Ch 1 · 17.1 Variation · AS & A Level Biology 9700
Topic 1 of 3 in Selection and evolution — AS & A Level Biology 9700 — 49 lessons.
Introduction
Variation between individuals is the raw material on which natural selection acts, and it arises from both genetic and environmental causes. This lesson distinguishes continuous from discontinuous variation, examines the genetic and environmental causes of variation, explains the genetic basis of each, and shows how to use the t-test to compare two samples. (Syllabus section 17.1; gene mutations are covered in 6.2.)
Key ideas — Continuous and discontinuous variation
Discontinuous variation produces a limited number of distinct phenotypic categories with no intermediates, for example human ABO blood group; it is usually controlled by one or a small number of genes (with little or no environmental influence) and shows a discrete distribution when plotted as a bar chart. Continuous variation produces a complete range of phenotypes between two extremes, with no distinct categories, for example human height or body mass; it is typically controlled by many genes acting together (polygenic inheritance) and is usually strongly influenced by the environment, giving a normal (bell-shaped) distribution when plotted as a histogram.
🧑🏫 Teacher's explanation
Imagine you are sorting a bag of marbles. If you only have red, blue, and green marbles, you have 'discontinuous' categories; you can't have a marble that is halfway between red and blue. This is like your blood type—you are either Type A, B, AB, or O. There is no 'Type A-and-a-half'.
📖 Definitions
🔍 Worked example — Easy
A researcher observes a population of beetles. Some are entirely green and some are entirely brown. No intermediate shades are observed. Identify the type of variation and the likely genetic basis.
🔍 Worked example — Standard
Calculate the percentage change in the mean height of a plant population when grown in nutrient-rich soil versus nutrient-poor soil, given the mean heights are 15 extcm and 12 extcm respectively.
🔍 Worked example — Exam-style
A student plots the mass of 100 sunflower seeds on a graph. The resulting graph shows a bell-shaped curve (normal distribution) centered around 0.5 extg. Explain why this variation is continuous rather than discontinuous.
⚠️ Common errors
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🎯 Exam question
(a) Identify the type of variation shown. [1] (b) Explain the genetic and environmental basis for this type of variation. [3] [4 marks]
Key ideas — Genetic and environmental causes of variation
Genetic causes of variation include mutation, the source of all new alleles, together with the crossing over, independent assortment and random fertilisation that occur during sexual reproduction, all of which produce new combinations of existing alleles. Environmental causes of variation are factors external to the genotype, such as diet, light, temperature and physical activity, that influence how a genotype is expressed as a phenotype. Most variation, particularly continuous variation, results from an interaction between genetic and environmental causes, since a genotype often sets the range within which a phenotype can vary, while the environment determines where within that range an individual phenotype actually lies.
🧑🏫 Teacher's explanation
Think of your phenotype (your observable traits) as a recipe. Your DNA is the recipe card (the genotype), telling you exactly what ingredients to use. However, how the final dish actually looks and tastes depends on the chef's skill and the quality of the ingredients—this is the environment. If you have the 'tall' recipe but don't get enough sunlight or nutrients, you won't actually grow tall.
📖 Definitions
🔍 Worked example — Easy
A student observes that in a population of pea plants, they can only be either purple or white. State the type of variation shown and explain the genetic basis for this.
🔍 Worked example — Standard
Explain why human height is an example of continuous variation.
🔍 Worked example — Exam-style
In a study of plant growth, the mean height of Group A is 15.2 cm and Group B is 18.5 cm. The researcher wants to determine if this difference is due to chance or the treatment. Name the statistical test required and state the null hypothesis.
⚠️ Common errors
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🎯 Exam question
(a) Explain why skin color is considered to show continuous variation. [2] (b) Describe how environmental factors might influence the phenotype of an individual's skin color. [2] [4 marks]
Key ideas — Genetic and environmental factors: examples
🧑🏫 Teacher's explanation
Think of your phenotype (your observable traits) as a final recipe. The 'genetic factors' are the ingredients you were born with—the flour, eggs, and sugar. The 'environmental factors' are how you cook them—the oven temperature, the brand of milk used, or how long you knead the dough. Even with the same ingredients, a different oven temperature will change the final cake.
📖 Definitions
🔍 Worked example — Easy
A researcher observes two traits in a population of pea plants: flower color (purple or white) and plant height (ranging from 10 cm to 50 cm). Classify these types of variation and state the likely genetic basis for each.
🔍 Worked example — Standard
Explain why identical twins might show different skin pigmentation levels despite having identical genotypes.
🔍 Worked example — Exam-style
In a population of birds, the beak length follows a normal distribution. A mutation occurs that affects only one gene responsible for beak length. Predict the change in the pattern of variation and explain why.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Name the type of variation shown. [1] (b) Explain the genetic and environmental basis of this type of variation. [3] [4 marks]
Key ideas — The genetic basis of the two types
🧑🏫 Teacher's explanation
Imagine you are organizing a party. Discontinuous variation is like the type of drink you serve: you either have Cola, Orange Juice, or Water. There is no 'half-orange-cola' option; you fall into one clear category. This is because the 'instruction' (the gene) is simple: it's either A or B.
📖 Definitions
🔍 Worked example — Easy
A researcher observes a population of beetles where individuals are either strictly black or strictly green. Identify the type of variation and explain the genetic basis.
🔍 Worked example — Standard
In a specific plant species, height is controlled by three genes (A, B, and C). Each dominant allele (A, B, or C) adds 5 cm to the height, while the recessive allele adds 0 cm. Calculate the height of a plant with the genotype AaBbCc.
🔍 Worked example — Exam-style
Compare the phenotypic distribution of human blood groups and human skin color.
⚠️ Common errors
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🎯 Exam question
Key ideas — The t-test
The t-test tells you whether the means of two samples of continuous data are significantly different, or whether the difference could be due to chance. The formula is given in the exam:
Worked example
Question. A student measured the length of 10 leaves from an ivy plant growing in shade and 10 from one growing in sun.
🔍 Worked example — Easy
A population of beetles shows two distinct colors: black and green. No intermediate colors are observed. Describe the genetic basis of this variation.
🔍 Worked example — Standard
Explain why human skin color is an example of continuous variation.
🔍 Worked example — Exam-style
Compare the variation seen in blood groups (ABO) and height in humans.
⚠️ Common errors
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Practice
Answers