Variation, Classification and Adaptation
Variation is the differences between individuals of the same species. Continuous variation gives a range of values with no gaps, such as height or mass, and is usually caused by many genes acting together with the environment. Discontinuous variation puts individuals into separate categories with nothing in between, such as blood group or having attached or free earlobes, and is controlled by genes alone. Classification is the system biologists use to sort living things into groups, from the five kingdoms down through phylum, class, order, family, genus and species. A species is a group whose members can breed together to produce fertile offspring, and every species has a two-word Latin binomial name. Adaptation is a feature that makes an organism better suited to its habitat. Natural selection links the three ideas: individuals vary, those whose variations suit the environment survive and breed more successfully, and over many generations the useful features become common in the population.
Method
- Decide first whether the data you are given is continuous or discontinuous. If the values could take any number in a range it is continuous; if they fall into named groups it is discontinuous.
- Choose the matching graph. Continuous data goes on a histogram or line graph; discontinuous data goes on a bar chart with gaps between the bars.
- For classification questions, work from the largest group inwards: kingdom, then phylum, then class, and so on. The further down two organisms share, the more closely related they are.
- Use a key by answering each either-or question about the organism in front of you and following the branch it sends you down, until you reach a name.
- When asked to explain an adaptation, name the feature, say what it does, and then say how that helps the organism survive or reproduce in its particular habitat. All three parts are needed for full marks.
- For natural selection, always tell the story in the same order: there is variation, there is a change or a pressure, the better suited individuals survive and breed, they pass on the useful gene, and over generations the population changes.
Worked example
Arctic foxes have thick white fur, small ears and a compact body. Explain how these features are adaptations to their habitat, and explain how natural selection produced the white coat.
- Take each feature in turn and name what it does. Thick fur traps a layer of air, which is a poor conductor, so it reduces heat loss.
- Small ears and a compact body reduce the surface area compared with the volume of the animal, so less heat is lost from the skin.
- White fur matches the snow, which is camouflage. Say what camouflage buys the fox: it can approach prey without being seen, and it is harder for predators to spot.
- Now the natural selection story. Start with variation: in the original fox population, coat colour varied from dark to pale.
- Add the selection pressure: in a snowy habitat, paler foxes were harder to see, so they caught more prey and survived to breed.
- Finish with inheritance and time: those foxes passed the alleles for pale fur to their cubs, so each generation had a higher proportion of pale foxes, until white fur became the normal coat colour.
Practice questions
Try each question, then tap to reveal the answer.
Q1Give one example of continuous variation and one example of discontinuous variation in humans.Show answer
Answer: Continuous: height, mass or hand span (any value in a range). Discontinuous: blood group, or whether earlobes are attached or free (separate categories).
Q2Why is a bar chart with gaps the right graph for blood group data but the wrong graph for height data?Show answer
Answer: Blood groups are separate categories with nothing in between, so gaps between bars are correct. Height is continuous, so the values run into each other and a histogram or line graph should be used instead.
Q3Define the term species.Show answer
Answer: A group of organisms whose members can breed with each other to produce fertile offspring.
Q4Two organisms are in the same family but different genera. Two others are in the same order but different families. Which pair is more closely related, and why?Show answer
Answer: The first pair. Family is a smaller, more specific group than order, so sharing a family means sharing more features and a more recent common ancestor.
Q5A cactus has a thick waxy stem, spines instead of leaves and roots that spread widely just below the surface. Explain how each feature helps it survive in a desert.Show answer
Answer: The thick waxy stem stores water and the wax reduces evaporation. Spines have a tiny surface area so very little water is lost, and they also deter animals from eating the plant. Wide shallow roots absorb as much water as possible from the brief rainfall before it drains away.
Q6Bacteria in a hospital become resistant to an antibiotic. Put these stages in order: the resistant bacteria reproduce; the antibiotic is used; some bacteria have a resistance gene by chance; most non-resistant bacteria die; nearly all the bacteria in the population are resistant.Show answer
Answer: Some bacteria have a resistance gene by chance; the antibiotic is used; most non-resistant bacteria die; the resistant bacteria reproduce; nearly all the bacteria in the population are resistant.
Q7Why does using a key give a more reliable identification than comparing an organism with photographs?Show answer
Answer: A key asks a fixed sequence of either-or questions about specific features, so two people following it should reach the same answer. Comparing with photographs relies on a general impression, and similar-looking species can easily be confused.
Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
A student measures the hand span of everyone in her class and records the mass of each pupil's school bag. She also records each pupil's eye colour. (a) State which of these three sets of data shows discontinuous variation, and give a reason. (b) Describe how she should display the hand span data.
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Peppered moths rest on tree trunks. Before the industrial revolution almost all were pale and speckled; the trunks were covered in pale lichen. During the industrial revolution soot killed the lichen and blackened the trunks, and dark moths became far more common in industrial areas. Explain, using natural selection, why the dark form became common.
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Free printable worksheet
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