Variation and Mutation

No two individuals of a species are exactly the same — even identical twins differ in small ways. The differences between members of the same species are called variation, and they are the raw material on which natural selection acts. This lesson, part of Topic B5 of OCR Gateway Science A, explains where variation comes from (your genes, your environment, or both), distinguishes continuous from discontinuous variation, and explains mutation — the random change to DNA that creates the new alleles ultimately responsible for all genetic variation. It is the bridge between the inheritance you have just studied and the evolution you will study next.

By the end of this lesson you should be able to describe genetic, environmental and combined causes of variation, distinguish continuous and discontinuous variation with examples and their graph shapes, explain what a mutation is and its effects, and link mutation to natural selection.

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What Is Variation?

Variation means the differences between the individuals of a species. Look around any classroom and you will see variation in height, hair colour, eye colour, blood group and countless other features. Variation has three possible causes:

• Genetic variation — caused by differences in the alleles (genes) individuals inherit. Examples: eye colour, blood group, natural hair colour, the ability to roll your tongue.
• Environmental variation — caused by the conditions an organism lives in, with no genetic basis. Examples: a scar, a suntan, a language you speak, the flattened shape of a plant grown in a windy spot.
• Variation caused by both genes and environment — most characteristics. Examples: height and body mass (you inherit genes that set a range, but diet and exercise decide where in that range you end up).

Exam Tip: Most human characteristics are caused by both genes and environment, so "both" is often the safest answer for features like height or weight. Reserve "purely genetic" for features like blood group and "purely environmental" for things like a scar or a learned skill.

Where does genetic variation come from?

Genetic variation arises because of sexual reproduction and mutation:

• Sexual reproduction mixes alleles from two parents. Meiosis shuffles the genes when gametes are made, and fertilisation combines two different gametes at random, so offspring get new combinations of alleles. (This is why sexually produced offspring are all different.)
• Mutation creates entirely new alleles — versions of genes that did not exist before. Mutation is the ultimate source of all genetic variation, because every allele began, at some point in the past, as a mutation.

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Continuous and Discontinuous Variation

Genetically influenced characteristics fall into two patterns, and OCR expects you to tell them apart and recognise their typical graph shapes.

• Continuous variation describes a characteristic that can take any value within a range — there are no distinct categories. Examples: height, body mass, hand span, leaf length. Continuous variation is usually controlled by many genes (and often influenced by the environment too). Plotted as a graph, it gives a smooth, bell-shaped curve, with most individuals near the middle and few at the extremes.
• Discontinuous variation describes a characteristic that falls into distinct categories with no in-between values. Examples: blood group (A, B, AB or O), tongue-rolling (you can or you can't), biological sex, eye colour (in broad categories). Discontinuous variation is usually controlled by a single gene (or a small number) and is not affected by the environment. Plotted as a graph, it gives a bar chart of separate categories.

Feature	Continuous variation	Discontinuous variation
Values	Any value in a range	Distinct categories only
Examples	Height, mass, hand span, leaf length	Blood group, tongue-rolling, sex
Usually controlled by	Many genes (+ environment)	One (or few) genes
Affected by environment?	Often yes	Usually no
Typical graph	Smooth bell-shaped curve	Bar chart of categories

Exam Tip: The deciding question is "are there in-between values?". Height has every value in between (continuous, bell curve); blood group has only A, B, AB or O with nothing in between (discontinuous, bar chart). Continuous data are plotted as a curve/histogram; discontinuous data as a bar chart.

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Mutation: The Source of New Alleles

A mutation is a random change to the DNA base sequence. Because the base sequence is the genetic code, changing it can produce a new allele — a new version of a gene. Mutations happen naturally at a low rate every time DNA is copied before a cell divides.

The effect of a mutation depends on where in the DNA it happens and how it changes the protein the gene codes for:

• Most mutations have no effect at all. Many fall in stretches of DNA that do not code for a protein, and even within a gene the code has some built-in "redundancy", so the protein may be unchanged.
• Some mutations are harmful. If a mutation changes the order of amino acids enough to alter a protein's shape, the protein may not work properly. A mutation can cause a genetic disorder, or increase the risk of a disease such as cancer.
• Very rarely, a mutation is beneficial. Occasionally a new allele produces a protein that gives the organism an advantage — for example, making it better able to survive a particular condition. These rare beneficial mutations are extremely important, because they provide the new variation on which natural selection acts.

flowchart TD
  A["Mutation:<br/>random change to DNA base sequence"] --> B["A new allele is produced"]
  B --> C["No effect<br/>(most common)"]
  B --> D["Harmful effect<br/>(protein altered / may not work)"]
  B --> E["Beneficial effect<br/>(very rare — gives an advantage)"]

What increases the mutation rate?

Mutations occur naturally, but their rate can be increased by certain factors in the environment:

• Ionising radiation, such as ultraviolet (UV) light, X-rays and gamma rays.
• Some chemicals, called mutagens — for example, some of the chemicals in tobacco smoke.

Agents that increase the mutation rate and so increase the risk of cancer are called carcinogens. This is part of the reason why exposure to UV light (linked to skin cancer) and tobacco smoke (linked to lung cancer) is dangerous: by raising the mutation rate, they raise the chance of a harmful mutation that can lead to cancer.

Exam Tip: Three facts about mutation reliably earn marks: a mutation is a random change to the DNA / base sequence; it produces a new allele; and most mutations have no effect, a few are harmful, and very rarely one is beneficial. The mutation rate is increased by radiation and some chemicals.

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Measuring Variation in a Population

To study variation, biologists often measure a characteristic across many individuals and look at how the values are spread. The pattern that emerges tells you whether the variation is continuous or discontinuous.