
B	161
C	165
D	170
E	172

Organised data are much easier to analyse.

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Presenting Data

Biologists often display variation using graphs.

Common graph types include:

• Histograms.
• Bar charts (for discontinuous variation).
• Frequency graphs.
• Line graphs (for changes over time).

Continuous variation is usually shown with a histogram, where the bars touch because the data form a continuous range.

Example of population height data

Illustrative frequency distribution showing continuous variation in a population.

 

Figure 2. A histogram helps display continuous variation within a population.

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Calculating the Mean (Average)

One of the most useful statistics is the mean, commonly called the average.

The mean is calculated using:

\( Mean = \frac{Total \ of \ all \ measurements}{Number \ of \ measurements} \)

​

The mean gives the typical value for a population.

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Worked Example – Calculating the Mean

Question

Five plants have the following heights:

18 cm, 20 cm, 21 cm, 19 cm, 22 cm

Find the mean height.

Solution

Step 1:

Add the measurements.

18 + 20 + 21 + 19 + 22 = 100

Step 2:

Divide by the number of plants.

\( \frac{100}{5} = 20 \)

Mean height = 20 cm

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Other Useful Statistics

Scientists also use:

Maximum

The largest measurement.

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Minimum

The smallest measurement.

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Range

The difference between the largest and smallest values.

Range = Maximum − Minimum

The range gives an idea of how much variation exists within the population.

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Interpreting Graphs

Graphs help scientists answer questions such as:

• What is the most common value?
• How much variation exists?
• Are most individuals similar?
• Are there any unusual values (outliers)?

When interpreting a graph, look for:

• The highest bars.
• The spread of the data.
• Any gaps or unusual patterns.

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Figure 3. Graphs help scientists identify patterns and compare populations.

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Drawing Conclusions

After analysing the data, scientists can draw conclusions.

For example:

• Most plants were between 15 cm and 20 cm tall.
• The average height was 18 cm.
• Very few plants were extremely short or extremely tall.
• The population showed continuous variation.

Conclusions should always be based on the evidence collected.

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Sources of Error

When collecting biological data, scientists should consider possible errors.

Common sources include:

• Measuring incorrectly.
• Small sample sizes.
• Recording mistakes.
• Using different measuring techniques.
• Biased sampling.

Reducing these errors improves the reliability of the results.

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Why Measuring Variation Is Important

Measuring variation helps scientists:

• Study evolution.
• Monitor endangered species.
• Improve crops.
• Investigate diseases.
• Compare populations.
• Understand biodiversity.

Accurate measurements provide evidence that supports scientific conclusions.

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Figure 4. Scientists measure variation to better understand populations and ecosystems.

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Worked Example

Question

A class measures the leaf lengths of 50 plants.

The histogram shows that most leaves are between 8 cm and 10 cm.

What conclusions can be drawn?

Solution

Possible conclusions:

• Most plants have leaves between 8 cm and 10 cm.
• Very short and very long leaves are less common.
• The population shows continuous variation.
• The average leaf length is likely close to the middle of the distribution.

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Real-World Connection

Conservation biologists measure variation in endangered animal populations to monitor their health. By collecting data on body size, weight, and genetic diversity, scientists can determine whether a population is thriving or declining and decide what conservation measures are needed to protect the species.

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Did You Know?

Modern biologists often use digital calipers, GPS devices, drones, and computer software to collect and analyse thousands of biological measurements. These technologies allow scientists to study variation in populations much more accurately and efficiently than ever before.

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Key Terms

Average (Mean) – The sum of all measurements divided by the number of measurements.

Frequency – The number of times a particular value or range of values occurs.

Histogram – A graph used to display continuous data, with bars that touch.

Population – A group of individuals of the same species living in the same area.

Range – The difference between the largest and smallest values in a dataset.

Sample – A smaller group selected to represent a population.

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

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Key Takeaways

• Scientists study variation by collecting measurements from many individuals within a population.
• Data should be organised using tables, frequency tables, or graphs.
• Histograms are commonly used to display continuous variation.
• The mean provides the average value, while the range indicates how much variation exists.
• Scientists interpret graphs to identify patterns, compare populations, and draw evidence-based conclusions.
• Careful data collection and analysis are essential for reliable biological investigations.