Ch 1 · Variation · O Level Biology 5090
Topic 1 of 3 in Variation and Selection — O Level Biology 5090 — 48 lessons.
Introduction
No two students in a classroom are identical: heights, masses and blood groups all differ. Variation is the name for these differences, and it is the raw material on which natural selection acts (lesson 24.18.3). Examiners expect you to say what kind of variation you are looking at, what causes it and how to display it.
Key ideas — Continuous and discontinuous variation
Cambridge defines variation as differences between individuals of the same species. There are two kinds.
🧑🏫 Teacher's explanation
Imagine you are looking at a group of students in a classroom. If you measure their height, you won't find just two or three heights; instead, you'll find a smooth spectrum where one student is 160.5 cm, another is 160.6 cm, and so on. This is like a smooth ramp or a slide. This is continuous variation.
📖 Definitions
🔍 Worked example — Easy
A researcher measures the height of 100 sunflowers. The heights range from 150 cm to 210 cm, with many plants measuring around 180 cm. What type of variation is this, and what is the most likely shape of the distribution graph?
🔍 Worked example — Standard
Compare the causes of variation for human skin color and human ABO blood groups.
🔍 Worked example — Exam-style
In a population of 500 beetles, 400 have red shells and 100 have green shells. A researcher suggests that shell color is a continuous trait. Evaluate this statement based on the data provided.
⚠️ Common errors
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🎯 Exam question
(a) State the type of variation shown by the height of the plants. [1] (b) Explain why this type of variation occurs. [2] [3 marks]
Key ideas — Mutation
A mutation is a genetic change. A gene mutation is a random change in the base sequence of DNA. Because the base sequence codes for a protein (lesson 24.17.1), a changed sequence may produce a changed protein, or no working protein.
🧑🏫 Teacher's explanation
Imagine you are typing a long essay on a computer. Most of the time, you type exactly what you intend. However, occasionally, you might accidentally hit the 's' key instead of the 'a' key. This tiny error changes the word, which might change the meaning of the sentence, or it might not matter at all. In biology, this 'typo' is a mutation.
📖 Definitions
🔍 Worked example — Easy
A student observes a population of beetles. Most are green, but one is bright red due to a sudden change in its DNA sequence. Describe the nature of this change.
🔍 Worked example — Standard
Compare the effect of UV radiation on the rate of mutation in skin cells versus a controlled environment with no radiation.
🔍 Worked example — Exam-style
An organism has a mutation in a somatic cell (skin cell) and another mutation in a gamete (sperm cell). Explain the difference in their evolutionary significance.
⚠️ Common errors
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🎯 Exam question
(b) Explain why ionising radiation is said to increase the rate of mutation. [2] [3 marks]
Key ideas — Mutation, new alleles and sickle cell anaemia
Mutation is the source of new alleles. An allele is a version of a gene. The only way a new allele can appear is by mutation altering the base sequence of an existing allele. Meiosis and sexual reproduction shuffle alleles into new combinations but cannot create new ones.
🧑🏫 Teacher's explanation
Think of your DNA as a massive instruction manual for building 'you'. Every time a cell copies this manual, there is a tiny chance of a typo—a single letter being swapped for another. This 'typo' is a mutation. While sexual reproduction (meiosis) is like shuffling a deck of cards to create new hands, mutation is like printing a brand new card that wasn't in the deck before. This is the only way to get a brand new allele.
📖 Definitions
🔍 Worked example — Easy
A student observes a population of beetles. Some are green and some are brown. The student claims that the appearance of a new 'blue' beetle in the population is due to sexual reproduction. Explain why this is incorrect.
🔍 Worked example — Standard
In a population of humans, the frequency of the Hb^ extS allele is 0.2 and the frequency of the Hb^ extA allele is 0.8. If a person has the genotype Hb^ extAHb^S, describe their phenotype and their fitness in a malaria-prone region.
🔍 Worked example — Exam-style
Calculate the percentage increase in mutation rate if a chemical increases the base substitution rate from 1 × 10^-9 per base per generation to 5 × 10^-9 per base per generation.
⚠️ Common errors
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🎯 Exam question
Key ideas — Investigating and presenting variation
A typical class investigation measures a continuous feature (height, hand span, mass of bean seeds) and records a discontinuous feature (blood group, tongue rolling).
🧑🏫 Teacher's explanation
Imagine you are looking at a crowd of people. If you measure their height, you won't find everyone is exactly 160 cm or 161 cm; instead, you get a smooth spectrum of values like 160.2 cm, 160.3 cm, etc. This is continuous variation. It is like a sliding scale or a ramp. Because there are no gaps between the values, we draw these as histograms where the bars touch to show the 'flow' of data.
📖 Definitions
🔍 Worked example — Easy
A student measures the mass of 50 bean seeds. The data is continuous. Describe how the student should present this data to show the distribution of mass.
🔍 Worked example — Standard
A class investigates blood groups (ABO) in 30 students. The results are: A (12), B (6), AB (2), O (10). Describe the most appropriate way to present this data.
🔍 Worked example — Exam-style
Calculate the percentage of a population that exhibits a specific discontinuous phenotype if 8 individuals out of a sample of 40 show it.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State whether height is an example of continuous or discontinuous variation. [1] (b) Describe how the researcher should present this data to show the distribution of heights. [3] [4 marks]
Worked example
Question. A student measured the height of 120 students in a year group and recorded the ABO blood group of each. Explain why height shows continuous variation while blood group shows discontinuous variation, and state how each set of data should be presented. [5]
🔍 Worked example — Easy
A researcher observes a population of beetles. Some are green, some are brown, and some are black. There are no beetles that are 'greenish-brown'. Identify the type of variation and state the cause.
🔍 Worked example — Standard
A student measures the mass of 50 sunflowers. The data shows a smooth distribution from 2.1 kg to 4.5 kg. What type of variation is this, and what two factors influence it?
🔍 Worked example — Exam-style
Compare the presentation of data for a study on human skin color (continuous) and human ABO blood groups (discontinuous).
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Practice
Answers