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Cambridge IGCSE · Biology 0610

Classification and identification keys

Classify organisms using shared observable features, the biological species concept, binomial names, dichotomous keys, DNA base-sequence evidence and the main features of the five kingdoms, named animal and plant groups, and viruses. This original practice pack is limited to Cambridge IGCSE Biology 0610 sections 1.2 and 1.3. It does not teach detailed taxonomic ranks, mechanisms of evolution, DNA replication, genetics, pathogen transmission or virus replication.

2026 / 2027 / 2028 · Academic review not recorded · Published 2026-09-08

AI-assisted practice — not independently academically reviewed. Answers may contain errors; check important results against your course materials.

Guided tutorial

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17 lesson sections

Lesson contents · 17 sections

What you will learn

  • Explain how shared features are used to place organisms into groups and why modern classification aims to reflect evolutionary relationships.
  • Describe a species as organisms that can reproduce with one another to produce fertile offspring.
  • Write and interpret a two-part binomial name that identifies genus and species.
  • Construct and use dichotomous keys based on clear, identifiable features.
  • Use similarities in DNA base sequences as evidence when classifying organisms and inferring how recently they shared an ancestor.
  • Distinguish animals, plants, fungi, prokaryotes and protoctists using their main features.
  • Classify vertebrates as mammals, birds, reptiles, amphibians or fish and arthropods as myriapods, insects, arachnids or crustaceans.
  • Distinguish ferns from flowering plants and monocotyledons from dicotyledons using observable features.
  • State that a virus consists of genetic material surrounded by a protein coat and distinguish viruses from cellular organisms.

Explanation

Classification organises organisms by shared features

Classification places organisms into groups using features they share. A useful feature is consistent, identifiable and able to separate one group from another. For example, feathers identify birds more reliably than colour, because bird colour varies widely and unrelated organisms can share the same colour. Scientists compare several features rather than relying on one superficial resemblance.

Modern classification systems aim to reflect evolutionary relationships. Organisms placed close together should share a more recent ancestor than organisms placed far apart. Observable anatomy remains useful, but molecular evidence can reveal relationships that appearance alone may hide. Classification is therefore an evidence-based model that can be revised when stronger evidence becomes available.

Worked example

Worked example: choose dependable grouping features

Four animals are described. P has feathers and two legs; Q has hair and four legs; R has feathers and two legs but a different colour from P; S has moist skin and four legs. To make a first division, use a stable biological feature: 'feathers present' separates P and R from Q and S. Colour would incorrectly split P from R even though both share the defining bird feature.

For Q and S, 'hair present' separates Q from S, and 'moist skin present' identifies S as an amphibian candidate. The method is to list observable features, reject vague or variable choices such as 'looks friendly', and select paired features that produce unambiguous groups. The classification is supported by the stated evidence only; extra features would be checked before making a final identification.

Explanation

Species and binomial names identify organisms precisely

A species is a group of organisms that can reproduce with one another to produce fertile offspring. Fertile means that the offspring can themselves reproduce. Similar appearance alone does not establish that two organisms belong to the same species, and members of one species can show variation in size, colour or other features.

The binomial system gives each species an internationally agreed two-part scientific name. The first word is the genus and begins with a capital letter; the second word identifies the species within that genus and begins with a lower-case letter. Both words are written in italics when typed, for example Homo sapiens. If handwritten, the two words are underlined separately.

Worked example

Worked example: decide species membership and format a name

Population A and population B look slightly different. When individuals from A and B reproduce, their offspring are healthy but cannot reproduce. Under the syllabus definition, this evidence does not place A and B in the same species because the offspring are not fertile. Appearance does not override the reproductive evidence provided.

A biologist records the scientific name Panthera leo for one population. Panthera is the genus name and receives the capital letter; leo is the species part and remains lower-case. Writing 'panthera Leo', using only one word, or reversing the words breaks the binomial convention and can make international identification less reliable.

Explanation

A dichotomous key offers two alternatives at every step

A dichotomous key identifies an organism through a sequence of paired statements. At each numbered step, exactly one of two contrasting alternatives should match the specimen. That choice either gives an identification or directs the user to another numbered step. 'Dichotomous' refers to this repeated division into two alternatives.

Good keys use observable, objective features such as number of legs, presence of wings or leaf-vein pattern. Avoid relative descriptions such as 'large' unless a measurable boundary is supplied, and avoid features that are hidden, temporary or dependent on opinion. The alternatives at one step should address the same feature and should not overlap.

Worked example

Worked example: construct a key for four arthropods

A collection contains W with three pairs of legs and one pair of antennae, X with four pairs of legs and no antennae, Y with many body segments, one pair of legs on each segment and one pair of antennae, and Z with more than four pairs of legs and two pairs of antennae. Begin with a clear split: 1a three or four pairs of legs — go to step 2; 1b more than four pairs of legs — go to step 3.

Complete it as follows: 2a three pairs of legs and antennae present — W; 2b four pairs of legs and antennae absent — X. Then use 3a one pair of antennae and many similar leg-bearing segments — Y; 3b two pairs of antennae — Z. Every route uses stated features, every step has two alternatives, and all four specimens finish at one identification.

Worked example

Worked example: follow a leaf key without skipping steps

Use this key: 1a veins parallel — species K; 1b veins form a branching network — go to 2. 2a leaf edge smooth — species L; 2b leaf edge toothed — species M. A leaf has branching veins and a toothed edge. At step 1 it follows 1b, then at step 2 it follows 2b, so the identification is species M.

The leaf must not jump directly to the first statement mentioning a toothed edge, because the route through the key supplies context. Record the route 1b → 2b so the result can be checked. If a specimen matches neither alternative at a step, do not force an answer; recheck the observation or conclude that the key may not include that specimen.

Explanation

DNA evidence can reveal evolutionary relationships

The sequence of bases in DNA can be compared as a means of classification. A comparison must use corresponding DNA regions. If two organisms have very similar base sequences in that region, the evidence supports a closer relationship than it would for organisms with many sequence differences.

Groups that share a more recent ancestor generally have more similar DNA base sequences than groups that share only a distant ancestor. This is evidence for relationship, not a claim that the organisms are identical. Sequence evidence is interpreted alongside other evidence, and the amount and quality of DNA compared affect the strength of the conclusion.

Worked example

Worked example: compare short DNA sequences

The same eight-base DNA region is recorded for three species: A = A C G T T A C G, B = A C G T T A T G, and C = T G A C C T A A. Comparing position by position, A and B differ only at position 7, so they match at 7 of 8 positions. A and C differ at every position in this short example.

For this corresponding region, A and B have the most similar base sequences and therefore the evidence supports them sharing a more recent ancestor with each other than either does with C. The conclusion should remain limited: one short region supports a relationship but is not, by itself, a complete classification of the species.

Explanation

The five kingdoms have contrasting cellular and nutritional features

Animals are multicellular, lack cell walls and chloroplasts, and obtain nutrients by feeding on other organisms. Plants are multicellular, have cellulose cell walls and usually contain chloroplasts for photosynthesis. Fungi have cell walls but no chloroplasts; many consist of hyphae forming a mycelium and obtain nutrients by secreting enzymes outside the body and absorbing soluble products.

Prokaryotes are usually unicellular and lack a nucleus; their DNA lies free in the cytoplasm, often with additional plasmids. Protoctists have cells with nuclei and are mostly microscopic; some are plant-like and contain chloroplasts, while others are animal-like. A single feature may be insufficient, so combine cell structure, number of cells and mode of nutrition.

Worked example

Worked example: place unfamiliar organisms into kingdoms

Organism J is multicellular, has chitin-containing cell walls, lacks chloroplasts and absorbs soluble products after external digestion. Those combined features place J in the fungi. Organism K is unicellular, has a cell wall and circular DNA but no nucleus. The absence of a nucleus is decisive evidence that K is a prokaryote rather than a protoctist.

Organism L is a single cell with a nucleus and chloroplasts. It is placed among the protoctists because it is a nucleated, mostly microscopic organism with plant-like features; having chloroplasts does not automatically make every organism a plant. Each answer names the observed evidence rather than relying on habitat or size alone.

Explanation

Vertebrate groups can be separated by covering, gas exchange and reproduction

Mammals have hair or fur and females produce milk for their young. Birds have feathers and a beak, and lay eggs with hard shells. Reptiles have dry, scaly skin and usually lay eggs with leathery shells. These body-covering features are especially useful because feathers and hair distinguish their groups clearly.

Amphibians have moist, permeable skin and typically lay jelly-coated eggs in water; their life cycle commonly includes an aquatic larval stage. Fish have scales, fins and gills and live in water. When classifying an unfamiliar vertebrate, use a combination of the stated features because one feature, such as egg-laying, is shared by several groups.

Worked example

Worked example: classify three unfamiliar vertebrates

Vertebrate P has dry scales and lays leathery-shelled eggs on land, so it is a reptile. Vertebrate Q has moist skin and lays jelly-coated eggs in water; it is an amphibian. Vertebrate R lives in water, has fins and uses gills throughout life; it is a fish. Habitat alone would not be enough because mammals and reptiles can also live in water.

A fourth vertebrate has wings, but the decisive stated feature is feathers, so it is a bird rather than a mammal such as a bat. The worked method is to identify the most diagnostic feature first, check it against a second feature where possible, and then name the group.

Explanation

Arthropod groups share jointed legs but differ in body plan

Arthropods have a segmented body, jointed legs and an external skeleton. Insects have three main body regions, three pairs of legs and one pair of antennae; many also have wings. Arachnids have two main body regions, four pairs of legs and no antennae.

Crustaceans commonly have more than four pairs of legs and two pairs of antennae. Myriapods have many similar body segments, many legs and one pair of antennae. Count pairs carefully: eight individual legs mean four pairs. Wings are helpful for many insects but cannot define the whole group because some insects are wingless.

Worked example

Worked example: distinguish four arthropod groups

Specimen A has six legs, three body regions and one pair of antennae: it is an insect. Specimen B has eight legs, two body regions and no antennae: it is an arachnid. Specimen C has two pairs of antennae and ten legs: it is a crustacean. Specimen D has many similar segments, each bearing legs, and one pair of antennae: it is a myriapod.

All four possess jointed legs and an external skeleton, so those shared features support placing them within the arthropods but do not separate the four groups. The group-level answer depends on leg number, antennae and body segmentation. Descriptions such as 'small' or 'lives under stones' are much less dependable.

Explanation

Plant groups differ in reproduction, leaves and seed structure

Ferns have roots, stems and leaves called fronds, but they do not produce flowers or seeds; they reproduce using spores. Flowering plants produce flowers and seeds. These features separate ferns from flowering plants more reliably than green colour, because both groups may be green and photosynthetic.

Among flowering plants, monocotyledons have one cotyledon in the seed, leaves with parallel veins, fibrous roots and flower parts commonly in multiples of three. Dicotyledons have two cotyledons, branching net-like leaf veins, a main taproot and flower parts commonly in multiples of four or five. Use several features if one is unavailable or unclear.

Explanation

Viruses have genetic material inside a protein coat

A virus consists of genetic material surrounded by a protein coat. It is not made of cells and does not have the cellular structures used to place organisms into the five kingdoms. In this topic, recognising those two stated components and the non-cellular nature of viruses is the required classification focus.

Do not identify a virus merely because something is microscopic; prokaryotes and many protoctists are also microscopic but are cellular. Likewise, a protein coat is not a cell wall. Detailed virus replication, named viral diseases and transmission routes belong outside this pack's mapped sections.

Original practice, not an official examination paper. Readnary is not affiliated with the awarding body. Prepared with AI assistance.