18.1 Variation Revision Notes

1. Overview

Variation is the engine of evolution and the reason why no two individuals (except identical twins) are exactly alike. Understanding variation allows us to categorize how characteristics are passed down and how environmental factors interact with our DNA to shape the physical traits (phenotypes) we see in a population.

Key Definitions

Variation: The differences between individuals of the same species.
Phenotype: The observable features of an organism.
Genotype: The genetic makeup of an organism in terms of the alleles present.
Mutation: A genetic change; the way in which new alleles are formed.
Gene Mutation: A random change in the base sequence of DNA.
Continuous Variation: Variation that results in a range of phenotypes between two extremes.
Discontinuous Variation: Variation that results in a limited number of phenotypes with no intermediates.

Core Content

Types of Variation

Variation can be classified into two main categories based on the distribution of phenotypes:

1. Continuous Variation

Description: Features that show a complete range of measurements from one extreme to the other. There are no distinct groups.
Causes: Influenced by both genes and the environment.
Examples:
- Body length/Height
- Body mass (weight)
Data Representation: Usually forms a "Bell Curve" (Normal Distribution) when plotted on a histogram.
📊A histogram showing a bell-shaped curve. The x-axis represents 'Height' and the y-axis represents 'Number of individuals'. The peak is in the middle, showing most people are of average height.

2. Discontinuous Variation

Description: Features that fall into distinct categories. You either have the characteristic or you don't.
Causes: Usually caused by genes only.
Examples:
- ABO Blood Groups (A, B, AB, or O)
- Seed shape in peas (round or wrinkled)
- Seed colour in peas (yellow or green)
Data Representation: Plotted as a bar chart with gaps between the bars.
📊A bar chart with four separate bars representing Blood Groups A, B, AB, and O. The bars do not touch, indicating distinct categories.

Mutations

What is it? A mutation is a change in the DNA of a cell.
The Result: Mutations are the only way new alleles are created. This provides the raw material for natural selection.
Increasing Mutation Rate: While mutations happen spontaneously, the rate can be increased by:
1. Ionising radiation: e.g., Gamma rays, X-rays, and Ultraviolet (UV) rays.
2. Chemicals: e.g., certain chemicals in tobacco smoke or industrial pollutants.

Extended Content (Extended Curriculum Only)

Gene Mutation at the Molecular Level

A gene mutation is defined specifically as a random change in the base sequence of DNA. Because the sequence of bases (A, T, C, G) codes for specific amino acids, a change in this sequence can change the protein produced, leading to a different phenotype.

Sources of Genetic Variation

While mutation is the primary source of new alleles, several processes during reproduction "shuffle" the existing alleles to create variation in a population:

Mutation: Creates brand new alleles.
Meiosis: During the formation of gametes (sperm and egg), alleles are redistributed through independent assortment and crossing over.
Random Mating: Any male in a population can potentially mate with any female; this mixes different combinations of alleles.
Random Fertilisation: Any one of the millions of sperm cells can fertilise the egg, ensuring the resulting zygote has a unique combination of DNA.

Key Equations

There are no specific mathematical formulas for topic 18.1. However, you should be able to:

Calculate Percentages: (Number of individuals with a trait ÷ Total population) × 100.
Calculate Means: For continuous variation data (Sum of all values ÷ Number of individuals).

Common Mistakes to Avoid

❌ Wrong: Saying continuous variation is only caused by the environment.
✓ Right: Continuous variation is caused by both genes and the environment (e.g., you may have the genes to be tall, but poor nutrition stops you from reaching that height).
❌ Wrong: Using a line graph to show discontinuous variation.
✓ Right: Use a bar chart for discontinuous data and a histogram for continuous data.
❌ Wrong: Thinking mutations are always "bad."
✓ Right: Mutations are "random"—they can be harmful, neutral, or occasionally beneficial to an organism's survival.

Exam Tips

Command Word: "State": This is the most common command word for this topic. Give a short, factual answer (e.g., "State two causes of mutation" -> "UV radiation and tobacco smoke").
Command Word: "Explain": If asked to explain why height is continuous variation, you must mention that it is controlled by multiple genes AND influenced by environmental factors like diet.
Look for Chemicals: Exam questions often use "chemicals" or "radiation" as a context for how mutations occur. Be ready to link these to DNA damage.
Data Analysis: Be prepared to identify the type of variation from a graph. If it's a smooth curve, it's continuous; if it's separate bars, it's discontinuous.