18.3 Selection
- Syllabus
- 0610–2026–2027
- Topic
- 18.3
- Level
- —
Learning objectives
18.3.1Natural selection with reference• Describe natural selection with reference to: (a) genetic variation within populations (b) production of many offspring (c) struggle for survival, including competition for resources (d) a greater chance of reproduction by individuals that are better adapted to the environment than others (e) these individuals pass on their alleles to the next generation18.3.2Selective breeding with reference• Describe selective breeding with reference to: (a) selection by humans of individuals with desirable features (b) crossing these individuals to produce the next generation (c) selection of offspring showing the desirable features18.3.3How selective breeding by artificial• Outline how selective breeding by artificial selection is carried out over many generations to improve crop plants and domesticated animals and apply this to given contexts18.3.4Adaptation as the process, resulting• Describe adaptation as the process, resulting from natural selection, by which populations become more suited to their environment over many generations18.3.5Development of strains of antibiotic• Describe the development of strains of antibiotic resistant bacteria as an example of natural selection18.3.6Differences between natural• Outline the differences between natural and artificial selection
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.