Edexcel IGCSE Biology · Spec 3.22, 3.33
Variation
Genetic and environmental variation, and the difference between continuous and discontinuous variation.
Biology revision video
Variation
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Variation, and where the differences come from
Variation is the differences between individuals of the same species. It has two sources, genes and environment, and most characteristics are shaped by both at once.
Genetic variation
You inherit alleles, which are different versions of the same gene, from your parents. Which combination you receive is a matter of chance, and that is what makes siblings different from each other.
Three processes create the raw material. Meiosis shuffles the chromosomes when gametes are made. Fertilisation combines the alleles of two different parent cells. Mutation introduces entirely new alleles.
Characteristics that are purely genetic include blood group, eye colour, and whether a person can roll their tongue. The environment does not change any of them.
Environmental variation
Surroundings and lifestyle produce differences too, and these are not passed on. A scar, a tattoo, a spoken language and a hairstyle are all environmental. A plant grown in the shade has a different appearance from a genetically identical cutting grown in full light, which is the standard way of demonstrating this.
Most characteristics are both
Height is the usual example. Your genes set a range and your diet determines where in that range you end up. Body mass, skin colour and intelligence all work the same way.
When a question asks for the cause of a characteristic, check whether it can be influenced after birth. If it can, the environment is involved.
Continuous and discontinuous
Continuous variation takes any value within a range, so measurements form a smooth distribution with no gaps. Height and body mass are continuous, and they are usually controlled by several genes together, often with an environmental contribution. Plotted, they give a bell shaped curve.
Discontinuous variation falls into distinct categories with nothing in between. Blood group is either A, B, AB or O, and there is no value halfway. This kind is usually controlled by a single gene and is unaffected by the environment. Plotted, it gives a bar chart.
The shape of the graph is often the question. Continuous data goes on a histogram or line graph; discontinuous data goes on a bar chart with gaps between the bars.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Biology describes a question on why sexual reproduction is beneficial. Strong answers referred to the generation of variation and then explained the consequence, that organisms are more able to adapt if the environment changes, or that some may survive an environmental change. Two marks, and the second one is the explanation rather than the fact.
On a neighbouring question about differences between sexual and asexual reproduction, the report records a precise error. A common answer referred to the need for two parents in sexual reproduction, and the report states that candidates should refer to the two parent cells. Two parents alone was insufficient.
The same report closes with advice that applies well beyond this topic: always be clear in answers about what you are referring to, and try not to say it or they.
Why variation matters
Without it, natural selection would have nothing to select from. If every individual in a population were identical and conditions changed, either all of them could cope or none of them could. Variation means some individuals happen to have characteristics suited to the new conditions, and those are the ones that survive and reproduce.
Spec 3.22, 3.33
What you need to know
- Explain what variation is
- Describe genetic and environmental causes of variation
- Know that most variation is caused by both together
Active recall
Quick check
Answer each question before opening the answer.
Give the difference between genetic and environmental variation, with an example.
Genetic variation is inherited (e.g. blood group); environmental variation is caused by surroundings (e.g. a scar or spoken language).
What is the difference between continuous and discontinuous variation?
Continuous varies over a range (e.g. height); discontinuous falls into distinct categories (e.g. blood group).
Biology revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.