Walk through any population of living organisms and you will find differences. Some plants grow taller than others. Some bacteria carry alleles that make them resistant to an antibiotic. Some animals are better camouflaged, tolerate heat better or obtain food more efficiently than other members of the same species.
Most of these differences seem small when we look at one generation. Over many generations, however, inherited differences can change an entire population. The key is that the environment does not affect every individual equally. When some inherited characteristics give their owners a better chance of reproducing, those characteristics tend to become more common. This process is natural selection.
Try these five questions before reading the chapter. They will help you identify whether you already understand variation, inheritance and the basic idea behind natural selection.
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Variation Provides the Raw Material for Natural Selection
Natural selection cannot operate if every individual is exactly the same. There must first be variation within the population.
Suppose a population of insects contains individuals with slightly different body colours. If birds can see pale insects more easily against dark tree bark, darker individuals may escape predation more often. Natural selection can therefore act because a difference already exists between individuals.
The variation exists first. The environment then determines which inherited variants give their owners an advantage under those particular conditions.
Continuous and discontinuous variation
Variation can be described as continuous or discontinuous. The distinction depends on the pattern of values observed.
| Type of variation | Pattern | Examples | Typical causes |
|---|---|---|---|
| Continuous variation | A range of values with many possible intermediate values | Height, body mass and hand span | Often influenced by many genes as well as environmental factors |
| Discontinuous variation | Individuals fall into distinct categories with no continuous range between them | ABO blood group; some genetically determined characteristics with distinct forms | Often strongly determined by genes |
Continuous variation is often influenced by both genes and the environment. For example, a person's potential height is influenced by inherited genes, while nutrition and health during development can also affect the height eventually reached.
This is important when thinking about natural selection. Natural selection can only cause inherited characteristics to become more common in later generations. A characteristic produced entirely by an individual's environment but not inherited cannot simply be passed to its offspring.
Where Does Genetic Variation Come From?
One important source of new genetic variation is mutation. A mutation is a change in genetic material. A mutation affecting the DNA sequence of a gene may create a new allele.
Mutations arise spontaneously, although certain environmental agents called mutagens can increase the rate at which mutations occur. Examples include ionising radiation and some chemicals.
| Type of mutation | What changes? | Possible consequence |
|---|---|---|
| Gene mutation | The DNA sequence within a gene changes | A new allele may be formed, possibly affecting the protein produced |
| Chromosome mutation | The structure or number of chromosomes changes | Many genes may be affected |
A mutation does not occur because an organism needs it. Nor is every mutation beneficial. A mutation may be harmful, have little detectable effect, or become advantageous under particular environmental conditions.
Mutations occur without regard to whether they will be useful. Natural selection happens afterwards, when the environment affects the reproductive success of individuals carrying different inherited variants.
Sexual reproduction also produces variation because offspring inherit different combinations of alleles from their parents. It therefore creates new combinations of existing alleles, while mutation can introduce genuinely new alleles.
How Natural Selection Works
Natural selection becomes easier to understand when we follow what happens to a population from one generation to the next.
1. Individuals in the population show inherited variation
Because individuals carry different combinations of alleles, they may differ in characteristics such as colour, resistance to disease, ability to tolerate temperature changes or efficiency in obtaining food.
2. More offspring may be produced than can all survive
Food, water, light, space and other resources are limited. Predators and diseases may also remove some individuals. As a result, not every organism survives long enough to reproduce successfully.
This produces competition, particularly when individuals depend on the same limited resources.
3. The environment acts as a selection pressure
A selection pressure is an environmental factor that affects an organism's chance of surviving and reproducing.
| Type of selection pressure | Examples | Possible characteristic favoured |
|---|---|---|
| Predation | Predators hunting prey | Better camouflage, speed or defensive characteristics |
| Disease | Pathogens spreading through a population | Inherited resistance to the disease |
| Competition | Limited food, water, light or space | Characteristics that improve access to the limiting resource |
| Temperature | Very cold or very hot conditions | Inherited characteristics that improve tolerance of those temperatures |
| Chemicals | Antibiotics or insecticides | Inherited resistance to the chemical |
4. Some individuals survive and reproduce more successfully
If an inherited characteristic gives an individual an advantage under the existing conditions, that individual is more likely to survive long enough to reproduce successfully.
However, survival alone is not enough. From the point of view of natural selection, the important outcome is reproductive success. An individual that survives but produces no offspring contributes no alleles to the next generation.
In evolution, being better fitted means being better suited to the environment in a way that increases reproductive success. A tiny bacterium resistant to an antibiotic can be better fitted than a susceptible bacterium even though physical strength has nothing to do with the difference.
5. Advantageous alleles are passed to offspring
When successful individuals reproduce, their offspring may inherit the alleles responsible for their advantageous characteristics.
Across many generations, these alleles may therefore become more common in the population. At the same time, alleles associated with less favourable characteristics may become less common.
6. The population becomes better adapted to its environment
After repeated selection over many generations, the population may contain a much greater proportion of individuals with characteristics suited to the prevailing conditions.
This is an important point: an individual organism does not evolve during its lifetime. The composition of the population changes from one generation to the next.
The sequence to remember
Inherited variation → selection pressure → differential survival and reproduction → inheritance of advantageous alleles → advantageous alleles become more common over generations.
This sequence explains natural selection without suggesting that organisms deliberately change or that the environment produces the exact mutation required.
Selection Depends on the Environment
There is no characteristic that is automatically advantageous in every environment. Whether a characteristic increases reproductive success depends on the conditions.
Imagine two forms of an insect: one dark and one pale. On dark tree bark, the darker form may be less visible to predators. On pale bark, the same dark colour might make the insect easier to see.
If the environment changes, the direction of natural selection can therefore change as well.
Natural selection favours characteristics that increase reproductive success under the current conditions. A characteristic that is useful today may become neutral or disadvantageous after the environment changes.
Example: Antibiotic Resistance in Bacteria
Antibiotic resistance provides one of the clearest examples of natural selection occurring around us.
Consider a population of disease-causing bacteria. Most bacteria may initially be susceptible to a particular antibiotic, but genetic variation means that a small number may already possess an allele that gives resistance.
When the antibiotic is used:
- susceptible bacteria are killed or prevented from reproducing;
- resistant bacteria are more likely to survive;
- the surviving resistant bacteria reproduce;
- their descendants inherit the resistance allele;
- the proportion of resistant bacteria increases over subsequent generations.
The antibiotic therefore acts as a selection pressure.
The antibiotic does not normally cause individual bacteria to develop the specific resistance mutation because they need it. Resistant variants may already be present before antibiotic treatment. The antibiotic selects among them by removing susceptible bacteria.
This also explains why unnecessary or inappropriate antibiotic use is a serious problem. The more frequently bacteria are exposed to an antibiotic, the more opportunities there are for resistant bacteria to gain a reproductive advantage over susceptible bacteria.
From Natural Selection to Evolution
Natural selection describes a mechanism by which inherited characteristics can change in frequency within a population.
Over many generations, the accumulation of such changes produces evolution. At O-Level, evolution can be understood as a gradual change in the inherited characteristics of a population over time.
Natural selection and evolution are related but not identical
Natural selection is a process in which individuals with certain inherited characteristics leave more offspring than others.
Evolution describes the resulting change in inherited characteristics of populations across generations.
Natural selection therefore explains how populations can become increasingly adapted to their environments without any organism planning, choosing or deliberately changing its genes.
Common Mistakes
| Common mistake | Why it is wrong | Better explanation |
|---|---|---|
| “The organism changed because it needed to survive.” | Need does not cause an individual to produce the required inherited characteristic. | Inherited variation already exists. Individuals with advantageous variants reproduce more successfully. |
| “The antibiotic caused the bacteria to become resistant.” | This suggests that the antibiotic deliberately produces the useful mutation. | Resistant variants may already exist. The antibiotic selects against susceptible bacteria. |
| “Only the strongest organisms survive.” | Strength is only one possible characteristic and may be irrelevant. | The better fitted individuals are those whose inherited characteristics increase reproductive success under the existing conditions. |
| “Natural selection changes an individual during its lifetime.” | Evolutionary change is measured across generations. | Individuals are selected; the inherited characteristics of the population change over generations. |
| “Every mutation is useful.” | Many mutations are neutral or harmful. | A mutation is advantageous only if its effect increases reproductive success under particular environmental conditions. |
| “Any characteristic developed during life is inherited.” | Many acquired characteristics do not involve an inherited change in DNA. | Natural selection requires variation that can be passed genetically to offspring. |
| “Survival alone determines evolutionary success.” | An organism that survives but leaves no offspring does not pass its alleles to the next generation. | Natural selection depends on differential survival and reproduction. |
| “The same characteristic is always advantageous.” | Environmental conditions can change. | A characteristic is advantageous only when it improves reproductive success in the current environment. |
What You Should Be Able to Do
Explain that variation refers to differences between individuals of the same species.
Recognise whether a characteristic forms a continuous range or falls into distinct categories.
State that mutation can introduce new alleles and therefore new genetic variation.
Recognise factors such as predation, disease, competition, temperature, antibiotics and insecticides.
Link inherited variation, selection pressure, survival, reproduction and inheritance in the correct sequence.
Understand why passing alleles to offspring is essential to natural selection.
Do not say that organisms mutate or change simply because they need to adapt.
Use the same reasoning for camouflage, disease resistance, antibiotic resistance or changing environmental conditions.
Describe how antibiotics select for bacteria carrying inherited resistance.
Explain how inherited characteristics of a population can change gradually over many generations.
These ten questions go beyond simple recall. Use the ideas of inherited variation, selection pressure, reproductive success and changing populations to work out what happens.
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Frequently asked questions
Frequently Asked Questions About Natural Selection
Natural selection becomes much easier once you separate variation, inheritance and environmental selection. These questions address the points students most often mix up.
What is the difference between variation and mutation?
Variation describes differences between individuals in a population. A mutation is a change in genetic material and is one possible source of inherited variation. In other words, mutation can create a new allele, while variation describes the differences that are present among individuals.
Does natural selection happen to individuals or populations?
Selection acts on individual organisms: some individuals survive and reproduce more successfully than others. However, the evolutionary result is seen in the population, because the frequencies of inherited characteristics change over generations.
Why must the variation be inherited?
For a characteristic to become more common through natural selection, it must be capable of passing from parents to offspring. An environmental change that affects only one individual's body but is not genetically inherited will not automatically appear in that individual's offspring.
Does the environment cause useful mutations?
Not in the purposeful sense often imagined. Mutations arise without regard to whether an organism needs them. An environmental condition can then act as a selection pressure, favouring individuals that happen to possess a useful inherited variant.
Are all mutations harmful?
No. A mutation can be harmful, neutral or advantageous. Its effect may also depend on the environment. An allele that has little effect under one set of conditions might become advantageous if conditions change.
What does “survival of the fittest” actually mean?
It does not simply mean that the physically strongest organisms survive. In evolutionary biology, greater fitness means greater success at surviving and reproducing under a particular set of environmental conditions. Reproductive success is essential because alleles must be passed to the next generation.
What is a selection pressure?
A selection pressure is an environmental factor that affects the survival or reproductive success of organisms. Examples include predators, pathogens, competition for food, changes in temperature, antibiotics and insecticides.
Why does competition occur during natural selection?
Populations may produce more offspring than the environment can support, while resources such as food, water, light and space are limited. Individuals therefore compete, directly or indirectly, for the resources required to survive and reproduce.
How does antibiotic resistance show natural selection?
A bacterial population may already contain resistant variants. When an antibiotic is applied, susceptible bacteria are killed more readily while resistant bacteria survive and reproduce. Their offspring inherit resistance, so resistant bacteria become a larger proportion of the population over successive generations.
Can a characteristic that is advantageous become disadvantageous later?
Yes. Whether a characteristic is advantageous depends on the environment. If environmental conditions change, the selection pressures change as well, so a different characteristic may provide the greater reproductive advantage.
What is the difference between natural selection and evolution?
Natural selection is a process in which inherited variants differ in reproductive success. Evolution describes the change in inherited characteristics of populations over generations. Natural selection is therefore one important mechanism that can produce evolutionary change.
Natural selection is not a story about organisms trying to change. It begins with variation that already exists. The environment then favours some inherited variants over others, causing their alleles to become more common as generations pass. Once you keep that sequence clear — variation, selection, reproduction and inheritance — even unfamiliar natural-selection questions become much easier to reason through.