AQA GCSE Variation(Biology)

Variation

Variation means the differences in characteristics between individuals of the same species.

For example:

Humans have different heights, eye colours, and blood groups

Plants may have different leaf sizes or flower colours

Variation is due to:

1. Genetic differences (inherting different genes)

2. Environmental causes (developing in different environments)

3. A mixture of genetic and environmental causes.

Genetic causes

These are caused by the genes you inherit from your parents.

Examples:

Eye colour

Blood group

Genetic disorders

There is usually a lot of genetic variation within a population

Environmental causes

These are caused by conditions around you.

Examples:

Diet affects weight

Sun exposure affects skin colour

Education affects language skills

Combination of both

Most characteristics are influenced by both genes AND environment.

Examples:

Height (genes + nutrition)

Body mass (genes + diet/exercise)

How our genome can determine our phenotype.

The genome is all the genetic material (DNA) in an organism.

The phenotype is the observable characteristics of the organism.

The genome and its interaction with the environment influence the development of the phenotype.

Mutations and Variation

A mutation is a random change in the DNA base sequence.

Mutations happen all the time (continuously)

Most mutations have no effect.

Some mutations change the phenotype slightly

Very few mutations cause a new phenotype

Mutations and evolution

Mutations sometimes create a new phenotype (a new characteristic).

If the environment changes, a mutation may give an organism an advantageous characteristic. If this happens it means the organism is more likely to survive and reproduce.

As a result:

-the organism passes the allele for the advantageous characteristic on to its offspring

-more individuals in the population inherit this allele for advantageous characteristic

-over time, the mutation becomes more common

This can cause a rapid change in the species.

Real life example

The peppered moth shows how variation can lead to changes in a species.

There are two forms of this moth:

a light (white) form
a dark form

These differences are caused by a mutation, which created a new phenotype (dark colour).

Before environmental change

Originally, tree bark was light-coloured.

Light moths were camouflaged
Dark moths were easily seen by predators (birds)

Light moths were more likely to survive and reproduce

After environmental change

During industrialisation, pollution covered trees in dark soot.

Dark moths became camouflaged. Light moths were easily seen and eaten

Dark moths were more likely to survive and reproduce

What happened over time?

The dark colour allele was passed on to offspring
More moths in the population became dark
The population changed over time

This is an example of natural selection

Key point

A mutation created variation, and when the environment changed, the advantageous phenotype became more common in the population.

Continuous vs Discontinuous variation

Variation can be described in two ways: continuous and discontinuous.

Discontinuous variation	Continuous variation
Discontinuous variation shows distinct categories.	Continuous variation shows a range of values between two extremes.
There are no intermediate values	Individuals can have any value within the range
It is usually controlled by genes only	It is usually influenced by both genes and the environment
E.g. Human blood group	E.g. Human height
Data represented by a bar chart	Data represented by a normal distribution curve

Speciation

Speciation is the formation of a new species

A species is a group of individuals that can breed together to produce fertile offspring.

Variation within a population means some individuals are better adapted to their environment. These individuals are more likely to survive and reproduce (natural selection). Over many generations, these changes can accumulate, and if populations become so different that they can no longer interbreed, a new species is formed. This process is called speciation.

Practice Questions

1.What is meant by the term variation in a population?

2.A population of rabbits lives in a forest. Some rabbits have slightly darker fur due to a genetic mutation. If the forest becomes darker over time, explain how this variation might affect which rabbits survive and reproduce.

3.A plant species shows a wide range of heights. Suggest two reasons why some plants are taller than others.