17.1 Variation

2026 Syllabus Objectives

By the end of this subtopic, you should be able to:

  1. Describe variation as differences between individuals of the same species
  2. Understand that continuous variation results in a range of phenotypes between two extremes, including body length and body mass
  3. Understand that discontinuous variation results in a limited number of phenotypes with no intermediates, including ABO blood groups, seed shape and seed colour in peas
  4. Understand that discontinuous variation is usually caused by genes only, and continuous variation is caused by genes and the environment
  5. Investigate and describe examples of continuous and discontinuous variation

Main Content

https://cambridgeassistant.com

1. What is Variation?

Variation means the differences that exist between individuals that belong to the same species.

Think about the students in your classroom. You are all human beings — the same species — yet you all look different. Some students are taller, some shorter. Some have curly hair, some have straight hair. Some have blood group A, others have blood group O. All of these differences between people of the same species are examples of variation.

Variation is not just about how things look. It can also include differences in behaviour, physiology (how the body works), and biochemistry (the chemicals inside organisms).

Key point: Variation always refers to differences within the same species — not between different species.

2. Types of Variation

There are two main types of variation:

  • Continuous variation
  • Discontinuous variation

Understanding the difference between these two types is very important.

3. Continuous Variation

Continuous variation is when individuals in a species show a full range of values between two extremes (a minimum and a maximum). There are no clear-cut groups — the measurements gradually change from one extreme to the other, with every value in between being possible.

Key features of continuous variation:

  • There is a smooth, gradual range of values.
  • There are no distinct categories — individuals fall anywhere along a scale.
  • Most individuals fall near the middle, with fewer at the extremes. This pattern produces a bell-shaped curve when drawn on a graph.

Examples of continuous variation:

  • Body length (height): People are not just "tall" or "short." Heights range smoothly from very short to very tall, with every measurement in between possible (e.g., 150 cm, 163 cm, 175 cm, 180 cm, etc.).
  • Body mass (weight): Like height, body mass covers a full range of values with no sharp cut-off points.
  • Other examples include hand span, foot length, and skin colour.

How to display continuous variation:

Because the data is a range of measurements, it is best shown on a histogram or a line graph with a smooth curve. The x-axis shows the measurement (e.g., height in cm), and the y-axis shows the number of individuals (frequency). The result is typically a bell-shaped (normal distribution) curve.

Sign in to view full notes