Populations, adaptation and variationIB MYP Sciences: Revision notes
Section 1
Competition
Organisms in a habitat compete for limited resources.
- Animals compete for food, water, shelter and mates.
- Plants compete for light, water, space and minerals.
Competition can be between members of the same species (usually the strongest) or between different species. When a resource is scarce, only some organisms get enough to survive and reproduce, so the population stops growing.
Section 2
Adaptations to a habitat
An adaptation is a feature that helps an organism survive and reproduce in its habitat. Adaptations can be structural (body features), behavioural or functional.
Fennec fox (desert): large ears give a large surface area to lose heat; pale fur gives camouflage; it is active at night to avoid daytime heat.
Polar bear (cold): thick fat and fur for insulation and white fur for camouflage.
Adaptations are inherited. They are not learned or gained during a lifetime.
Organisms do not change themselves to fit their habitat. Helpful features are already present in some individuals and are inherited.
Section 3
Variation
Variation means differences between individuals of the same species. It is caused by genes, the environment, or both.
- Continuous variation: a feature that can take any value within a range, with most individuals in the middle, such as height, mass or beak depth.
- Discontinuous variation: a feature with a few clear groups and no values in between, such as blood group or whether you can roll your tongue.
Continuous features are usually shown on a histogram or line graph; discontinuous features on a bar chart.
Section 4
Natural selection
Natural selection explains how species change over many generations.
- Individuals in a population vary.
- They compete for limited resources, and more offspring are born than can survive.
- Individuals with features that suit the environment are more likely to survive.
- They reproduce and inherit the helpful features (genes) to their young.
- Over many generations the helpful feature becomes more common.
Example: in a drought, finches with deeper beaks crack hard seeds, survive and breed, so the next generation has deeper beaks on average.
Use the sequence: variation, competition, survival, reproduction and inheritance.
Section 5
Extinction and biodiversity
A species becomes extinct when no living members remain. Causes include:
- Hunting or overfishing
- Habitat loss, such as deforestation
- New predators, diseases or competitors
- Environmental change faster than natural selection can keep up with
Biodiversity is the variety of living organisms in a habitat or on Earth. High biodiversity makes ecosystems more stable, and extinction reduces it.
Section 6
Sampling: quadrats and transects
Scientists cannot count every organism, so they sample a habitat.
- A quadrat is a square frame (often 1 m by 1 m) placed at random positions. Count the organisms inside and find the mean per quadrat.
- A transect is a line across a habitat, with quadrats placed at regular intervals. It shows how the numbers change, for example from the shade into the open.
Population estimate = mean number per m² × total area (m²)
Worked example: the mean is 5 daisies per m² in a field of 3200 m², so the estimate is 5 × 3200 = 16 000.
To make the estimate more reliable, use more quadrats and place them at random.
Must Know
- Organisms compete for resources such as food, water, light and space
- Adaptations are inherited features that help survival
- Continuous variation has a range; discontinuous has clear groups
- Natural selection: variation, competition, survival, inheritance
- Extinction happens when no members of a species remain
- Population estimate = mean per m² × area
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Populations, adaptation and variation
- The fennec fox lives in the Sahara Desert. It has very large ears, pale sandy fur and hairy soles on its paws. It hunts at night and shelters in a burrow during the day. Water is scarce and daytime temperatures are very high.Suggest how the fox's habit of hunting at night and sheltering in a burrow by day helps it to survive in the desert.2 marks
- On a small Galápagos island, ground finches feed on seeds. Scientists measured the beak depth of many birds and found values between 7 mm and 12 mm, with most birds near 9 mm. During a severe drought the small, soft seeds ran out and only large, hard seeds were left. Birds with deeper beaks could crack these seeds. Many birds with shallow beaks starved. When the survivors bred, the chicks had a greater average beak depth than the previous generation.Explain why the average beak depth of the finches increased in the next generation.2 marks
- Students want to estimate the number of daisy plants in a school field that measures 80 m by 40 m. They place ten quadrats, each 1 m by 1 m, at different positions in the field and count the daisies inside each one. The counts are 4, 7, 3, 6, 5, 8, 2, 5, 6 and 4.Calculate the mean number of daisies per quadrat and use it to estimate the total number of daisies in the field.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).