Variation with a Species

Section 1 of 7What variation is

Definition

• Definition Variation means the differences between individuals of the same species.

Three possible causes

• Three possible causes Differences in genes, which arise from the random fusion of gametes at fertilisation.
• Differences in the environment the individual lives in.
• A combination of genes and the environment acting together.

Section 2 of 7Environmental variation

Environmental causes

What 'environmental' means

• What 'environmental' means It simply means anything outside the organism, including climate, diet, culture, lifestyle and accidents.

Accidents

• Accidents An injury can leave a scar on the body.

Diet and lifestyle

• Diet and lifestyle Eating too much and taking little exercise leads to weight gain.

Culture

• Culture The country where a person grows up determines the language and accent they speak.

Plants

• Plants A plant growing in the shade of a large tree grows taller and thinner to reach more light.

Section 3 of 7Discontinuous and continuous variation

Feature	Discontinuous variation	Continuous variation
Appearance	Distinct categories with no in-between values	A smooth range of values between two extremes
Main cause	Genes alone	Genes and the environment together
Genes involved	Usually a single gene (monogenic)	Often many genes (polygenic)
Examples	Blood group, tongue rolling, ability to taste a bitter chemical	Height, body mass, skin colour
Graph	Bar chart	Line graph or histogram forming a bell shape

Section 4 of 7Worked example: height

1Genes inherited

• Genes inherited Tall parents pass on many alleles for height to their children.

2Genetic potential

• Genetic potential The children have the genetic potential to be tall.

3Environment acts

• Environment acts If the diet is poor, the children cannot make enough protein and grow poorly.

4Outcome

• Outcome Their final height is lower than their potential.
  • Genes decide the characteristics we inherit, but the environment affects how they develop.

Section 5 of 7Deciding which cause applies

1

Ask whether the characteristic falls into clear categories, such as blood group A, B, AB or O. If so, it is discontinuous and mostly genetic.

2

Ask whether the values form a range, such as heights from 140 cm to 190 cm. If so, it is continuous and probably affected by several genes and the environment.

3

Ask whether the difference could appear during a person's life, such as a scar or a suntan. If so, it is environmental.

4

If identical twins raised apart still differ in a characteristic, the difference must be caused by the environment, because their genes are the same.

5

Always explain your choice by naming the cause: genes, environment or both.

Section 6 of 7Key terms

Term	Meaning
Variation	Differences between individuals of the same species.
Discontinuous variation	Variation where individuals fall into distinct categories, with no intermediates.
Continuous variation	Variation where individuals show a full range of values between two extremes.
Polygenic	Controlled by many genes acting together.
Environmental variation	Differences caused by factors outside the organism, such as diet or climate.

Section 7 of 7Exam technique

1

Saying that variation is always inherited; scars, accents and weight gain are environmental.

2

Confusing continuous with discontinuous variation. Height is continuous because values form a smooth range.

3

Writing that the environment changes the genes. It affects how the inherited characteristic develops.

4

Suggesting that blood group is affected by diet; it is controlled by genes alone.