What you'll learn
- How to describe habitats, populations, communities and ecological niches.
- How abiotic and biotic factors affect where organisms live.
- How adaptations increase survival and reproductive success.
- How natural selection changes allele frequencies over generations.
Starting point: organisms in ecosystems
Before you tackle niches and selection, you need a few ecology terms.
A species is a group of organisms that can usually interbreed to produce fertile offspring. A population is all the organisms of one species living in the same area at the same time. A community is all the populations of different species living and interacting in an area. An ecosystem is the community plus the non-living environment it interacts with.
A habitat is the place where an organism lives, such as a pond, woodland, salt marsh or rocky shore.
Ecological niche
An ecological niche is the role of a species in its ecosystem, including the resources it uses, the conditions it tolerates, and how it interacts with other organisms.
A niche is more than an “address”. The habitat tells you where an organism lives; the niche tells you how it lives there.
The diagram shows how a niche depends on both physical conditions and interactions with other organisms.
Abiotic and biotic factors
An abiotic factor is a non-living environmental factor, such as temperature, light intensity, water availability, oxygen concentration, pH or mineral ion concentration.
A biotic factor is a living factor, or a factor caused by living organisms, such as food availability, predators, competitors, parasites or pathogens.
Together, these factors affect whether a species can survive, reproduce and maintain a population in a particular habitat.
Habitat versus niche
A habitat is where an organism lives. A niche is its way of life: what it needs, what it does, and how it fits into the ecosystem.
Comparing two niches
Two bird species live in the same woodland. Species A feeds on insects under tree bark during the day. Species B feeds on flying insects above the canopy at dusk.
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Both species have the same broad habitat because they live in the same woodland.
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Their feeding locations are different: Species A uses tree bark, while Species B uses open air above the canopy.
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Their feeding times are different: Species A feeds during the day, while Species B feeds at dusk.
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Their niches overlap only slightly, so direct competition for food is likely to be lower than if both species fed in the same place at the same time.
Competition and niche overlap
Competition occurs when organisms need the same limited resource. Intraspecific competition is competition between members of the same species. Interspecific competition is competition between members of different species.
If two species have very similar niches, they may compete strongly. Over time, one species may outcompete the other, or the species may use the habitat differently. This reduction in niche overlap is called niche partitioning.
Same habitat does not mean same niche
Do not assume that two organisms have the same niche just because they live in the same place. To compare niches, look at food source, timing, microhabitat, predators, competitors and abiotic tolerances.
Adaptation: features that fit the niche
Adaptation
An adaptation is a feature that increases an organism’s chance of survival and reproduction in its environment.
Adaptations are often grouped into three types.
Anatomical adaptations
An anatomical adaptation is a structural feature of the body. Examples include thick fur in Arctic mammals, webbed feet in aquatic birds, thorns on plants, or a large surface area of root hairs for water uptake.
Physiological adaptations
A physiological adaptation is a feature involving internal processes or biochemistry. Examples include venom production, salt excretion in marine birds, enzyme activity at high temperatures, or the ability of some bacteria to break down unusual food sources.
Behavioural adaptations
A behavioural adaptation is something an organism does. Examples include migration, hibernation, courtship displays, nocturnal feeding or basking in the sun to warm up.
Linking an adaptation to a selection pressure
Marram grass lives on sand dunes, where water is often difficult to obtain and wind increases water loss. Its leaves can roll up, and its stomata are sunken in grooves.
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The main selection pressure is water loss from the leaves, especially in windy, dry conditions.
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Rolling the leaf traps moist air inside the leaf, reducing the water vapour concentration gradient between the leaf air spaces and the outside air.
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Sunken stomata reduce air movement around the stomata, so water vapour diffuses away more slowly.
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Plants with these features are more likely to survive and reproduce on dunes, so these features are adaptive in that niche.
Organisms do not adapt by trying
Avoid saying “the organism changed because it needed to”. Adaptations arise because individuals with inherited features that already help them survive are more likely to reproduce.
Variation: the raw material for selection
Variation means differences between individuals. In a population, variation may be:
- genetic, caused by different alleles
- environmental, caused by conditions such as diet, temperature or light
- caused by both genes and environment
An allele is a version of a gene. A gene is a length of DNA that codes for a polypeptide or functional RNA. A mutation is a change in the DNA base sequence, which can create new alleles.
For natural selection to cause evolution, the useful variation must be heritable, meaning it can be passed from parents to offspring.
Only heritable variation drives evolution
Natural selection can only increase the frequency of features that are passed on genetically. Acquired features, such as stronger muscles from training, are not inherited as alleles.
Natural selection
Natural selection
Natural selection is the process where individuals with advantageous inherited characteristics are more likely to survive, reproduce and pass on their alleles, causing allele frequencies in a population to change over generations.
A selection pressure is an environmental factor that affects survival or reproductive success. It may be abiotic, such as low temperature or drought, or biotic, such as predation, disease or competition.
The diagram summarises the logic of natural selection from variation to evolution.
The sequence you need to explain
Natural selection usually follows this chain:
- Individuals in a population show genetic variation.
- A selection pressure acts on the population.
- Some individuals have alleles that give them an advantage.
- These individuals are more likely to survive and reproduce.
- They pass the advantageous alleles to their offspring.
- The frequency of those alleles increases over many generations.
- The population becomes better adapted to its environment.
Evolution is the change in allele frequencies in a population over time.
Explaining pesticide resistance
A farmer uses a pesticide on a population of insects. A few insects already carry an allele that gives resistance.
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There is genetic variation in the insect population: some insects are resistant, while others are not.
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The pesticide is the selection pressure because it kills insects without resistance more effectively.
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Resistant insects are more likely to survive and reproduce after spraying.
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Resistant survivors pass the resistance allele to their offspring.
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Over generations, the resistance allele becomes more common, so the population evolves increased pesticide resistance.
Individuals are selected; populations evolve
Selection acts on individuals because individuals survive or die. Evolution happens to populations because allele frequencies change across generations.
Tracking allele frequency
An allele frequency is the proportion of all alleles at a gene locus that are a particular allele.
For a diploid organism, each individual has two alleles at each gene locus. So in a population of 100 individuals, there are 200 allele copies for that locus.
p=number of copies of a particular alleletotal number of alleles at that gene locusp = \frac{\text{number of copies of a particular allele}}{\text{total number of alleles at that gene locus}}p=total number of alleles at that gene locusnumber of copies of a particular alleleCalculating allele frequency change
In a beetle population of 100 individuals, body colour is affected by two alleles: B and b.
Before a drought: 30 beetles are BB, 50 are Bb, and 20 are bb.
After several generations: 55 beetles are BB, 35 are Bb, and 10 are bb.
Calculate the frequency of allele B before and after the drought.
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Work out the total number of alleles in each population. The beetles are diploid, so 100 individuals have 2×100=2002 \times 100 = 2002×100=200 alleles at this gene locus.
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Count B alleles before the drought. BB beetles each have two B alleles, and Bb beetles each have one: 2×30+50=1102 \times 30 + 50 = 1102×30+50=110.
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Calculate the frequency before the drought: p=110200=0.55p = \frac{110}{200} = 0.55p=200110=0.55.
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Count B alleles after the drought: 2×55+35=1452 \times 55 + 35 = 1452×55+35=145.
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Calculate the frequency after the drought: p=145200=0.725p = \frac{145}{200} = 0.725p=200145=0.725.
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Compare the frequencies. The B allele increased from 0.55 to 0.725, suggesting that conditions during the drought favoured beetles carrying B.
Patterns of selection
Natural selection can affect phenotypes in different ways.
Directional selection favours one extreme phenotype. For example, pesticide resistance may become more common after pesticide use.
Stabilising selection favours intermediate phenotypes and selects against extremes. For example, very low and very high human birth masses are associated with higher risk, so intermediate birth masses tend to have higher survival.
Disruptive selection favours both extremes and selects against intermediates. This can happen when different phenotypes are suited to different niches within the same environment.
Recognising the selection pattern
If the average shifts, think directional. If the range narrows around the middle, think stabilising. If both extremes are favoured, think disruptive.
Studying niches in the field
You can investigate niches by measuring where organisms are found and which abiotic factors are associated with their distribution.
A quadrat is a square frame used to sample organisms in a known area. A transect is a line or belt used to sample across an environmental gradient, such as from shaded woodland to open grassland.
Useful practical measurements include:
- abundance of a species, such as percentage cover or number of individuals per square metre
- light intensity
- soil moisture
- temperature
- pH
- wind exposure
Choosing a sampling method
You want to investigate whether a small plant becomes less abundant as light intensity decreases from the edge of a woodland to the centre.
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The independent variable is distance into the woodland, which is linked to changing light intensity.
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Because there is a clear environmental gradient, a transect is more suitable than purely random quadrats.
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Place quadrats at regular intervals along the transect and record plant abundance in each quadrat.
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Measure light intensity at each quadrat so you can test whether plant abundance changes with the abiotic factor, not just with distance.
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Repeat the transect or use parallel transects to improve reliability and reduce the effect of chance variation.
In the exam
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When explaining natural selection, include variation, selection pressure, differential survival and reproduction, inheritance, and changing allele frequency.
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Link every adaptation to a specific advantage in a specific environment; do not just name the feature.
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Use “population evolves” rather than “individual evolves”, and avoid implying that organisms change because they need to.
Check yourself
- What is the difference between a habitat and a niche?
- Why must variation be heritable for natural selection to cause evolution?
- How could you investigate whether soil moisture affects the distribution of a plant species?
