Niche, adaptation and natural selectionEdexcel A-Level Biology A: Revision notes
Section 1
The concept of niche
The niche of a species is its role in the ecosystem. It includes:
- its habitat, where it lives
- its feeding relationships, what it eats and what eats it
- its interactions with other species, such as competition and mutualism
- the abiotic conditions it tolerates, such as temperature, light and water availability
The habitat is only one part of the niche. Species with similar niches compete strongly, so species living together usually differ in their niche, for example by feeding on different foods or in different places.
Do not write that the niche is where an organism lives. That is the habitat. The niche is the whole role of the organism in the ecosystem.
Section 2
Adaptation to the environment
An adaptation is an inherited feature that increases an organism's chance of survival and reproduction in its environment. There are three types:
- Anatomical (structural): a physical feature of the body. Examples: the rolled leaves and hair-lined stomatal pits of marram grass, which trap humid air and reduce transpiration; the thick layer of fat and dense feathers of penguins; the deep or narrow beaks of finches.
- Physiological: a process inside the body. Examples: antifreeze proteins in the blood of Antarctic fish; a lowered metabolic rate during fasting; the production of venom.
- Behavioural: the way an organism acts. Examples: penguins huddling together; basking by reptiles to warm up; migration; an animal playing dead.
Most adaptations help an organism to fit its niche.
To classify an adaptation, ask: is it a body structure (anatomical), a body process (physiological) or an action (behavioural)?
Section 3
Variation and selection pressure
Natural selection depends on variation between individuals of a species. Variation is caused by differences in alleles, which arise by mutation, and by the mixing of alleles in sexual reproduction. It may also be affected by the environment.
Organisms produce more offspring than the environment can support, so individuals face a selection pressure such as predation, disease, competition for food or a change in climate. A selection pressure is a factor that affects the survival and reproduction of individuals.
Section 4
Natural selection
Natural selection works in steps:
- Individuals in a population show variation in their phenotypes, due to different alleles.
- More offspring are produced than can survive, so there is a struggle for survival.
- Individuals with phenotypes that suit the environment better are more likely to survive and reproduce; they have a selective advantage.
- They pass on the advantageous alleles to their offspring.
- Over many generations the frequency of the advantageous alleles increases in the population.
Natural selection acts on the phenotypes of individuals, but it is the population whose allele frequencies change.
Individuals do not adapt during their lifetime because they need to. Mutations arise randomly, and the environment selects among the variation already there.
Section 5
From selection to adaptation and evolution
Repeated natural selection over many generations makes a population better adapted to its environment, because advantageous alleles become more common.
Evolution is the change in the allele frequencies of a population over generations. Natural selection is one cause of evolution.
Examples:
- Warfarin resistance in rats. A rare resistance allele gave a selective advantage once warfarin was used, so it became common.
- Antibiotic resistance in bacteria. Antibiotics kill most bacteria, but any with a resistance allele survive and divide, so the allele frequency rises.
- Finch beaks. After a drought, finches with deeper beaks survived to breed, and the mean beak depth of the next generation increased.
Section 6
Writing a natural selection answer
For a six-mark answer, tell the story in order:
- variation exists, due to different alleles, which arose by mutation
- the selection pressure is named
- individuals with the advantageous allele survive and breed
- the allele is passed on to offspring
- the allele frequency increases in the next generation
Use the terms allele, selection pressure and allele frequency rather than words such as 'gene' or 'trait'.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Niche, adaptation and natural selection
- Marram grass grows on exposed sand dunes, where water drains away quickly and strong winds dry the air. Its leaves are rolled lengthways, with the stomata on the inner surface in pits lined with hairs. It also has long roots that reach deep into the sand.Explain how the rolled leaves and stomata in hair-lined pits help marram grass to survive on the dunes.2 marks
- Male emperor penguins incubate their eggs through the Antarctic winter. The males huddle tightly together and take turns to stand at the cold outer edge of the huddle. Each penguin has densely packed, overlapping feathers and a thick layer of fat under the skin. During incubation the males fast and lower their metabolic rate.Explain how huddling increases the chance of a male penguin surviving the winter and breeding successfully.2 marks
- In the 1950s warfarin, a poison that causes fatal internal bleeding, was used to kill brown rats on farms in Britain. Within a few years warfarin-resistant rats were found in some areas. Resistance is caused by an allele of one gene that arose by mutation and is inherited. Before warfarin was used, the allele was very rare in the rat population.Explain how the frequency of the resistance allele in the rat population increased.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).