17.2.217-18 · Selection, Evolution, Classification and Conservation
Natural Selection- Types of Selection — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 9 parts Lesson 8 of 36 Unit quiz
Lesson summary
17.2.2 Natural Selection: Types of Selection — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Compare stabilising, directional and disruptive selection and their effects on allele frequencies.Selection pressures — environmental factors that affect the chance of survival; e.g.
- Stabilising selection
- Effect on the curve
- Human birth weight
- Other examples
- Directional selection
- Polymorphism
Part 1 of 7Selection pressures and the three types
Selection pressures and the three types
Selection pressures
- Selection pressures environmental factors that affect the chance of survival;
- e.g. little prey selects for faster, more powerful lions.
Selection acts on phenotypes, but the frequency of alleles changes.
- For continuous characteristics such as height, birth mass or beak size, the population shows a normal distribution.
- Selection can change the position or the shape of this distribution.
There are three types of selection:
- Stabilising.
- Directional.
- Disruptive.
Part 2 of 7Stabilising selection
Stabilising selection
- Stabilising selection natural selection that keeps allele frequencies relatively constant over generations.
- It occurs when the environment is stable and favours the intermediate phenotype.
- Allele frequencies stay constant unless the environment changes.
Effect on the curve
- Effect on the curve the mean stays the same but the range narrows, so the peak is taller and narrower.
Human birth weight
- Human birth weight very low and very high weights are selected against.
- Low weight increases the risk of health complications for the baby.
- High weight increases the risk of birth complications.
Other examples
- Other examples clutch size in birds and length of gestation.
Part 3 of 7Directional selection
Directional selection
- Directional selection natural selection producing a gradual change in allele frequencies over several generations.
- It usually follows a change in the environment or a new selection pressure that makes an allele advantageous.
Effect on the curve
- Effect on the curve the mean shifts towards one extreme.
Fish body size in warming seas:
- Warmer seas speed up metabolism and raise oxygen need, while oxygen levels are lower.
- Larger fish have greater metabolic needs, so they are affected more.
- Smaller fish are fitter, reproduce more and pass on alleles for small body size.
- Over generations the alleles for small size increase and those for large size decrease.
Other examples
- Other examples peppered moths, antibiotic resistance and pesticide resistance.
Part 4 of 7Disruptive selection
Disruptive selection
- Disruptive selection natural selection that maintains high frequencies of two different sets of alleles by acting against intermediates.
- It can occur in an environment with variation, e.g. two habitats or two food types.
Effect on the curve
- Effect on the curve a bimodal distribution with two peaks.
It maintains polymorphism, the continued existence of two or more distinct phenotypes in a species.
Beak size in Galapagos birds:
- Different seed sizes are foraged more efficiently by short or long beaks than by medium beaks.
- Beaks are small or large and the medium beak is selected against.
If the two forms stop interbreeding it can lead to sympatric speciation.
Part 5 of 7Comparing the three types
| Type | Favours | Environment | Effect on the curve |
|---|---|---|---|
| Stabilising | Mean (intermediate) | Stable | Narrower peak, same mean |
| Directional | One extreme | Changing in one direction | Mean shifts |
| Disruptive | Both extremes | Varied or diverse | Two peaks |
Part 6 of 7Interpreting distribution graphs
1
Compare the position of the peak (the mean), the width of the curve and the number of peaks before and after selection.
- A narrower peak with the same mean shows stabilising selection.
- A peak that has moved shows directional selection.
- Two peaks show disruptive selection.
2
Stabilising selection reduces variation, directional selection shifts the mean, and disruptive selection increases variation.
3
Sexual selection, e.g. peacock tails driven by mate choice, is a form of directional selection.
4
Selection works only on variation with a genetic basis; changes caused only by the environment are not inherited.
Part 7 of 7More from the specification
More from the specification
Stabilising, Directional, and Disruptive Selection
- Natural selection acts on phenotypic variations among individuals in a population;
- directional selection favours an extreme phenotype under environmental change,
- while stabilising selection favours intermediate traits.
Hardy-Weinberg and Recessive Phenotypes
- In population genetics, individuals displaying the homozygous recessive phenotype allow determination of the recessive allele frequency q,
- which enables calculation of heterozygous carrier frequencies.
Artificial Selection (Selective Breeding)
- Humans deliberately cross individual animals or crop plants possessing desirable growth traits or yields to produce offspring with enhanced phenotypic characteristics over successive generations.
Stabilising, Directional, and Disruptive Selection
- Natural selection acts on phenotypic variations among individuals in a population;
- directional selection favours an extreme phenotype under environmental change,
- while stabilising selection favours intermediate traits.
Hardy-Weinberg and Recessive Phenotypes
- In population genetics, individuals displaying the homozygous recessive phenotype allow determination of the recessive allele frequency q,
- which enables calculation of heterozygous carrier frequencies.
Artificial Selection (Selective Breeding)
- Humans deliberately cross individual animals or crop plants possessing desirable growth traits or yields to produce offspring with enhanced phenotypic characteristics over successive generations.
Hardy-Weinberg principle
- Equations predicting allele and genotype frequencies in a non-evolving population.
Growth
- A permanent increase in size and dry mass.
Recessive
- Expressed only when no dominant allele is present.
Organism
- A living thing, such as a plant or an animal.
Dominant
- Expressed when present in the genotype.
Artificial selection
- Selective breeding of organisms with desirable traits by humans.
Genetic drift
- Change in allele frequency due to chance, most important in small populations.
Founder effect
- Loss of genetic variation when a few individuals found a new population.
Bottleneck effect
- Loss of genetic variation after a sudden fall in population size.
Key terms
| Term | Meaning |
|---|---|
| Stabilising selection | Selection against both extremes that keeps the intermediate phenotype. |
| Directional selection | Selection favouring one extreme phenotype, shifting the mean. |
| Disruptive selection | Selection favouring both extremes and acting against intermediates. |
| Polymorphism | Continued existence of two or more distinct phenotypes in a species. |
| Selection pressure | An environmental factor that affects the chance of survival. |
| Bimodal distribution | A distribution with two peaks. |
Avoid these mistakes
1
Saying stabilising selection removes all variation — it removes the extremes and keeps the intermediate phenotype.
2
Saying directional selection needs a stable environment — it follows an environmental change.
3
Thinking disruptive selection favours the mean — it acts against intermediates and favours both extremes.
4
Describing graphs without comparing peak position, width and number of peaks.
Last updated: