Ch 2 · Concept and Uses of Classification Systems · O Level Biology 5090
Topic 2 of 4 in Characteristics and Classification of Living Organisms — O Level Biology 5090 — 48 lessons.
Introduction
Around two million species have been named and many more remain undiscovered. To study them, biologists sort organisms into groups and give each species a name that means the same thing in every country. This lesson covers the idea behind classification, the Cambridge definition of a species, the rules of the binomial naming system, and, for candidates, how DNA and protein sequences are now used to show how closely organisms are related.
Key ideas — Classifying by shared features
Organisms are classified into groups by the features they share. Organisms with many features in common are placed in the same small group; organisms sharing only a few general features are placed together only in a large group.
🧑🏫 Teacher's explanation
Imagine you are sorting a massive box of mixed Lego bricks. You might first separate the red ones from the blue ones (Kingdom). Then, within the red bricks, you separate the long beams from the small squares (Phylum, Class). Finally, you group identical red 2x4 beams together (Species). Classification works exactly like this: it sorts living things into nested groups based on shared features. The more features two organisms share, the deeper in the hierarchy they sit together. If a lion and a tiger share many features, they are in the same Genus. If a lion and a human share fewer features, they are only grouped together in a much larger category, like Kingdom or Phylum.
📖 Definitions
🔍 Worked example — Easy
An unknown organism is found to have a backbone, breathe air, and produce milk for its young. It is placed in the same group as a cat. Which taxonomic rank is the smallest group that contains both the unknown organism and the cat?
🔍 Worked example — Standard
Write the correct scientific name for the domestic dog, given its genus is *Canis* and its species is *lupus familiaris*.
🔍 Worked example — Exam-style
Two species of birds, *A* and *B*, have DNA sequences that are 98% identical. Two other species, *C* and *D*, have DNA sequences that are 92% identical. Which pair is more closely related evolutionarily, and why?
⚠️ Common errors
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🎯 Exam question
(b) Explain why the binomial system of naming is preferred over common names. (c) A new organism is discovered. It has a backbone, lays eggs, and breathes with lungs. It is placed in the same class as a frog. State the name of this class. (d) Scientists compare the amino acid sequences of a protein in the new organism and a frog. The sequences are 95% identical. Compare the amino acid sequences of the new organism and a human, which are 80% identical. Which organism is more closely related to the frog, and explain your reasoning. [8 marks]
Key ideas — What a species is
A species is a group of organisms that can reproduce to produce fertile offspring.
🧑🏫 Teacher's explanation
Think of a species as a closed biological family. If two individuals can have babies, and those babies can *also* have babies, they belong to the same species. If the babies are sterile (like a mule), the parents are different species. This is the biological species concept, and it is the gold standard for animals.
📖 Definitions
🔍 Worked example — Easy
A lion and a tiger can mate to produce a 'liger'. Ligers are almost always sterile. Are lions and tigers the same species?
🔍 Worked example — Standard
Write the correct scientific name for the domestic dog, given its genus is *Canis* and its species epithet is *lupus familiaris*.
🔍 Worked example — Exam-style
Two species of bacteria, Species A and Species B, are compared. Species A and B share 95% of their DNA base sequences. Species A and Species C share 60% of their DNA base sequences. Which pair is more closely related, and why?
⚠️ Common errors
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🎯 Exam question
Key ideas — The binomial system
The binomial system is an internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and species.
🧑🏫 Teacher's explanation
Imagine you are trying to order a specific type of apple from a global supplier. If you just say "apple", they might send you a Fuji, a Granny Smith, or a Gala. To be precise, you need a two-part code: the first part tells them the *brand* or *family* (like the genus), and the second part tells them the *specific model* (the species). This is exactly how the binomial system works. It is a universal address for every living thing, ensuring that a scientist in Japan and a scientist in Brazil are talking about the exact same organism, regardless of their native language.
📖 Definitions
🔍 Worked example — Easy
Write the correct scientific name for the domestic cat, given that its genus is *Felis* and its specific epithet is *catus*. Then, write the abbreviated form used after the first mention.
🔍 Worked example — Standard
Two students are discussing the relationship between the lion (*Panthera leo*) and the cheetah (*Acinonyx jubatus*). Student A says, "They are in the same genus, so they are very closely related." Student B says, "They are in different genera, so they are not as closely related as two lions would be." Who is correct and why?
🔍 Worked example — Exam-style
An examiner marks a student's answer where the student wrote *homo Sapiens* for humans. Identify the two specific errors in this notation and explain the correct convention.
⚠️ Common errors
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🎯 Exam question
(a) Identify which two organisms are most closely related and explain your reasoning using the binomial system. [2] (b) Explain why the use of binomial names is more reliable than common names for identifying organisms. [2] [4 marks]
Key ideas — Classification and evolutionary relationships
Classification now reflects evolution. Modern classification systems aim to reflect evolutionary relationships: the organisms in a group should share a common ancestor. Early classification relied on morphology (the visible form of an organism) and anatomy (its internal structure). These can mislead, because unrelated organisms living in similar ways can evolve similar features, such as the wings of birds and bats.
🧑🏫 Teacher's explanation
Imagine you are sorting a huge box of Lego bricks. Early on, you might sort them by colour or shape. But if you want to know which bricks were actually made from the same mould, you need to look at the microscopic mould marks. Biology works the same way. We used to sort animals by how they look (morphology), but that is like sorting by colour. A bird and a bat both have wings, so they look similar, but they are not close relatives. They evolved wings separately to fly. This is called convergent evolution, and it tricks us if we only look at the outside.
📖 Definitions
🔍 Worked example — Easy
Write the correct scientific name for the domestic dog, given its genus is *Canis* and its species is *lupus familiaris*.
🔍 Worked example — Standard
Two species, Species A and Species B, have cytochrome c protein sequences. Species A has 0 differences from the reference sequence. Species B has 12 differences. Explain what this suggests about their evolutionary relationship.
🔍 Worked example — Exam-style
Explain why the wings of a bird and the wings of a bat are not evidence of a close evolutionary relationship, despite their similar function.
⚠️ Common errors
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🎯 Exam question
Deduce which organism is more closely related to the reference and explain your reasoning. [6 marks]
Worked example
Lions, Panthera leo, and tigers, Panthera tigris, occasionally mate in captivity. The offspring, called ligers, are infertile.
🔍 Worked example — Easy
Easy: Write the correct scientific name for the domestic cat, given the genus Felis and the species epithet catus. Explain why the first letter of the second word is lowercase.
🔍 Worked example — Standard
Standard: Two bacteria, Bacterium A and Bacterium B, are found in the same pond. They look identical under a microscope. However, when scientists try to cross-breed them, no offspring are produced. Are they the same species? Explain your reasoning using the biological definition of species.
🔍 Worked example — Exam-style
Exam-style: The table shows the number of amino acid differences in cytochrome c between humans and three other organisms: Chimpanzee (0), Dog (11), Yeast (44). (a) Which organism is most closely related to humans? (b) Explain how the amino acid sequence relates to the DNA sequence and why this indicates evolutionary closeness.
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Practice
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