17.2.117-18 · Selection, Evolution, Classification and Conservation
Natural Selection — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 10 parts Lesson 7 of 36 1 quiz
Lesson summary
17.2.1 Natural Selection — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe natural selection as the differential survival and reproduction of individuals that changes allele frequencies over generations.Every individual has the potential to reproduce and add …
- Environmental factors
- Biotic factors
- Competition for food
- Competition for a mate
- Supply of water
- Temperature
Part 1 of 8Population growth and environmental limits
Population growth and environmental limits
Every individual has the potential to reproduce and add to population growth.
- If all offspring survived and reproduced, the population would grow exponentially.
- This happens only when nothing checks the population, e.g. 24 European rabbits introduced into Australia in the 1800s had abundant resources, no predators and little competition.
In reality environmental factors stop every individual reaching adulthood and reproducing.
- Most wild populations produce far more offspring than survive to adulthood.
Part 2 of 8Environmental factors
Environmental factors
- Environmental factors limit population size by reducing growth rate whenever a population reaches a certain size.
Biotic factors
- Biotic factors involve other living organisms.
- Predation, competition for resources and disease.
Abiotic factors
- Abiotic factors involve the non-living parts of the environment.
- Light availability, water supply and soil pH.
Under these factors not all individuals survive, e.g. when food is limited some animals starve.
Part 3 of 8Limits on African lion populations
Competition for food
- Competition for food prey is limited and other lions and carnivores hunt the same prey, so a lion that cannot hunt starves.
Competition for a mate
- Competition for a mate females often outnumber males, so males fight and the loser may be injured or killed and fails to pass on his alleles.
Supply of water
- Supply of water the dry season is arid and water sources may be miles apart, so lions die of dehydration if a source dries up.
Temperature
- Temperature extreme heat can cause overheating and prevents hunting during the day, reducing food intake.
The combined effect is that fewer individuals survive to adulthood and reproduce, so population growth falls.
Part 4 of 8Natural selection and survival
Variation exists within a population, so individuals have different phenotypes.
- Members of one species have the same genes but different alleles.
Environmental factors affect the chance of survival, so they act as selection pressures.
Fitness
- Fitness the ability to survive and pass on alleles to offspring.
- Individuals with higher fitness have adaptations that suit them to their environment.
Natural selection
- Natural selection the process by which individuals with a fitter phenotype survive and pass on their alleles so advantageous alleles increase in frequency over generations.
Part 5 of 8Sequence of natural selection (Darwin and Wallace)
1
Individuals in a population vary, and much of the variation is inherited.
2
Organisms overproduce offspring, so there is a struggle for limited resources such as food, water, light, mates and space.
3
A selection pressure (predator, disease, climate) acts on the population.
4
Individuals with advantageous alleles and phenotypes are more likely to survive and reproduce (differential survival and reproduction).
5
They pass the advantageous alleles on to offspring.
6
Over many generations the frequency of the advantageous alleles rises and less favourable alleles become rarer.
Part 6 of 8Adaptation and selection pressures
Adaptation
- Adaptation an inherited anatomical, physiological or behavioural feature that increases survival and reproduction in a particular environment.
Variation arises from mutation, together with meiosis and random fertilisation, and the environment determines which phenotypes survive.
- Individuals do not change their alleles in response to the environment;
- allele frequencies in the population change.
Examples of adaptations produced include the waxy cuticle of xerophytes, the camouflage of prey and the long legs of cheetahs.
Fitness is measured by reproductive success, not strength or size.
Part 7 of 8Examples of natural selection
Examples of natural selection
Rabbit fur colour
- Rabbit fur colour brown fur camouflages better than white fur (homozygous recessive).
- (dominant allele)
- Predators such as foxes and stoats see white rabbits more easily.
- Over many generations the brown allele increases and the white allele decreases.
Peppered moth (Biston betularia)
- Peppered moth (Biston betularia) industrial soot blackened trees and killed lichens,
- so dark moths were camouflaged and pale moths were eaten by birds.
- Dark allele frequency rose to over 90% in industrial areas, and fell again after clean-air laws.
- Kettlewell's mark-release-recapture experiments recaptured more dark moths in polluted woods, supporting bird predation as the selection pressure.
Selection also explains the sickle cell allele in malaria regions and pesticide resistance in insects.
Part 8 of 8More from the specification
More from the specification
Principles of natural selection
- Overproduction of offspring leads to competition for limited resources;
- individuals possessing advantageous phenotypic traits have higher reproductive success,
- increasing favoured allele frequencies.
Evolution of antibiotic resistance in bacteria
- Random mutations conferring antibiotic degradation or efflux mechanisms allow resistant bacteria to survive clinical treatments, rapidly dominating populations via binary fission.
Stabilizing, directional, and disruptive selection
- Directional selection favours one phenotypic extreme in changing environments;
- stabilizing selection preserves intermediate phenotypes;
- disruptive selection favours both extremes.
Selective breeding for crop yield
- Selective breeding for crop yield artificial selection in agricultural plants selects individuals displaying desired traits over several generations to maximise crop yield.
Antibiotic resistance by natural selection
- Antibiotic resistance by natural selection bacterial populations undergo natural selection when antibiotic exposure favours mutant individuals carrying protective resistance alleles.
Environmental selection on genetic traits
- Environmental selection on genetic traits varying environmental selection pressures alter population allele frequencies,
- resulting in adaptive evolutionary changes over generations.
Bottleneck effect
- Loss of genetic variation after a sudden fall in population size.
Directional selection
- Selection favouring one extreme phenotype.
Hardy-Weinberg principle
- Equations predicting allele and genotype frequencies in a non-evolving population.
Stabilising selection
- Selection favouring the average phenotype.
Disruptive selection
- Selection favouring both extreme phenotypes.
Antibiotic
- A drug that kills bacteria or stops their growth and does not affect viruses.
Selective breeding
- Humans choosing which individuals breed.
Artificial selection
- Selective breeding of organisms with desirable traits by humans.
Antibiotic resistance
- The ability of bacteria to survive an antibiotic, arising by mutation and spreading by natural selection.
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.
Key terms
| Term | Meaning |
|---|---|
| Selection pressure | An environmental factor that affects the chance of survival and reproduction. |
| Fitness | Ability of an organism to survive and pass on its alleles to offspring. |
| Adaptation | An inherited feature that increases survival and reproduction in a particular environment. |
| Natural selection | Differential survival and reproduction by phenotype, changing allele frequencies. |
| Biotic factor | A factor involving other living organisms, such as predation, competition or disease. |
| Abiotic factor | A non-living environmental factor such as light, water or soil pH. |
Avoid these mistakes
1
Saying organisms try to adapt — adaptation arises from selection of existing variation, not need.
2
Saying the strongest survive — fitness means reproductive success and passing on alleles.
3
Saying individuals evolve — populations evolve as allele frequencies change; individuals do not change their alleles.
4
Forgetting that survival is not guaranteed — less suited organisms can still survive but with a lower chance.
5
Saying mutation is caused by the selection pressure — mutations arise randomly beforehand.
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