18. Variation and selection
- Syllabus
- 0610–2026–2027
- Section
- 18
- Level
- —
Exam analysis
Most tested topics
18.1 Variation
- Syllabus
- 0610–2026–2027
- Topic
- 18.1
- Level
- —
Define variation within a species
Variation means differences between individuals of the same species.
Individuals can differ in measured features such as body mass or in categories such as blood group. These differences may arise from genes, the environment, or both.
Differences between two different species are not variation within a species; the comparison must be among members of the same species.
Recognise continuous variation
Continuous variation produces a range of phenotypes between two extremes, with many possible intermediate values.
Body length and body mass are continuous: measurements can take many values, such as masses throughout a range rather than only a few fixed categories.
When many individuals are measured, adjacent measurement intervals form a continuous distribution. The extremes describe the lowest and highest observed values.
A measurement being rounded into groups does not make the underlying variation discontinuous; intermediate values still exist.
Recognise discontinuous variation
Discontinuous variation produces a limited number of distinct phenotypes with no intermediates between the categories.
| Characteristic | Separate phenotypes |
|---|---|
| ABO blood group | A, B, AB or O |
| pea seed shape | round or wrinkled |
| pea seed colour | distinct colour categories |
Different frequencies do not create intermediates: even if one blood group is much more common, every individual still belongs to one category.
Link variation type to genes and environment
| Variation type | Usual cause | Why |
|---|---|---|
| discontinuous | genes only | allele combinations place individuals into distinct categories |
| continuous | genes and environment | many genetic and environmental influences contribute to a measured range |
ABO blood group is genetically determined and discontinuous. Body mass varies continuously because inherited features and environmental factors such as nutrition can both affect it.
These are usual patterns: do not claim that the environment changes an individual's ABO blood-group category or that continuous variation is environmental only.
Investigate and describe variation
- Choose one clearly defined feature and a representative sample of the same species.
- Measure continuous features consistently, or record discontinuous categories using an unambiguous key.
- Tally every individual and present frequencies.
- Describe the pattern using the range and distribution for continuous data, or the number and frequency of categories for discontinuous data.
Use adjacent measurement intervals for a continuous distribution; use separate bars for discontinuous categories. State units for measurements and keep sampling and measurement methods consistent.
A graph alone does not establish cause. It describes the observed variation; genetic and environmental explanations require additional evidence or controlled comparison.
Define mutation as genetic change
A mutation is a genetic change.
Because genetic information is carried in DNA, a mutation changes genetic material and can create a difference that was not present in the original DNA.
A change in appearance caused only by the environment is not automatically a mutation; the change must be genetic.
Explain how mutation forms new alleles
Mutation is the way new alleles are formed. When the genetic information in a gene changes, the changed version is a new allele of that gene.
mutation in a gene → changed genetic information → new allele → possible new inherited variation
Selection does not create the new allele needed by an organism. Mutation creates alleles; selection can later change their frequencies in a population.
Identify factors that increase mutation rate
Ionising radiation and some chemicals increase the rate of mutation.
They increase the probability that genetic material will change, so mutations occur more frequently in an exposed group than they otherwise would.
They increase mutation rate but do not direct a specific useful mutation, and not every exposed cell necessarily mutates.
Define a gene mutation precisely
A gene mutation is a random change in the base sequence of DNA.
Changing the order of bases can create a new allele. If that gene codes for a protein, the changed base sequence may alter the amino-acid sequence and therefore the protein's shape or function.
Random means the change does not occur because the organism needs it. A gene mutation changes the DNA base sequence, not an amino-acid sequence inside DNA.
Identify four sources of genetic variation
| Source | Contribution to genetic variation |
|---|---|
| mutation | creates new alleles |
| meiosis | produces genetically different gametes |
| random mating | combines alleles from different parents |
| random fertilisation | one of many possible sperm and eggs fuse |
Together these processes create and reshuffle allele combinations, so individuals in a population can have different genotypes.
Mitosis and asexual reproduction normally produce genetically identical cells or offspring, so they do not reshuffle alleles in the same way.
18.2 Adaptive features
- Syllabus
- 0610–2026–2027
- Topic
- 18.2
- Level
- —
Define an adaptive feature by fitness
An adaptive feature is an inherited feature that helps an organism survive and reproduce in its environment.
A feature increases fitness only if it improves the probability of survival and reproduction under the conditions of that environment. For example, camouflage can reduce detection by predators or prey, increasing the chance of surviving to reproduce.
A useful change acquired during an individual's lifetime is not automatically an adaptive feature: the feature must be inherited and linked to survival and reproduction in the environment.
Explain adaptive features from evidence
Interpret an image or description in three linked moves: 1. state the feature you can actually observe; 2. explain the physical or biological function it could provide; 3. connect that function to survival and reproduction in the stated environment.
Observed feature: a cheetah has a streamlined body. Function: reduced air resistance helps it run quickly. Environmental advantage: catching prey improves its chance of surviving and reproducing.
Do not name a hidden feature that the image does not show, and do not stop at appearance. The explanation must connect evidence to a functional advantage in the given habitat.
Explain hydrophyte and xerophyte adaptations
Hydrophytes live in water, where support and water supply are abundant but gas exchange may be limited by submergence. Xerophytes live where water is scarce, so they must obtain water and reduce loss by transpiration.
| Plant and feature | Mechanism and environmental advantage |
|---|---|
| hydrophyte: large air spaces | lower density for buoyancy and provide internal routes for gas diffusion |
| floating hydrophyte: stomata on upper surface | exposes stomata to air for gas exchange |
| hydrophyte: thin or absent cuticle | little need to prevent water loss; allows easier exchange |
| submerged hydrophyte: narrow or divided leaves | reduces resistance to water flow |
| hydrophyte: reduced roots, xylem and support tissue | water surrounds the plant and provides support |
| xerophyte: thick waxy cuticle | reduces evaporation from the leaf surface |
| xerophyte: small, rolled or spine-like leaves | reduces exposed surface area and traps humid air |
| xerophyte: hairs and sunken or fewer stomata | traps still humid air and reduces the water-vapour gradient or diffusion |
| xerophyte: deep or widespread roots | reaches deep water or rapidly absorbs water over a wide area after rain |
| xerophyte: succulent stem or leaves | stores water for dry periods |
State the mechanism for the observed plant: not every hydrophyte floats and not every xerophyte has every listed feature. A feature is adaptive only when its effect matches the habitat.
18.3 Selection
- Syllabus
- 0610–2026–2027
- Topic
- 18.3
- Level
- —
Trace natural selection as a causal chain
A population contains genetic variation, so individuals carry different alleles and may show different inherited phenotypes. Organisms produce more offspring than the environment can support, creating competition for limited resources.
- A selection pressure acts in the environment. 2. Individuals with an inherited feature that suits that environment are more likely to survive. 3. They are therefore more likely to reproduce. 4. They pass the advantageous allele to offspring. 5. Over generations, that allele and phenotype become more common in the population.
If darker tree trunks make dark-winged insects harder for predators to see, dark insects have a survival and reproductive advantage. Their allele is inherited by more offspring, so dark wings become more frequent over generations.
The environment does not create a needed allele or make an individual change deliberately. Heritable variation exists first; selection changes allele frequencies across generations.
Run the selective-breeding cycle
Selective breeding is artificial selection in which humans choose which organisms reproduce because they show desirable inherited features.
- Identify the desired feature. 2. Select parents that show it strongly. 3. Cross those parents. 4. Examine the offspring and select those showing the desired feature. 5. Cross the selected offspring and repeat the cycle.
To breed wheat that flowers with fewer hours of daylight, select plants that flower under the shortest day length, cross them, then select and cross the offspring that flower under similarly short days.
Choosing one useful parent is not enough. Selective breeding requires crossing selected individuals and selecting suitable offspring for further breeding.
Improve crops and animals over many generations
Artificial selection repeats the select–cross–select cycle for many generations. Each generation, humans keep breeders with the strongest desired inherited feature, so alleles contributing to that feature become more common.
| Context | Feature selected | How the cycle is applied |
|---|---|---|
| crop plant | high yield or disease resistance | cross plants with the target feature; grow seeds; retain the best offspring as the next parents |
| domesticated animal | high milk yield, fast growth or suitable wool | cross animals showing the target feature; measure offspring; breed from the best performers |
Repeating the cycle can strengthen and stabilise the desired feature because selected offspring become the parents of the next generation. In a given context, name a measurable feature and apply every step to that feature.
Artificial selection does not improve every feature or guarantee identical offspring in one cross. It shifts inherited characteristics through repeated human selection over many generations.
Explain adaptation across generations
Adaptation is the process, resulting from natural selection, by which a population becomes more suited to its environment over many generations.
When an inherited feature gives a reproductive advantage under an environmental pressure, its allele is passed to a greater proportion of the next generation. Repeated selection makes the advantageous allele and phenotype more common, so the population becomes better suited to that environment.
The change is measured in the population: later generations contain a higher proportion of individuals with the advantageous inherited feature.
An individual does not genetically adapt during its lifetime. Individuals may acclimatise, but evolutionary adaptation is a population-level change over generations.
Explain how antibiotic-resistant strains develop
Before treatment, random mutation has produced genetic variation: a small number of bacteria may already carry an allele that gives resistance. The antibiotic does not cause a useful mutation because the bacteria need it.
- Antibiotic treatment kills susceptible bacteria. 2. Resistant bacteria survive, so they face less competition. 3. Survivors reproduce rapidly and pass the resistance allele to their offspring. 4. After repeated generations, resistant bacteria form a larger proportion of the population and a resistant strain develops.
An increasing percentage of resistant bacteria over time shows natural selection: the antibiotic is the selection pressure, but the resistant variation existed before it acted.
People do not become resistant to antibiotics, and antibiotics do not train individual bacteria. The bacterial population evolves because resistant variants survive and reproduce.
Distinguish natural and artificial selection
Both processes act on inherited genetic variation, allow only some individuals to contribute more offspring, pass alleles to the next generation and alter populations over generations.
| Feature | Natural selection | Artificial selection |
|---|---|---|
| selector | environmental selection pressure | humans |
| criterion | features that improve survival and reproduction in that environment | features desired by humans |
| mating and reproduction | better-adapted individuals reproduce more successfully | humans choose parents and offspring for crossing |
| usual direction and pace | environment-dependent and often slower | human-directed and often faster |
| outcome | population becomes better suited to its environment | crop or domesticated-animal population shows more of the desired feature |
Ask who or what determines reproductive success. Environmental survival and competition indicate natural selection; deliberate human choice of breeders indicates artificial selection.
Human involvement in observing or treating organisms does not automatically make selection artificial. Artificial selection requires humans to choose which individuals reproduce.