Look around any group of people and the first thing you notice is that no two individuals are exactly alike. People differ in height, body mass, blood group, skin pigmentation and countless other characteristics. The same is true of other species: plants of the same species may differ in height, while animals in the same population may differ in speed, colour or resistance to disease.
These differences are not merely interesting details. Variation provides the raw material on which natural selection acts. To understand how populations change over generations, we therefore need to connect three ideas: variation, mutation and natural selection.
Try these five questions before reading the chapter. They will check whether you can already distinguish variation, mutation and natural selection.
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1. What Is Variation?
Variation is the occurrence of differences between individuals of the same species. A characteristic that varies between individuals is sometimes called a variable characteristic.
Some differences are easy to place into separate groups. For example, a person's ABO blood group is A, B, AB or O. Other characteristics, such as height, show many possible values with individuals spread across a range. This gives us two important patterns of variation: discontinuous variation and continuous variation.
Discontinuous variation
Discontinuous variation occurs when individuals can be placed into distinct categories and there are no intermediate forms between those categories.
ABO blood group is a clear example. A person's blood group is classified as A, B, AB or O. There is no blood group halfway between A and B.
Think in categories: if the characteristic places individuals into clearly separated groups rather than along a numerical scale, the variation is discontinuous.
Discontinuous variation is usually controlled mainly by genetic factors. Environmental conditions generally have much less effect on which category an individual belongs to.
Continuous variation
Continuous variation occurs when a characteristic can take many values across a range, including many intermediate values.
Human height is a familiar example. People cannot be divided naturally into just a few categories such as "short" and "tall". Between the shortest and tallest individuals are many intermediate heights.
Continuous characteristics are often influenced by many genes and may also be strongly affected by the environment. For example, a person's potential height depends partly on inherited genes, but nutrition during growth can also affect the height actually reached.
| Feature | Discontinuous variation | Continuous variation |
|---|---|---|
| Pattern | Distinct categories | A continuous range of values |
| Intermediate forms | Absent between categories | Many intermediate values occur |
| Typical genetic control | Often controlled mainly by one or a small number of genes | Often influenced by many genes |
| Environmental influence | Usually relatively small | Often important |
| Examples | ABO blood group; flower colour when distinct genetic categories occur | Height, body mass and skin pigmentation |
2. Where Does Variation Come From?
Variation does not have a single cause. Depending on the characteristic, differences between individuals may arise from genetic factors, environmental factors, or an interaction between the two.
Genetic causes of variation
Individuals inherit alleles from their parents. Different individuals may inherit different combinations of these alleles, giving them different genotypes and therefore potentially different phenotypes.
Sexual reproduction is especially important because offspring receive genetic material from two parents. Processes involved in sexual reproduction generate different combinations of alleles, so genetically different offspring can be produced.
However, sexual reproduction mainly reshuffles alleles that already exist. For genuinely new alleles to appear, a change in genetic material must occur. That is where mutation becomes important.
Environmental causes of variation
The environment can also affect phenotype. Factors such as nutrition, exercise, light intensity, temperature and exposure to disease may influence how a characteristic develops.
Two people may have similar genetic potential for height, but if one experiences severe malnutrition during childhood, that person's final height may be reduced. The observed phenotype therefore reflects both genetic and environmental influences.
Genotype: the genetic constitution of an organism — the alleles it possesses.
Phenotype: the observable characteristics of an organism, produced through the interaction of its genotype with the environment.
3. Mutation Creates New Genetic Variation
A mutation is a change in genetic material. Mutations are important because they can create new genetic variants that were not previously present in a population.
Mutations occur naturally and are described as random. This does not mean that every possible mutation is equally likely. It means that useful mutations do not appear simply because an organism needs them.
Mutation produces variation first; natural selection acts on that variation afterwards. The environment does not instruct an organism to produce the particular mutation that would help it survive.
Gene mutations
A gene mutation is a change in the DNA nucleotide sequence of a gene. Because a gene contains information used in making a polypeptide, changing its DNA sequence may alter the resulting protein and therefore the phenotype.
A gene mutation can create a new allele. Its effect may be harmful, neutral or beneficial, depending on what the mutation changes and the environment in which the organism lives.
Changes involving chromosomes
Genetic variation can also result from changes involving chromosomes. These may involve chromosome structure or chromosome number.
One important example is Down syndrome. It is most commonly caused by trisomy 21, in which there are three copies of chromosome 21 instead of two. The affected person's body cells therefore usually contain 47 chromosomes rather than 46.
This can arise when chromosomes fail to separate correctly during the formation of gametes. After fertilisation, the resulting zygote contains an extra copy of chromosome 21.
| Type of genetic change | What changes? | Possible consequence |
|---|---|---|
| Gene mutation | The DNA nucleotide sequence within a gene | A new allele may be formed |
| Chromosome structure change | Part of a chromosome | Many genes may potentially be affected |
| Chromosome number change | The number of chromosomes | For example, trisomy 21 in Down syndrome |
4. Mutagens Increase the Chance of Mutation
Mutations can occur spontaneously, but certain environmental agents increase the rate at which they occur. Such agents are called mutagens.
Examples include ionising radiation, such as X-rays and gamma rays, and certain chemical mutagens, including some chemicals found in tobacco smoke.
A mutagen increases the probability of mutation. Exposure does not mean that a particular useful or harmful mutation is guaranteed to occur.
Ultraviolet radiation can also damage DNA and cause mutations. However, ultraviolet radiation should not be described as ionising radiation; these are different categories of radiation.
5. Mutations Are Not Automatically Harmful
Students sometimes associate the word mutation only with disease, but the effect of a mutation depends on its consequences.
Many mutations have little or no noticeable effect. Some are harmful. Occasionally a mutation produces a characteristic that gives an organism an advantage under particular environmental conditions.
This last possibility is particularly important in evolution. If a new allele improves the reproductive success of the organisms carrying it, natural selection may cause that allele to become more common over successive generations.
6. Natural Selection Acts on Existing Variation
A population normally contains individuals with different inherited characteristics. At the same time, organisms often produce more offspring than the environment can support. Resources such as food, water, space and mates may therefore be limited.
These environmental conditions create selection pressures. Predators, diseases, climate and competition for resources can also act as selection pressures.
If some individuals possess an inherited characteristic that gives them an advantage under those conditions, they may be more likely to survive long enough to reproduce successfully.
The sequence of natural selection
The process is easiest to understand as a chain of cause and effect:
Variation → selection pressure → differential survival and reproduction → inheritance → change in the population over generations
First, individuals in the population show heritable variation.
Next, the environment creates selection pressures. Individuals whose inherited characteristics make them better suited to those conditions are more likely, on average, to survive and reproduce.
When these individuals reproduce, they pass their advantageous alleles to their offspring. Over many generations, those alleles may therefore become more common in the population.
Notice that natural selection does not require the individuals to know what is happening. There is no deliberate choice to change. Individuals vary first, and the environment affects which variants leave more offspring.
What does “fittest” really mean?
The phrase survival of the fittest can be misleading if “fittest” is interpreted as strongest, fastest or largest.
In evolutionary biology, greater fitness means greater reproductive success in a particular environment. A characteristic is advantageous only if it helps an organism leave more surviving offspring under those particular conditions.
A thick coat may be advantageous in a cold climate but disadvantageous in a very hot climate. Whether a characteristic is advantageous depends on the selection pressures present in the environment.
7. A Classic Example: Antibiotic Resistance
Antibiotic resistance provides a clear example of natural selection. Imagine a bacterial population containing genetic variation. Most bacteria may be susceptible to an antibiotic, while a small number possess an allele that makes them resistant.
When the antibiotic is applied, susceptible bacteria are killed or have their reproduction greatly reduced. Resistant bacteria are more likely to survive and reproduce. Their resistance alleles are passed to their descendants.
After repeated generations under this selection pressure, resistant bacteria may form a much larger proportion of the population.
The antibiotic does not cause bacteria to mutate because they “need” resistance. Resistant variants arise through genetic variation, and the antibiotic then selects for bacteria that already possess resistance.
8. How Natural Selection Leads to Evolution
Evolution refers to changes in the inherited characteristics of populations over generations. At the genetic level, this involves changes in how common particular alleles are within a population.
Natural selection can therefore produce evolutionary change because some inherited variants contribute more offspring to later generations than others.
The important word is population. An individual organism does not evolve during its lifetime. Instead, the genetic composition of a population changes over many generations.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Saying that continuous variation means there are several distinct categories | Distinct categories describe discontinuous variation. | Continuous variation shows a range with many intermediate values. |
| Assuming all variation is genetic | Environmental factors can also affect phenotype. | Decide whether the characteristic is influenced by genes, environment, or both. |
| Saying sexual reproduction creates new alleles | Sexual reproduction mainly produces new combinations of existing alleles. | Remember that mutation is the ultimate source of genuinely new alleles. |
| Saying organisms mutate because they need to adapt | Mutations do not arise in response to what would be useful to the organism. | Variation arises first; natural selection then acts on that variation. |
| Assuming every mutation is harmful | Mutations may be harmful, neutral or beneficial depending on their effects and the environment. | Describe the effect of the mutation rather than assuming it is automatically harmful. |
| Describing UV radiation as ionising radiation | UV radiation can damage DNA, but it is not classified as ionising radiation. | Use X-rays and gamma rays as examples of ionising radiation. |
| Saying the strongest individuals are always the “fittest” | Evolutionary fitness refers to reproductive success, not simply physical strength. | Link the advantageous characteristic to survival and successful reproduction in the particular environment. |
| Saying an individual evolves | Evolution describes inherited changes in populations over generations. | State that individuals are selected, while populations evolve. |
What you should be able to do
Define variation
Explain that variation refers to differences between individuals of the same species.
Distinguish continuous and discontinuous variation
Recognise whether a characteristic forms distinct categories or a continuous range of values.
Explain the causes of variation
Describe how genetic factors, environmental factors, or both can produce differences in phenotype.
Explain mutation
State that mutations are changes in genetic material and can introduce new alleles into a population.
Recognise chromosome-number changes
Relate Down syndrome to trisomy 21 and the presence of an extra chromosome 21.
Identify mutagens
Give examples such as ionising radiation and certain chemical mutagens, and explain that they increase mutation rate.
Explain natural selection
Link heritable variation, selection pressure, differential reproduction and inheritance over generations.
Connect natural selection to evolution
Explain how advantageous inherited characteristics can become more common in a population over many generations.
These ten questions test whether you can apply ideas about continuous and discontinuous variation, mutation and natural selection to unfamiliar situations.
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Frequently asked questions
Frequently Asked Questions About Variation
Variation, mutation and natural selection are closely connected, so many examination mistakes arise from mixing up what each process actually does. These questions clarify the distinctions that matter most.
What is the simplest way to distinguish continuous from discontinuous variation?
Ask whether the characteristic forms distinct categories or a measurable range. ABO blood groups form separate categories, so they show discontinuous variation. Height can take many values between two extremes, so it shows continuous variation.
Is continuous variation caused only by the environment?
No. Continuous characteristics are often influenced by many genes as well as environmental factors. Human height, for example, has a strong genetic component but can also be affected by nutrition during growth.
Is discontinuous variation always controlled by only one gene?
Not necessarily. The important feature of discontinuous variation is that phenotypes fall into distinct categories. Such variation is usually controlled mainly by genetic factors and often involves one or a small number of genes, but the definition is based on the pattern of phenotypes rather than simply counting genes.
What is the difference between mutation and variation?
Variation describes the differences that exist between individuals. Mutation is one process that can produce genetic variation by changing genetic material and creating new alleles. Variation may also arise from different combinations of existing alleles and from environmental effects.
Are all mutations harmful?
No. A mutation may be harmful, neutral or beneficial. Its effect depends on what has changed and on the environment. A mutation that is advantageous in one environment may provide no advantage, or even be disadvantageous, in another.
What is a mutagen?
A mutagen is an agent that increases the rate of mutation. Examples include ionising radiation such as X-rays and gamma rays, as well as certain chemicals. A mutagen increases the chance of a mutation but does not guarantee a particular mutation.
Why does natural selection require variation?
Selection can operate only if individuals differ. When some of those differences are heritable and improve reproductive success, the alleles responsible can be passed to more offspring and become more common over generations. If every individual were genetically identical for the relevant characteristic, there would be no alternative variants for natural selection to favour.
Does the environment cause organisms to develop useful mutations?
No. Mutations do not occur because an organism needs a particular adaptation. Genetic variants arise first. Environmental selection pressures then determine which variants tend to survive and reproduce more successfully.
What does “survival of the fittest” mean in an examination answer?
It should not simply be written as “the strongest survive”. Individuals that are better adapted to a particular environment are more likely to survive and reproduce successfully. Their advantageous alleles are therefore passed to more offspring.
What is the difference between natural selection and evolution?
Natural selection is a process that causes individuals with certain heritable characteristics to contribute more offspring to later generations. Evolution is the resulting change in inherited characteristics of a population over generations. Natural selection is therefore one important mechanism that can drive evolution.
Variation gives populations their differences, mutation supplies new genetic possibilities, and natural selection changes which inherited variants become common over time. Once you connect those three ideas as one continuous story, evolution becomes much easier to explain: variation exists first, the environment selects among that variation, and populations change across generations.